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  TUNNELING:
  A PRACTICAL TREATISE

  BY

  CHARLES PRELINI, C. E.

  AUTHOR OF “EARTH AND ROCK EXCAVATION,” “DREDGES AND DREDGING,”
  “EARTH SLOPES, RETAINING WALLS AND DAMS,” ETC. PROFESSOR
  OF CIVIL ENGINEERING IN MANHATTAN COLLEGE,
  NEW YORK

  _167 ILLUSTRATIONS_

  SIXTH EDITION, REVISED AND ENLARGED

  [Illustration]

  NEW YORK
  D. VAN NOSTRAND COMPANY
  TWENTY-FIVE PARK PLACE
  1912


  COPYRIGHT, 1912,
  BY
  D. VAN NOSTRAND COMPANY
  NEW YORK

  Stanhope Press
  F. H. GILSON COMPANY
  BOSTON, U.S.A.




PREFACE TO THE SIXTH EDITION


During the few years that have elapsed since the publication of the
first edition of this work, the art of tunneling through different soils
and especially under large bodies of water, has made considerable
progress. During the last ten years, no less than eight subaqueous
tunnels involving the construction of sixteen tubes have been
constructed for the service of the city of New York alone. The reader
will, no doubt, also recall the tunnels under the Boston Harbor, the St.
Clair, the Charles and Detroit Rivers in our own country as well as the
tunnels under the Thames and the Seine in Europe. Engineers, contractors
and workmen have acquired such experience in these difficult underground
and under-river construction that the work is now undertaken without any
of the fear and hesitation that were associated with the earlier
enterprises.

As entirely new methods have been introduced by professional men, it was
found necessary to arrange the presentation of the subject in this sixth
edition so as to give due prominence to these recent methods.

Besides this, other changes have been made in order to give greater
attention to American method of excavating tunnels through rock and
loose soil. This will explain the treatment of the crown-bar and also
the extensive illustration of the heading and bench method as well as
the drift method of driving tunnels which is followed in the United
States.

Space has also been given to important tunnels recently built mainly for
the purpose of illustrating the various methods discussed in the text
and also to bring out more clearly the characteristics of the different
methods of tunnel excavation.

The author hopes that these added features will meet the present
requirements of engineers and students.

  CHARLES PRELINI.

  MANHATTAN COLLEGE,
  NEW YORK CITY.




CONTENTS


                                                                    PAGE

  INTRODUCTORY--THE HISTORICAL DEVELOPMENT OF TUNNEL BUILDING       xiii

  CHAPTER

      I. PRELIMINARY CONSIDERATIONS; CHOICE BETWEEN A TUNNEL AND AN
         OPEN CUT; GEOLOGICAL SURVEYS                                  1

     II. METHODS OF DETERMINING THE CENTER LINE AND FORMS AND
         DIMENSIONS OF CROSS-SECTION                                   9

    III. EXCAVATING MACHINES AND ROCK DRILLS; EXPLOSIVES AND
         BLASTING                                                     22

     IV. GENERAL METHODS OF EXCAVATION; SHAFTS; CLASSIFICATION OF
         TUNNELS                                                      36

      V. METHODS OF TIMBERING OR STRUTTING TUNNELS                    47

     VI. METHODS OF HAULING IN TUNNELS                                59

    VII. TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL
         LININGS OF MASONRY                                           66

   VIII. METHODS OF LINING TUNNELS                                    72

     IX. TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION;
         REPRESENTATIVE MECHANICAL INSTALLATIONS FOR TUNNEL WORK      84

      X. TUNNELS THROUGH HARD ROCK (_continued_); EXCAVATION BY
         DRIFTS; THE SIMPLON AND MURRAY HILL TUNNELS                 102

     XI. TUNNELS THROUGH HARD ROCK (_continued_); EXCAVATION BY
         HEADINGS                                                    130

    XII. EXCAVATING TUNNELS THROUGH SOFT GROUND; GENERAL
         DISCUSSION; THE BELGIAN METHOD                              143

   XIII. THE GERMAN METHOD--EXCAVATING TUNNELS THROUGH SOFT GROUND
         (_continued_); BALTIMORE BELT LINE TUNNEL                   155

    XIV. THE FULL SECTION METHOD OF TUNNELING; ENGLISH METHOD;
         AMERICAN METHOD; AUSTRIAN METHOD                            166

     XV. SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD;
         QUICKSAND TUNNELING; PILOT METHOD                           182

    XVI. OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS;
         BOSTON SUBWAY AND NEW YORK RAPID TRANSIT                    195

   XVII. SUBMARINE TUNNELING; GENERAL DISCUSSION; THE SEVERN TUNNEL  218

  XVIII. SUBMARINE TUNNELING (_continued_); THE COMPRESSED AIR
         METHOD; THE MILWAUKEE WATER-WORKS TUNNEL                    225

    XIX. SUBMARINE TUNNELING (_continued_); THE SHIELD SYSTEM        238

     XX. SUBMARINE TUNNELING (_continued_); THE SHIELD AND
         COMPRESSED AIR METHOD; THE HUDSON RIVER TUNNEL OF THE
         PENNSYLVANIA RAILROAD                                       263

    XXI. SUBMARINE TUNNELING (_continued_); TUNNELS AT VERY SHALLOW
         DEPTH; THE COFFERDAM METHOD; THE PNEUMATIC CAISSON METHOD;
         THE JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND      281

   XXII. ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER
         CONSTRUCTION                                                301

  XXIII. RELINING TIMBER-LINED TUNNELS WITH MASONRY                  315

   XXIV. THE VENTILATION AND LIGHTING OF TUNNELS DURING
         CONSTRUCTION                                                325

    XXV. THE COST OF TUNNEL EXCAVATION AND THE TIME REQUIRED FOR
         WORK                                                        336




LIST OF ILLUSTRATIONS


  FIGURE                                                            PAGE

    1. Diagram Showing Manner of Lining in Rectilinear Tunnels        10

    2. B. R. Value’s Device for Locating the Center Line Inside of
       a Tunnel                                                       11

    3. Triangulation System for Establishing the Center Line of the
       St. Gothard Tunnel                                             12

    4. Method of Transferring the Center Line down Center Shafts      13

    5. Method of Transferring the Center Line down the Side Shafts    14

    6. Method of Laying out the Center Line of Curvilinear Tunnels    15

    7. Diagram of Polycentric Sectional Profile                       19

    8, 9 and 10. Typical Sectional Profiles for Tunnel                20

   11. Soft Ground Bucket Excavating Machine; Central London
       Underground Railway                                            22

   12. Column Mounting for Percussion Drill; Ingersoll Sargent Drill
       Co.                                                            26

   13. Sketch of Diamond Drill Bit                                    27

   14. Diagram Showing Sequence of Excavation for St. Gothard
       Tunnel                                                         36

   15. Diagram Showing Manner of Determining Correspondence of
       Excavation to Sectional Profile                                38

   16. Polar Protractor for Determining Profile of Excavated Cross-
       Section                                                        39

   17. Joining Tunnel Struts by Halving                               48

   18. Round Timber Post and Cap Bearing                              48

   19. Ceiling Strutting for Tunnel Roofs                             49

   20. Ceiling Strutting with Side Post Supports                      49

   21. Sill, Side Post and Cap Cross Frame Strutting                  49

   22. Reinforced Cross Frame Strutting for Treacherous Materials     49

   23. Longitudinal Poling-Board System of Roof Strutting             50

   24. Transverse Poling-Board System of Roof Strutting               50

   25. Shaft with Single Transverse Strutting                         52

   26. Rectangular Frame Strutting for Shafts                         53

   27. Reinforced Rectangular Frame Strutting for Shafts in
       Treacherous Materials                                          53

   28. Strutting of Timber Posts and Railway Rail Caps                56

   29. Strutting Made Entirely of Railway Rails                       56

   30. Rziha’s Combined Strutting and Centering of Cast Iron          57

   31. Cast-Iron Segment of Rziha’s Strutting and Centering           57

   32. Cast-Iron Segmental Strutting for Shafts                       58

   33. Platform Car for Tunnel Work                                   59

   34. Iron Dump-Car for Tunnel Work                                  60

   35. Wooden Dump-Car for Tunnel Work                                60

   36. Box-Car for Tunnel Work                                        61

   37. Elevator Car for Tunnel Shafts                                 65

   38. Ground Mold for Constructing Tunnel Invert Masonry             67

   39. Combined Ground Mold and Leading Frame for Invert and Side
       Wall Masonry                                                   67

   40. Leading Frame for Constructing Side Wall Masonry               68

   41. Plank Center for Constructing the Roof Arch                    69

   42. Trussed Center for Constructing the Roof Arch                  70

   43 and 44. A Typical Form of Timber Lining for Tunnels             73

   45. Diagram Showing Forms adopted for Side-Wall Foundations        76

   46 and 47. Transverse Sections of Tunnels Showing Methods for
       Increasing the Thickness of the Lining at Different Points     79

   48. Refuge Niche in St. Gothard Tunnel                             81

   49. East Portal of Hoosac Tunnel                                   82

   50, 51 and 52. Arrangement of Drill Holes in the Heading of
       Turchino Tunnel                                                91

   53 and 54. Arrangement of Drill Holes in the Heading of the Fort
       George Tunnel                                                  91

   55. Diagram Showing Sequence of Excavations in Drift Method of
       Tunneling Rock                                                102

   56. Sketches Showing Sequence of Work in Excavating and Lining
       the Simplon Tunnel                                            111

   57. General Details of the Brandt Rotary Drills Employed at the
       Simplon Tunnel                                                112

   58. Sequence of Excavation in the Murray Hill Tunnel              124

   59. Traveling Platform for the Excavation of the Upper Side of
       the Murray Hill Tunnel                                        125

   60. Timbering Used in the Murray Hill Tunnel                      126

   61. Diagram Showing Sequence of Excavation in Heading Method of
       Tunneling Rock                                                132

   62. Method of Strutting Roof, St. Gothard Tunnel                  135

   63. Sketch Showing Arrangement of Tracks, St. Gothard Tunnel      135

   64. Arrangement of Drill Holes in the Fort George Tunnel          137

   65. Longitudinal Section of the Heading and Bench Excavation at
       the Fort George Tunnel                                        137

   66. Diagram Showing the Arrangement of Drill Holes in the
       Heading and Bench of the Gallitsin Tunnel                     140

   67. Diagram Showing Modification of the Heading and Bench Method  140

   68 and 68A. Diagrams Showing Sequence of Excavation in the
       Belgian Method                                                145

   69. Sketch Showing Radial Roof Strutting, Belgian Method          147

   70. Sketch Showing Roof Arch Center, Belgian Method               147

   71. Sketch Showing Method of Underpinning Roof Arch with the
       Side Wall Masonry                                             149

   72. Longitudinal Section Showing Construction by the Belgian
       Method                                                        149

   73. Diagram Showing Sequence of Excavation in Modified Belgian
       Method                                                        152

   74. Sketch Showing Failure of Roof Arch by Opening at Crown       153

   75. Sketch Showing Methods of Repairing Roof Arch Failures        154

   76. Diagrams Showing Sequence of Excavation in German Method of
       Tunneling                                                     155

   77. Diagram Showing Sequence of Excavation in Water Bearing
       Material, German Method                                       156

   78. Sketch Showing Work of Excavating and Timbering Drifts and
       Headings                                                      157

   79. Sketch Showing Method of Roof Strutting                       157

   80. Sketch Showing Roof Arch Centers and Arch Construction        158

   81. Sketch Showing Method of Excavating and Strutting Baltimore
       Belt Line Tunnel                                              162

   82. Roof Arch Construction with Timber Centers, Baltimore Belt
       Line Tunnel                                                   163

   83. Roof Arch Construction with Iron Centers, Baltimore Belt
       Line Tunnel                                                   164

   84. Diagram Showing Sequence of Excavation in English Method of
       Tunneling                                                     167

   85. Sketches Showing Construction of Strutting, English Method    168

   86 and 87. Sketches of Typical Timber Roof-Arch Centers, English
       Method                                                        169

   88. Sequence of Excavation in the American Method                 172

   89. Strutting the Heading in the American Method                  172

   90. Temporary Timbering of the Roof in the American Method        173

   91. Showing Crown Bars Supported by Segmental Arches              173

   92. Transversal and Longitudinal Section of a Tunnel Excavated
       and Strutted According to the American Method                 174

   93 and 94. Diagrams Showing Sequence of Excavation in Austrian
       Method of Tunneling                                           177

   95, 96 and 97. Sketches Showing Construction of Strutting,
       Austrian Method                                               178

   98. Sketch Showing Manner of Constructing the Lining Masonry,
       Austrian Method                                               179

   99. Diagram Showing Sequence of Excavation in Italian Method of
       Tunneling                                                     183

  100. Sketch Showing Strutting for Lower Part of Section            183

  101 and 101A. Sketches Showing Construction of Centers, Italian
       Method                                                        184

  102. Sketch Showing Invert and Foundation Masonry, Italian Method. 185

  103. Sketch Showing Longitudinal Section of a Tunnel under
       Construction, Italian Method                                  186

  104. Sketch Showing Sequence of Excavation, Stazza Tunnel          186

  105. Sketch Showing Method of Strutting First Drift, Stazza
       Tunnel                                                        187

  106 and 107. Sketches Showing Temporary Strutting Arch
       Construction, Stazza Tunnel                                   187

  108. Sketch Showing Preliminary Drainage Galleries, Quicksand
       Method                                                        190

  109. Sketch Showing Construction of Roof Strutting, Quicksand
       Method                                                        190

  110. Sketch Showing Construction of Masonry Lining, Quicksand
       Method                                                        191

  111. Sketch Showing Pilot Method of Tunneling                      193

  112. Diagram Showing Sequence of Construction in Open-Cut Tunnels  197

  113. Sketch Showing Method of Timbering Open-Cut Tunnels, Double
       Parallel Trench Method                                        198

  114. Side-Wall Foundation Construction Open-Cut Tunnels            198

  115. Wide-Arch Section, Boston Subway                              204

  116. Double-Barrel Section, Boston Subway                          205

  117. Four-Track Rectangular Section, Boston Subway                 206

  118. Section Showing Slice Method of Construction, Boston Subway   206

  119. Double-Track Section, New York Rapid Transit Railway          212

  120. Park Avenue Deep Tunnel Construction, New York Rapid Transit
       Railway                                                       214

  121. Harlem River Tunnel, New York Rapid Transit Railway           215

  122. Sketch Showing Underground Stream, Milwaukee Water-Works
       Tunnel                                                        229

  123. Sketch Showing Methods of Lining, Milwaukee Water-Works
       Tunnel                                                        232

  124. Longitudinal Section of Brunel’s Shield, First Thames Tunnel  241

  125. First Shield Invented by Barlow                               242

  126. Second Shield Invented by Barlow                              243

  127. Shield Suggested by Greathead for the Proposed North and
       South Woolwich Subway                                         245

  128. Beach’s Shield Used on Broadway Pneumatic Railway Tunnel      245

  129. Shield for City and South London Railway                      246

  130. Shield for St. Clair River Tunnel                             247

  131. Shield for Blackwall Tunnel                                   248

  132. Elliptical Shield for Clichy Sewer Tunnel, Paris              249

  133. Semi-Elliptical Shield for Clichy Sewer Tunnel                250

  134. Roof Shield for Boston Subway                                 251

  135. Transversal and Longitudinal Section of Prelini’s Shield      252

  136. Elevation and Section of Hydraulic Jack, East River Gas
       Tunnel                                                        260

  137. Cast-Iron Lining, St. Clair River Tunnel                      262

  138. General Elevations and Sections of Shields                    270

  139. Plan and Elevation of First Bulkhead Wall in South Tube,
       Manhattan                                                     273

  140. Typical Cross-Sections of One Tube of Pennsylvania Railroad
       Tunnel under the Hudson River                                 278

  141. Sections of Cofferdam, Van Buren St. Tunnel, Chicago          283

  142. Showing Working Platforms and Piles Sunk in Dredged Channel   286

  143. Showing Sheeting-Piles for the Sides of the Caisson and
       Trussed Beam for the Roof                                     287

  144. Showing the Caisson with the Working-Chamber                  287

  145. Showing the Tunnel Constructed within the Caisson             289

  146. Showing Sides of the Caisson and Supports for the Roof        290

  147. Showing the Roof of the Caisson Formed by the Upper Half of
       the Tunnel                                                    291

  148. Showing the Tunnel Completed by Building the Lower Half
       within the Caisson                                            292

  149. Transversal Section of the Caissons for the Tunnel under the
       Seine River                                                   294

  150. Showing the Joining of the Caissons at the Pont Mirabeau
       Tunnel under the Seine River                                  295

  151. Cross-Sections and Plans of the Detroit River Tunnel          298

  152. Tunneling through Caved Material by Heading                   306

  153. Tunneling through Caved Material by Drifts                    307

  154 and 155. Filling in Roof Cavity Formed by Falling Material     307

  156. Timbering to Prevent Landslides at Portal                     308

  157. Shortening Tunnel Crushed by Landslide at Portal              308

  158. Extending Tunnel through Landslide at Portal                  309

  159 and 160. Relining Timber-Lined Tunnel                          316

  161. Relining Timber-Lined Tunnel, Great Northern Ry               317

  162. Relining Timber-Lined Tunnel, Great Northern Ry               318

  163. Relining Timber-Lined Tunnel, Great Northern Ry               319

  164. Construction of Centering Mullan Tunnel                       320

  165. Centering Mullan Tunnel                                       321

  166. Relining Timber-Lined Tunnel, Norfolk & Western Ry            322

  167. Relining Timber-Lined Tunnel, Norfolk & Western Ry            323




INTRODUCTION

THE HISTORICAL DEVELOPMENT OF TUNNEL BUILDING.


A tunnel, defined as an engineering structure, is an artificial gallery,
passage, or roadway beneath the ground, under the bed of a stream, or
through a hill or mountain. The art of tunneling has been known to man
since very ancient times. A Theban king on ascending the throne began at
once to drive the long, narrow passage or tunnel leading to the inner
chamber or sepulcher of the rock-cut tomb which was to form his final
resting-place. Some of these rock-cut galleries of the ancient Egyptian
kings were over 750 ft. long. Similar rock-cut tunneling work was
performed by the Nubians and Indians in building their temples, by the
Aztecs in America, and in fact by most of the ancient civilized peoples.

The first built-up tunnels of which there are any existing records were
those constructed by the Assyrians. The vaulted drain or passage under
the southeast palace of Nimrud, built by Shalmaneser II. (860-824 B.C.),
is in all essentials a true soft-ground tunnel, with a masonry lining. A
much better example, however, is the tunnel under the Euphrates River,
which may quite accurately be claimed as the first submarine tunnel of
which there exists any record. It was, however, built under the dry bed
of the river, the waters of which were temporarily diverted, and then
turned back into their normal channel after the tunnel work was
completed, thus making it a true submarine tunnel only when finished.
The Euphrates River tunnel was built through soft ground, and was lined
with brick masonry, having interior dimensions of 12 ft. in width and
15 ft. in height.

Only hand labor was employed by these ancient peoples in their tunnel
work. In soft ground the tools used were the pick and shovels, or
scoops. For rock work they possessed a greater range of appliances.
Research has shown that among the Egyptians, by whom the art of
quarrying was highly developed, use was made of tube drills and saws
provided with cutting edges of corundum or other hard, gritty material.
The usual tools for rock work were, however, the hammer, the chisel, and
wedges; and the excellence and magnitude of the works accomplished by
these limited appliances attest the unlimited time and labor which must
have been available for their accomplishment.

The Romans should doubtless rank as the greatest tunnel builders of
antiquity, in the number, magnitude, and useful character of their
works, and in the improvements which they devised in the methods of
tunnel building. They introduced fire as an agent for hastening the
breaking down of the rock, and also developed the familiar principle of
prosecuting the work at several points at once by means of shafts. In
their use of fire the Romans simply took practical advantage of the
familiar fact that when a heated rock is suddenly cooled it cracks and
breaks so that its excavation becomes comparatively easy. Their method
of operation was simply to build large fires in front of the rock to be
broken down, and when it had reached a high temperature to cool it
suddenly by throwing water upon the hot surface. The Romans were also
aware that vinegar affected calcareous rock, and in excavating tunnels
through this material it was a common practice with them to substitute
vinegar for water as the cooling agent, and thus to attack the rock both
chemically and mechanically. It is hardly necessary to say that this
method of excavation was very severe on the workmen because of the heat
and foul gases generated. This was, however, a matter of small concern
to the builders, since the work was usually performed by slaves and
prisoners of war, who perished by thousands. To be sentenced to labor on
Roman tunnel works was thus one of the severest penalties to which a
slave or prisoner could be condemned. They were places of suffering and
death as are to-day the Spanish mercury mines.

Besides their use of fire as an excavating agent, the Romans possessed a
very perfect knowledge of the use of vertical shafts in order to
prosecute the excavation at several different points simultaneously.
Pliny is authority[1] for the statement that in the excavation of the
tunnel for the drainage of Lake Fucino forty shafts and a number of
inclined galleries were sunk along its length of 3¹⁄₂ miles, some of the
shafts being 400 ft. in depth. The spoil was hoisted out of these shafts
in copper pails of about ten gallons’ capacity by windlasses.

  [1] “Tunneling,” Encly. Brit., 1889, vol. xxiii., p. 623.

The Roman tunnels were designed for public utility. Among those which
are most notable in this respect, as well as for being fine examples of
tunnel work, may be mentioned the numerous conduits driven through the
calcareous rock between Subiaco and Tivoli to carry to Rome the pure
water from the mountains above Subiaco. This work was done under the
Consul Marcius. The longest of the Roman tunnels is the one built to
drain Lake Fucino, as mentioned above. This tunnel was designed to have
a section of 6 ft. × 10 ft.; but its actual dimensions are not uniform.
It was driven through calcareous rock, and it is stated that 30,000 men
were employed for eleven years in its construction. The tunnels which
have been mentioned, being designed for conduits, were of small section;
but the Romans also built tunnels of larger sections at numerous points
along their magnificent roads. One of the most notable of these is that
which gives the road between Naples and Pozzuoli passage through the
Posilipo hills. It is excavated through volcanic tufa, and is about 3000
ft. long and 25 ft. wide, with a section of the form of a pointed arch.
In order to facilitate the illumination of this tunnel, its floor and
roof were made gradually converging from the ends toward the middle; at
the entrances the section was 75 ft. high, while at the center it was
only 22 ft. high. This double funnel-like construction caused the rays
of light entering the tunnel to concentrate as they approached the
center, and thus to improve the natural illumination. The tunnel is on a
grade. It was probably excavated during the time of Augustus, although
some authorities place its construction at an earlier date.

During the Middle Ages the art of tunnel building was practiced for
military purposes, but seldom for the public need and comfort. Mention
is made of the fact that in 1450 Anne of Lusignan commenced the
construction of a road tunnel under the Col di Tenda in the Piedmontese
Alps to afford better communication between Nice and Genoa; but on
account of its many difficulties the work was never completed, although
it was several times abandoned and resumed. For the most part,
therefore, the tunnel work of the Middle Ages was intended for the
purposes and necessities of war. Every castle had its private
underground passage from the central tower or keep to some distant
concealed place to permit the escape of the family and its retainers in
case of the victory of the enemy, and, during the defense, to allow of
sorties and the entrance of supplies.

The tunnel builders of the Middle Ages added little to the knowledge of
their art. Indeed, until the 17th century and the invention of gunpowder
no practical improvement was made in the tunneling methods of the
Romans. Engravings of mining operations in that century show that
underground excavation was accomplished by the pick or the hammer and
chisel, and that wood fires were lighted at the ends of the headings to
split and soften the rocks in advance. Although gunpowder had been
previously employed in mining, the first important use of it in tunnel
work was at Malpas, France, in 1679-81, in the tunnel for the Languedoc
Canal. This tunnel was 510 ft. long, 22 ft. wide, and 29 ft. high, and
was excavated through tufa. It was left unlined for seven years, and
then was lined with masonry.

With the advent of gunpowder and canal building the first strong impetus
was given to tunnel building, in its modern sense, as a commercial and
public utilitarian construction, since the days of the Roman Empire.
Canal tunnels of notable size were excavated in France and England
during the last half of the 17th century. These were all rock or
hard-ground tunnels. Indeed, previous to 1800 the soft-ground tunnel was
beyond the courage of engineer except in sections of such small size
that the work better deserves to be called a drift or heading than a
tunnel. In 1803, however, a tunnel 24 ft. wide was excavated through
soft soil for the St. Quentin Canal in France. Timbering or strutting
was employed to support the walls and roof of the excavation as fast as
the earth was removed, and the masonry lining was built closely
following it. From the experience gained in this tunnel were developed
the various systems of soft-ground subterranean tunneling since
employed.

It was by the development of the steam railway, however, that the art of
tunneling was to be brought into its present prominence. In 1820-26 two
tunnels were built on the Liverpool & Manchester Ry. in England. This
was the beginning of the rapid development which has made the tunnel one
of the most familiar of engineering structures. The first railway tunnel
in the United States was built on the Alleghany & Portage R. R. in
Pennsylvania in 1831-33; and the first canal tunnel had been completed
about 13 years previously (1818-21) by the Schuylkill Navigation Co.,
near Auburn, Pa. It would be interesting and instructive in many
respects to follow the rise and progress of tunnel construction in
detail since the construction of these earlier examples, but all that
may be said here is that it was identical with that of the railway.

The art of tunneling entered its last and greatest phase with the
construction of the Mont Cenis tunnel in Europe and the Hoosac tunnel in
America, which works established the utility of machine rock-drills and
high explosives. The Mont Cenis tunnel was built to facilitate railway
communication between Italy and France, or more properly between
Piedmont and Savoy, the two parts of the kingdom of Victor Emmanuel II.,
separated by the Alps. It is 7.6 miles long, and passes under the Col di
Fréjus near Mont Cenis. Sommeiller, Grattoni, and Grandis were the
engineers of this great undertaking, which was begun in 1857, and
finished in 1872. It was from the close study of the various
difficulties, the great length of the tunnel, and the desire of the
engineers to finish it quickly, that all the different improvements were
developed which marked this work as a notable step in the advance of the
art of tunneling. Thus the first power-drill ever used in tunnel work
was devised by Sommeiller. In addition, compressed air as a motive power
for drills, aspirators to suck the foul air from the excavation, air
compressors, turbines, etc., found at Mont Cenis their first application
to tunnel construction. This important rôle played by the Mont Cenis
tunnel in Europe in introducing modern methods had its counterpart in
America in the Hoosac tunnel completed in 1875. In this work there were
used for the first time in America power rock-drills, air compressors,
nitro-glycerine, electricity for firing blasts, etc.

There remains now to be noted only the final development in the art of
soft-ground submarine tunneling, namely, the use of the shield and metal
lining. The shield was invented and first used by Sir Isambard Brunel in
excavating the tunnel under the River Thames at London, which was begun
in 1825, and finished in 1841. In 1869 Peter William Barlow used an iron
lining in connection with a shield in driving the second tunnel under
the Thames at London. From these inventions has grown up one of the most
notable systems of tunneling now practiced, which is commonly known as
the shield system.

In closing this brief review of the development of modern methods of
tunneling, to the presentation of which the remainder of this book is
devoted, mention should be made of a form of motive power which promises
many opportunities for development in tunnel construction. Electricity
has long been employed for blasting and illuminating purposes in tunnel
work. It remains to be extended to other uses. For hauling and for
operating certain classes of hoisting and excavating machinery it is one
of the most convenient forms of power available to the engineer. Its
successful application to rock-drills is another promising field. For
operating ventilating fans it promises unusual usefulness.




TUNNELING




CHAPTER I.

PRELIMINARY CONSIDERATIONS. CHOICE BETWEEN A TUNNEL AND OPEN CUT.
GEOLOGICAL SURVEYS.


CHOICE BETWEEN A TUNNEL AND AN OPEN CUT.

When a railway line is to be carried across a range of mountains or
hills, the first question which arises is whether it is better to
construct a tunnel or to make such a détour as will enable the
obstruction to be passed with ordinary surface construction. The answer
to this question depends upon the comparative cost of construction and
maintenance, and upon the relative commercial and structural advantages
and disadvantages of the two methods. In favor of the open road there
are its smaller cost and the decreased time required in its
construction. These mean that less capital will be required, and that
the road will sooner be able to earn something for its builders. Against
the open road there are: its greater length and consequently its heavier
running expenses; the greater amount of rolling-stock required to
operate it; the heavy expense of maintaining a mountain road; and the
necessity of employing larger locomotives, with the increased expenses
which they entail. In favor of the tunnel there are: the shortening of
the road, with the consequent decrease in the operating expenses and
amount of rolling-stock required; the smaller cost of maintenance,
owing to the protection of the track from snow and rain and other
natural influences causing deterioration; and the decreased cost of
hauling due to the lighter grades. Against the tunnel, there are its
enormous cost as compared with an open road and the great length of time
required to construct it.

To determine in any particular case whether a tunnel or an open road is
best, requires a careful integration of all the factors mentioned. It
may be asserted in a general way, however, that the enormous advance
made in the art of tunnel building has done much to lessen the strength
of the principal objections to tunnels, namely, their great cost and the
length of time required for their construction. Where the choice lies
between a tunnel or a long détour with heavy grades it is sooner or
later almost always decided in favor of a tunnel. When, however, the
conditions are such that the choice lies between a tunnel or a heavy
open cut with the same grades the problem of deciding between the two
solutions is a more difficult one.

It is generally assumed that when the cut required will have a vertical
depth exceeding 60 ft. it is less expensive to build a tunnel unless the
excavated material is needed for a nearby embankment or fill. This rule
is not absolute, but varies according to local conditions. For instance,
in materials of rigid and unyielding character, such as rock, the
practical limit to the depth of a cut goes far beyond that point at
which a tunnel would be more economical according to the above rule. In
soils of a yielding character, on the other hand, the very flat slope
required for stability adds greatly to the cost of making a cut.

It may be noted in closing that the same rule may be employed in
determining the location of the ends of the tunnel, for assuming that it
is more convenient to excavate a tunnel than an open cut when the depth
exceeds 60 ft., then the open cut approaches should extend into the
mountain- or hill-sides only to the points where the surface is 60 ft.
above grade, and there the tunnel should begin. If, therefore, we draw
on the longitudinal profile of the tunnel a line parallel to the plane
of the tracks, and 60 ft. above it, this line will cut the surface at
the points where the open-cut approaches should cease and the tunnel
begin. This is a rule-of-thumb determination at the best, and requires
judgment in its use. Should the ground surface, for example, rise only a
few feet above the 60 ft. line for any distance, it is obviously better
to continue the open cut than to tunnel.


THE METHOD AND PURPOSE OF GEOLOGICAL SURVEYS.

When it has been decided to build a tunnel, the first duty of the
engineer is to make an accurate geological survey of the locality. From
this survey the material penetrated, the form of section and kind of
strutting to be used, the best form of lining to be adopted, the cost of
excavation, and various other facts, are to be deduced. In small tunnels
the geological knowledge of the engineer should enable him to construct
a geological map of the locality, or this knowledge may be had in many
cases by consulting the geological maps issued by the State or general
government surveys. When, however, the tunnel is to be of great length,
it may be necessary to call in the assistance of a professional
geologist in order to reconstruct accurately the interior of the
mountain and thereby to ascertain beforehand the different strata and
materials to be excavated, thus obtaining the data for calculating both
the time and cost of excavating the tunnel.

The geological survey should enable the engineer to determine, (1) the
character of the material and its force of cohesion, (2) the inclination
of the different strata, and (3) the presence of water.


=Character of Material.=--The character of the material through which
the proposed tunnel will penetrate is best ascertained by means of
diamond rock-drills. These machines bore an annular hole, and take away
a core for the whole depth of the boring, thus giving a perfect
geological section showing the character, succession, and exact
thickness of the strata. By making such borings at different points
along the center line of the projected tunnel, and comparing the
relative sequence and thickness of the different strata shown by the
cores, the geological formation of the mountain may be determined quite
exactly. Where it is difficult or impracticable to make diamond drill
borings on account of the depth of the mountain above the tunnel, or
because of its inaccessibility, the engineer must resort to other
methods of observation.

The present forms of mountains or hills are due to weathering, or the
action of the destructive atmospheric influences upon the original
material. From the manner in which the mountain or hill has resisted
weathering, therefore, may be deduced in a general way both the nature
and consistency of the materials of which it is composed. Thus we shall
generally find mountains or hills of rounded outlines to consist of soft
rocks or loose soils, while under very steep and crested mountains hard
rock usually exists. To the general knowledge of the nature of its
interior thus afforded by the exterior form of the mountain, the
engineer must add such information as the surface outcroppings and other
local evidences permit.

For the purposes of the tunnel builder we may first classify all
materials as either, (1) hard rock, (2) soft rock, or (3) soft soil.

Hard rocks are those having sufficient cohesion to stand vertically when
cut to any depth. Many of the primary rocks, like granite, gneiss,
feldspar, and basalt, belong to this class, but others of the same group
are affected by the atmosphere, moisture, and frost, which gradually
disintegrate them. They are also often found interspersed with pyrites,
whose well-known tendency to disintegrate upon exposure to air
introduces another destructive agency. For these reasons we may divide
hard rocks into two sub-classes; viz., hard rocks unaffected by the
atmosphere, and those affected by it. This distinction is chiefly
important in tunneling as determining whether or not a lining will be
required.

Soft rocks, as the term implies, are those in which the force of
cohesion is less than in hard rocks, and which in consequence offer less
resistance to attacks tending to break down their original structure.
They are always affected by the atmosphere. Sandstones, laminated clay
shales, mica-schists, and all schistose stones, chalk and some volcanic
rocks, can be classified in this group. Soft rocks require to be
supported by timbering during excavation, and need to be protected by a
strong lining to exclude the air, and to support the vertical pressures,
and prevent the fall of fragments.

Soft soils are composed of detrital materials, having so little cohesion
that they may be excavated without the use of explosives. Tunnels
excavated through these soils must be strongly timbered during
excavation to support the vertical pressure and prevent caving; and they
also always require a strong lining. Gravel, sand, shale, clay,
quicksand, and peat are the soft soils generally encountered in the
excavation of tunnels. Gravels and dry sand are the strongest and
firmest; shales are very firm, but they possess the great defect of
being liable to swell in the presence of water or merely by exposure to
the air, to such an extent that they have been known to crush the
timbering built to support them. Quicksand and peat are proverbially
treacherous materials. Clays are sometimes firm and tenacious, but when
laminated and in the presence of water are among the most treacherous
soils. Laminated clays may be described as ordinary clays altered by
chemical and mechanical agencies, and several modifications of the same
structure are often found in the same locality. They are composed of
laminæ of lenticular form separated by smooth surfaces and easily
detached from each other. Laminated clays generally have a dark color,
red, ocher or greenish blue, and are very often found alternating with
strata of stiatites or calcareous material. For purposes of construction
they have been divided into three varieties.

Laminated clays of the first variety are those which alternate with
calcareous strata and are not so greatly altered as to lose their
original stratification. Laminated clays of the second variety are those
in which the calcareous strata are broken and reduced to small pieces,
but in which the former structure is not completely destroyed; the clay
is not reduced to a humid state. Laminated clays of the third variety
are those in which the clay by the force of continued disturbance, and
in the presence of water, has become plastic. Laminated clays are very
treacherous soils; quicksand and peat may be classed, as regards their
treacherous nature, among the laminated clays of the third variety.


=Inclination of Strata.=--Knowing the inclination of the strata, or the
angle which they make with the horizon, it is easy to determine where
they intersect the vertical plane of the tunnel passing through the
center line, thus giving to a certain extent a knowledge of the
different strata which will be met in the excavation. On the inclination
of the strata depend: (1) The cost of the excavation; the blasting, for
instance, will be more efficient if the rocks are attacked perpendicular
to the stratification; (2) The character of the timbering or strutting;
the tendency of the rock to fall is greater if the strata are horizontal
than if they are vertical; (3) The character and thickness of the
lining; horizontal strata are in the weakest position to resist the
vertical pressure from the load above when deprived of the supporting
rock below, while vertical strata, when penetrated, act as a sort of
arch to support the pressure of the load above. The foregoing remarks
apply only to hard or soft rock materials.

In detrital formations the inclination of the strata is an important
consideration, because of the unsymmetrical pressures developed. In
excavating a tunnel through soft soil whose strata are inclined at 30°
to the horizon, for instance, the tunnel will cut these strata at an
angle of 30°. By the excavation the natural equilibrium of the soil is
disturbed, and while the earth tends to fall and settle on both sides at
an angle depending upon the friction and cohesion of the material, this
angle will be much greater on one side than on the other because of the
inclination of the strata; and hence the prism of falling earth on one
side is greater than on the other, and consequently the pressures are
different, or in other words, they are unsymmetrical. These
unsymmetrical pressures are usually easily taken care of as far as the
lining is concerned, but they may cause serious cave-ins and badly
distort the strutting. Caving-in during excavation may be prevented by
cutting the materials according to their natural slope; but the
distortion of the strutting is a more serious problem to handle, and one
which oftentimes requires the utmost vigilance and care to prevent
serious trouble.


=Presence of Water.=--An idea of the likelihood of finding water in the
tunnel may be obtained by studying the hydrographic basin of the
locality. From it the source and direction of the springs, creeks,
ravines, etc., can be traced, and from the geological map it can be seen
where the strata bearing these waters meet the center line. Not only
ought the surface water to be attentively studied, but underground
springs, which are frequently encountered in the excavation of tunnels,
require careful attention. Both the surface and underground waters
follow the pervious strata, and are diverted by impervious strata. Rocks
generally may be classed as impervious; but they contain crevices and
faults, which often allow water to pass through them; and it is,
therefore, not uncommon to encounter large quantities of water in
excavating tunnels through rock. As a rule, water will be found under
high mountains, which comes from the melted ice and snow percolating
through the rock crevices.

Some detrital soils, like gravel and sand, are pervious, and others,
like clay and shale, are impervious. Detrital soils lying above clay are
almost certain to carry water just above the clay stratum. In tunnel
work, therefore, when the excavation keeps well within the clay stratum,
little trouble is likely to be had from water; should, however, the
excavation cut the clay surface and enter the pervious material above,
water is quite certain to be encountered. The quantity of water
encountered in any case depends upon the presence of high mountains near
by, and upon other circumstances which will attract the attention of the
engineer.

A knowledge of the pressure of the water is desirable. This may be
obtained by observing closely its source and the character of the strata
through which it passes. Water coming to the excavation through rock
crevices will lose little of its pressure by friction, while that which
has passed some distance through sand will have lost a great deal of its
pressure by friction. Water bearing sand, and, in fact, any water
bearing detrital material, has its fluidity increased by water pressure;
and when this reaches the point where flow results, trouble ensues. The
streams of water met in the construction of the St. Gothard tunnel had
sufficient pressure to carry away timber and materials.




CHAPTER II.

METHODS OF DETERMINING THE CENTER LINE AND FORMS AND DIMENSIONS OF
CROSS-SECTION.


DETERMINING THE CENTER LINE.

Tunnels may be either curvilinear or rectilinear, but the latter form is
the more common. In either case the first task of the engineer, after
the ends of the tunnel have been definitely fixed, is to locate the
center line exactly. This is done on the surface of the ground; and its
purpose is to find the exact length of the tunnel, and to furnish a
reference line by which the excavation is directed.


=Rectilinear Tunnels.=--In short tunnels the center line may be
accurately enough located for all practical purposes by means of a
common theodolite. The work is performed on a calm, clear day, so as to
have the instrument and observations subjected to as little atmospheric
disturbance as possible. Wooden stakes are employed to mark the various
located points of the center line temporarily. The observations are
usually repeated once at least to check the errors, and the stakes are
altered as the corrections dictate; and after the line is finally
decided to be correctly fixed, they are replaced by permanent monuments
of stone accurately marked. The method of checking the observations is
described by Mr. W. D. Haskoll[2] as follows:

  “Let the theodolite be carefully set up over one of the stakes, with
  the nail driven into it, selecting one that will command the best
  position so as to range backwards and forwards over the whole length
  of line, and also obtain a view of the two distant points that range
  with the center line; this being done, let the centers of every stake
  ... be carefully verified. If this be carefully done, and the centers
  be found correct, and thoroughly in one visual line as seen through
  the telescope, there will be no fear but that a perfectly straight
  line has been obtained.”

  [2] “Practical Tunneling,” by F. W. Simms.

[Illustration: FIG. 1.--Diagram Showing Manner of Lining in Rectilinear
Tunnels.]

The center line which has thus been located on the ground surface has to
be transposed to the inside of the tunnel to direct the excavation. To
do this let _A_ and _B_ be the entrances and _a_ and _b_ be the two
distinct fixed points which have been ranged in with the center line
located on the ground surface over the hill _A f B_, Fig. 1. The
instrument is set up at _V_, any point on the line _A a_ produced, and a
bearing secured by observation on the center line marked on the surface.
This bearing is then carried into the tunnel by plunging the telescope,
and setting pegs in the roof of the heading. Lamps hung from these pegs
furnish the necessary sighting points. This same operation is repeated
on the opposite side of the hill to direct the excavation from that end
of the tunnel. These operations serve to locate only the first few
points inside the tunnel. As the excavation penetrates farther into the
hill, it becomes impossible to continue to locate the line from the
outside point, and the line has to be run from the points marked on the
roof of the heading. Great accuracy is required in all these
observations, since a very small error at the beginning becomes greater
and greater as the excavation advances. To facilitate the accurate
location of points on the roof of the tunnel, a simple device was
designed by Mr. Beverley R. Value, shown in Fig. 2. Two iron spikes,
each having a small hole in the flat end, are driven into the rock about
9 ins. apart. A brass bar, 1 in. high, ¹⁄₄ in. thick and 10 ins. long,
having a hole near one end and a 1 in. slot at the other, is screwed
tightly into the head of the spikes. The middle part of the bar is
divided into inches and tenths of an inch. A separate brass hanger is
fitted to the bar, having a vernier with its zero at the middle of the
hanger and corresponding to a plumb line attached below. The hanger is
moved back and forth until it coincides with the line of sight of the
transit, and then the readings of the vernier are recorded. Any time
that the hanger is placed on the bar and the vernier marks the same
reading, the plumb line will indicate the center line of the tunnel.
When, instead of one bar, two are inserted at a distance of 20 or 30 ft.
apart, the plumb lines suspended from the hangers will represent the
vertical plane passing through the axis of the tunnel in coincidence
with the one staked out on the surface ground.

[Illustration: FIG. 2.--B. R. Value’s Device for Locating the Center
Line Inside of a Tunnel.]

The location of the center line of a long tunnel, which is to be
excavated under high mountains, is a very difficult operation, and the
engineers usually leave this part of the work to astronomers, who fix
the stations from which the engineers direct the work of construction.
The center lines of all the great Alpine tunnels were located by
astronomers who used instruments of large size. Thus, in ranging the
center line of the St. Gothard tunnel, the theodolite used had an object
glass eight inches in diameter.[3] Instead of the ordinary mounting a
masonry pedestal with a perfectly level top is employed to support the
instrument during the observations. The location is made by means of
triangulation. The various operations must be performed with the
greatest accuracy, and repeated several times in such a way as to reduce
the errors to a minimum, since the final meeting of the headings
depends upon their elimination.

  [3] See also the Simplon Tunnel, Chapter X.

[Illustration: FIG. 3.--Triangulation System for Establishing the Center
Line of the St. Gothard Tunnel.]

The St. Gothard tunnel furnishes perhaps the best illustration of
careful work in locating the center line of long rectilinear tunnels of
any tunnel ever built. The length of this tunnel is 9.25 miles, and the
height of the mountain above it is very great. The center line was
located by triangulation by two different astronomers using different
sets of triangles, and working at different times. The set or system of
triangles used by Dr. Koppe, one of the observers, is shown by Fig. 3;
it consists of very large and quite small triangles combined, the latter
being required because the entrances both at Airolo and Goeschenen were
so low as to permit only of a short sight being taken. The apices of the
triangles were located by means of the contour maps of the Swiss Alpine
Club. Each angle was read ten times, the instrument was collimated four
times for each reading, and was afterwards turned off 5° or 10° to avoid
errors of graduation. The average of the errors in reading was about one
second of arc. The triangulation was compensated according to the method
of least squares. The probable error in the fixed direction was
calculated to be 0.8″ of arc at Goeschenen and 0.7″ of arc at Airolo.
From this it was assumed that the probable deviation from the true
center would be about two inches at the middle of the tunnel, but when
the headings finally met this deviation was found to reach eleven
inches.

Comparatively few tunnels are driven by working from the entrances
alone, the excavation being usually prosecuted at several points at once
by means of shafts. In these cases, in order to direct the excavation
correctly, it is necessary to fix the center line on the bottom of the
shaft. This is accomplished in two ways,--one being employed when the
shaft is located directly over the center line, and the other when the
shaft is located to one side of the center line.

When the shaft is located on the center line two small pillars are
placed on opposite edges of the shaft and collimating with the center
line as shown by Fig. 4. On these two pillars the points corresponding
to the center line are correctly marked, and connected by a wire
stretched between them. To this wire two plumb bobs are fastened as far
apart as possible. These plumb bobs mark two points on the center line
at the bottom of the shaft, and from them the line is extended into the
headings as the work advances. In these operations, heavy plumb bobs are
used. In the New York subway plumb bobs of steel, weighing 25 lbs. each,
were used, and to prevent rotation they were made with cross-sections,
in the shape of a Greek cross, and were sunk in buckets filled with
water. Owing to the difference between the temperature at the top and
that at the bottom of the shaft, strong currents of air are produced,
which keep in constant oscillation the wires to which the bobs are
suspended.

[Illustration: FIG. 4.--Method of Transferring the Center Line down
Center Shafts.]

To determine the center line at the bottom of the shaft, the headings
are first driven from both sides of the shaft, after which a transit is
set up on the same alignment with the two wires, and this will indicate
the vertical plane passing through the axis of construction. Two points
are then fixed on the roof of the tunnel in continuation of this
vertical plane. When the plumb bobs are removed from the shaft and two
small plumb bobs are suspended to the two points mentioned, they will
always give the same vertical plane passing through the axis of
construction transferred from the surface.

Because of the continuous moving of the wires, the fixing of the points
on the roof of the tunnel is very troublesome, and the operation should
be repeated by different men at different times before the points are
permanently fixed.

[Illustration: FIG. 5.--Method of Transferring the Center Line down Side
Shafts.]

When the shaft is placed at one side of the tunnel the pillars or bench
marks are placed normal to the center line on the edges of the shaft as
shown by Fig. 5. Between the points _A_ and _B_ a wire is stretched, and
from it two plumb bobs are suspended, as described in the preceding
case; these plumb bobs establish a vertical plane normal to the axis of
the tunnel. The excavation of the side tunnel is carried along the line
_BW_ until it intersects the line of the main tunnel, whose center line
is determined by measuring off underground a distance equal to the
distance _BO_ on the surface. By setting the instrument over the
underground point _O_, and turning off a right angle from the line _BO_,
the center line of the tunnel is extended into the headings.


=Curvilinear Tunnels.=--There are various methods of locating the center
lines of curvilinear tunnels, but the method of tangent offsets is the
one most commonly employed.

At the beginning the excavation is conducted as closely as may be to
the line of the curve, and as soon as it has progressed far enough the
tangent _AT_, Fig. 6, is ranged out. At _B_ a point is located over
which to set the instrument, and the distance _AB_ is measured for the
purpose of finding the ordinate of the right angle triangle _OAB_. Now
_OA_ = _r_, _AB_ = _d_, and φ = angle _ABO_. Then:

            _r_
  Tang. φ = ---.
            _d_

[Illustration: FIG. 6.--Method of Laying Out the Center Line of
Curvilinear Tunnels.]

Doubling the value of φ and making the angle _ABC_ = 2 φ, the line _BC_
will be fixed and the point _C_ located by taking _AB_ = _BC_. On _BC_
the ordinates are laid off to locate the curve. Prolong _CB_ so that
_CD_ = _CB_. Then the portion of the curve _CF_ is symmetrical with
_CE_, and the ordinates used to locate _EC_ may be employed to locate
_CF_, by laying them off in the reverse order.

In curvilinear tunnels several cases may be considered.

(1) When the tunnel for almost its entire length is driven on a tangent
with a curve at each end.

(2) When the tunnel begins with a curve and ends with a straight line.

(3) When the whole tunnel is in curve from portal to portal.

(4) The helicoidal or corkscrew tunnel.

       *       *       *       *       *

(1) The axis of every one of the great Alpine tunnels is a straight
line, with a curve at each end. To range out the center line of one of
these long tunnels from a curve, no matter how accurately laid out, will
certainly cause an error, which, magnified with the distance, may
produce serious results. To avoid these inconveniences, the
determination of the axis of the tunnel should be made from a straight
line. This means that the tunnel is at first excavated on a straight
line for its entire length and after the headings driven from both
portals have met, the two portions of the tunnel or curve are excavated
and constructed. The portions of the tunnel excavated on straight lines
for conveniences of construction may then be abandoned or used in cases
of accidents or repairs.

When the axis of a short tunnel has a curve at each end and a straight
line in the middle, it is driven directly from the entrances; first,
however, excavating the curvilinear portions of the tunnel. In such a
case it would be advisable to proceed in the following manner. Drive the
headings on the curvilinear portions of the tunnel, staking out the
center line by means of the offsets from the tangents. At the ends of
the curves lay out from both fronts the rectilineal portion of the
tunnel. Only very narrow headings should be excavated at first while the
whole section could be enlarged near the entrances. The excavation of
the headings at the front should advance very rapidly, in order that the
headings may meet in the shortest possible time. When communication is
established, it is comparatively easy to correct an error resulting from
driving the tunnel from the curves.

(2) When a tunnel begins with a curve and ends with a straight line, the
work of excavation should proceed from both ends. From the straight end
of the tunnel only the heading should be driven, while from the
curvilinear end the whole section could be opened at once. By this
arrangement the excavation progresses slowly from the curvilinear end
and rapidly from the straight end of the tunnel. Once communication has
been established and any error corrected, the work of enlarging the
profile of the tunnel may be pushed with the same activity from both
ends.

(3) When the center line of the entire tunnel is a curve, there is more
probability of slight deviations from the true axis of the proposed
work. In such a case it would be advisable to first excavate a narrow
heading and to concentrate all the efforts in driving the headings as
rapidly as possible in order that they may meet in the shortest time.
The center line of these headings is staked out by the usual method of
the offsets from the tangent. The enlarging of the section of the tunnel
could be commenced at both portals and be driven slowly until the
headings have met and any errors corrected, when the work could be
pushed with the greatest activity all along the line.

(4) In corkscrew or helicoidal tunnels the entire center line is on a
curve. In these tunnels, as a rule, there is a great difference of level
between the two portals, one being much higher than the other, so
careful attention should be paid to the tunnel grade. Working in the
limited spaces afforded by narrow headings it is very probable that
errors may be made in fixing both the alignment and the grade of the
tunnel. To prevent these almost unavoidable errors, it would be well to
excavate at first only the headings, to stake the center line in the
roof of these headings and then to lay the grade of the tunnel as
accurately as possible. The work on the headings should be pushed as
rapidly as possible in order that they may meet quickly, so that the
center line, as temporarily laid out, may be corrected and permanently
fixed for the direction of successive operations. In these tunnels the
headings should be excavated near the center of the tunnel cross-section
so that the sides and roof of the heading would be at some distance from
the sides and roof of the proposed tunnel. This arrangement will easily
permit corrections to be made in case any slight difference from the
true line was erroneously made during the excavation of the headings.


FORM AND DIMENSIONS OF CROSS-SECTION.

In deciding upon the sectional profile of a tunnel two factors have to
be taken into consideration: (1) The form of section best suited to the
conditions, and (2) the interior dimensions of this section.


=Form of Section.=--The form of the sectional profile of a tunnel should
be such that the lining is of the best form to resist the pressures
exerted by the unsupported walls of the tunnel excavation, and these
vary with the character of the material penetrated. These pressures are
both vertical and lateral in direction; the roof, deprived of support by
the excavation, tends to fall, and the opposite sides for the same
reason tend to slide inward along a plane more or less inclined,
depending upon the friction and cohesion of the material. In some rocks
the cohesion is so great that they will stand vertically, while it may
be very small in loose earth which slides along a plane whose
inclination is directly proportional to the cohesion.

From the theory of resistance of profiles we know that the resistance of
a line to exterior normal forces is directly proportional to its degree
of curvature, and consequently inversely proportional to the radius of
the curve. Hence the sectional profile of a tunnel excavated through
hard rock, where there are no lateral pressures owing to the great
cohesion of the material, and having to resist only the vertical
pressure, should be designed to offer the greatest resistance at its
highest point, and the curve must, therefore, be sharper there, and may
decrease toward the base. In quicksand, mud, or other material
practically without cohesion, the pressures will all be normal to the
line of the profile, and a circular section is the one best suited to
resist them. These theoretical considerations have been proved correct
by actual experience, and they may be employed to determine in a general
way the form of section to be adopted. Applying them to very hard rock,
they give us a section with an arched roof and vertical side walls. In
softer materials they give us an elliptical section with its major axis
vertical, and in very soft quicksands and mud they give us the circular
section. These three forms of cross-section and their modifications are
the ones commonly employed for tunnels. An important exception to this
general practice, however, is met with in some of the city underground
rapid-transit railways built of late years, where a rectangular or box
section is employed. These tunnels are usually of small depth, so that
the vertical pressures are comparatively light, and the bending strains,
which they exert upon the flat roof, are provided for by employing steel
girders to form the roof lining.

[Illustration: FIG. 7.--Diagram of Polycentric Sectional Profile.]

From what has been said it will be seen that it is impossible to
establish a standard sectional profile to suit all conditions. The best
one for the majority of conditions, and the one most commonly employed,
is a polycentric figure in which the number of centers and the length of
the radii are fixed by the engineer to meet the particular conditions
which exist. In a general way this form of center may be considered as
composed of two parts symmetrical in respect to the vertical axis. Fig.
7 shows such a profile, in which _DH_ is the vertical axis. The section
is unsymmetrical in respect to the horizontal axis _GE_. The upper part
forming the roof arch is usually a semi-circle or semi-oval, while the
lower part, comprising the side walls and invert of floor, varies
greatly in outline. Sometimes the side walls are vertical and the invert
is omitted, as shown by Fig. 8; and sometimes the side walls are
inclined, with their bottoms braced apart by the invert, as shown by
Fig. 9. In more treacherous soils the side walls are curved, and are
connected by small curved sections to the invert, as shown by Fig. 10.
In the last example the side walls are commonly called skewbacks, and
the lower part of the section is a polycentric figure like the upper
part, but dissimilar in form.

In a tunnel section whose profile is composed entirely of arcs the
following conditions are essential: The centers of the springer arcs
_Ga_ and _Ea′_, Fig. 7, must be located on the line _GE_; the center of
the roof arc _bDb′_ must be located on the axis _HD_; the total number
of centers must be an odd number; the radii of the succeeding arcs from
_G_ toward _D_ and _E_ toward _D_ must decrease in length, and finally
the sum of the angles subtended by the several arcs must equal 180°.

[Illustration:

~Fig. 8~

~Fig. 9~

~Fig. 10~

FIGS. 8 to 10.--Typical Sectional Profiles for Tunnel.]


=Dimensions of Section.=--The dimensions to be given to the
cross-section of a tunnel depend upon the purpose for which it is to be
used. Whatever the purpose of the tunnel, the following three points
have to be considered in determining the size of its cross-section: (1)
The size of clear opening required; (2) the thickness of lining masonry
necessary; and (3) the decrease in the clear opening from the
deformation of the lining.

Railway tunnels may be built either to accommodate one or two, three and
four tracks. In single-track tunnels a clear space of at least 2¹⁄₂ ft.
on each side should be allowed for between the tunnel wall and the side
of the largest standard locomotive or car, and a clear space of at least
3 ft. should be allowed for between the roof and the top of the same
locomotive or car. Since the roof of the tunnel is arch-shaped, to
secure a clearance of 3 ft. at every point will necessitate making the
clearance at the center greater than this amount. In double-track
tunnels the same amounts of side and roof clearances have to be provided
for, and, in addition, there has to be a clearance of at least 2 ft.
between trains. On the three- and four-track tunnels only the width
varies while the height remains almost equal to the two track. Referring
to Fig. 7, and assuming the line _AB_ to represent the level of the
tracks, then the ordinary dimensions in feet required for both single-
and double-track tunnels are as follows:--

  +--------------+--------+----------+----------+---------------+
  |            |  HEIGHT, |  WIDTH,  |  HEIGHT, |   HEIGHT,     |
  |            |   D. F.  |   G. E.  |   C. F.  |    C. H.      |
  |            |   FEET.  |   FEET.  |   FEET.  |    FEET.      |
  +------------+----------+----------+----------+---------------+
  |Single track|17.6 to 18|16.5 to 18|6   to 7.4|¹⁄₄ to ¹⁄₈ _AB_|
  |Double track|26.6 to 28|26.6 to 28|6.3 to 6.9|¹⁄₄ to ¹⁄₈ _AB_|
  +------------+----------+----------+----------+---------------+

The dimensions of tunnels built for aqueduct purposes are determined so
as to have an area of cross-section equal to the required waterway. In
the Croton Aqueduct two different types of cross-sections were used, the
circular one for tunnels through rock and the horseshoe section for
tunnels through loose materials. In the Catskill aqueduct three
different cross-sections have been selected, the circular one for
tunnels under pressure and the horseshoe for tunnels at the hydraulic
gradient. These, however, through rock have a cross-section formed of a
semi-circular arch and vertical side walls, while through earth the
semi-circular arch is supported by skewback walls.

In tunnels built for railroad aqueduct sewer and any other purpose the
thickness of the masonry lining to be allowed for varies with the
material penetrated, as will be explained in a succeeding chapter where
the dimensions for various ordinary conditions are given in tabular
form. The lining masonry is subject to deformation in three ways: by the
sinking of the whole masonry structure, by the squeezing together of the
side walls by the lateral pressures, and by the settling of the
roof-arch. The whole masonry structure never sinks more than three or
four inches, and merits little attention. The movement of the side walls
towards each other, which may amount to three or four inches for each
wall without endangering their stability, has, however, to be allowed
for; and similar allowance must be made for the settling of the
roof-arch, which may amount to from nine inches to two feet, when the
arch is built first as in the Belgian system and rests for some time
upon the loose soil.




CHAPTER III.

EXCAVATING MACHINES AND ROCK DRILLS: EXPLOSIVES AND BLASTING.


=Earth-Excavating Machines.=--Comparatively few of the labor-saving
machines employed for breaking up and removing loose soil in ordinary
surface excavation are used in tunnel excavation through the same
material. Several forms of tunnel excavating machines have been tried at
various times, but only a few of them have attained any measure of
success, and these have seldom been employed in more than a single work.
In the Central London underground railway work through clay a continuous
bucket excavator (Fig. 11) was employed with considerable saving in time
and labor over hand work. In some recent tunnel work in America the
contractors made quite successful use of a modified form of steam
shovel. These are the most recent attempts to use excavating machines in
soft ground, and they, like all previous attempts, must be classed as
experiments rather than as examples of common practice. The Thomson
machine,[4] however, can be employed in any tunnel driven through loose
soil. It occupies a comparatively small space and may easily work when
the timbering is used to support the roof of the tunnel. Steam shovel
instead may give efficient result only in the case that the whole
section of the tunnel is open at once and there are no timbers to
prevent the free swinging of the dipper handle and boom. But in tunnels
through loose soils it is almost impossible to open the whole section at
once without the necessity of supporting the roof. Consequently the use
of steam shovel in loose tunnels is very limited. The shovel, the spade,
and the pick, wielded by hand, are the standard tools now, as in the
past, for excavating soft-ground tunnels.

  [4] The machine was designed by Mr. Thomas Thomson, Engineer for
  Messrs. Walter Scott & Co.

[Illustration: FIG. 11.--Soft Ground Bucket Excavating Machine: Central
London Underground Railway.]


=Rock-Excavating Machines.=--At one period during the work of
constructing the Hoosac tunnel considerable attention was devoted to the
development of a rock excavating, boring, or tunneling machine. This
device was designed to cut a groove around the circumference of the
tunnel thirteen inches wide and twenty-four feet in diameter by means of
revolving cutters. It proved a failure, as did one of smaller size,
eight feet in diameter, tried subsequently. During and before the Hoosac
tunnel work a number of boring-machines of similar character were
experimented with at the Mont Cenis tunnel and elsewhere in Europe; but,
like the American devices, they were finally abandoned as impracticable.


=Hand Drills.=--Briefly described, a drill is a bar of steel having a
chisel-shaped end or cutting-edge. The simplest form of hand drill is
worked by one man, who holds the drill in one hand, and drives it with a
hammer wielded by his other hand. A more efficient method of hand-drill
work is, however, where one man holds the drill, and another swings the
hammer or sledge. Another form of hand drill, called a churn drill,
consists of a long, heavy bar of steel, which is alternately raised and
dropped by the workman, thus cutting a hole by repeated impacts.

In drilling by hand the workman holding the drill gives it a partial
turn on its axis at every stroke in order to prevent wedging and to
offer a fresh surface to the cutting-edge. For the same reason the chips
and dust which accumulate in the drill-hole are frequently removed. The
instruments used for this purpose are called scrapers or dippers, and
are usually very simple in construction. A common form is a strong wire
having its end bent at right angles, and flattened so as to make a sort
of scoop by which the drillings may be scraped or hoisted out of the
hole. It is generally advantageous to pour water into the drill-hole
while drilling to keep the drill from heating.


=Power Drills.=--When the conditions are such that use can be made of
them, it is nearly always preferable to use power drills, on account of
their greater speed of penetration and greater economy of work. Power
drills are worked by direct steam pressure, or by compressed air
generated by steam or water power, and stored in receivers from which it
is led to the drills through iron pipes. A great variety of forms of
power drills are available for tunnel work in rock, but they can nearly
all be grouped in one of two classes: (1) Percussion drills, and (2)
Rotary drills.


_Percussion Drills._--The first American percussion drill was patented
by Mr. J. J. Couch of Philadelphia, Penn., in March, 1849. In May of the
same year, Mr. Joseph W. Fowle, who had assisted Mr. Couch in developing
his drill, patented a percussion drill of his own invention. The Fowle
drill was taken up and improved by Mr. Charles Burleigh, and was first
used on the Hoosac tunnel. In Europe Mr. Cavé patented a percussion
drill in France in October, 1851. This invention was soon followed by
several others; but it was not until Sommeiller’s drill, patented in
1857 and perfected in 1861, was used on the Mont Cenis tunnel, that the
problem of the percussion drill was practically solved abroad. Since
this time numerous percussion drill patents have been taken out in both
America and Europe.

A percussion drill consists of a cylinder, in which works a piston
carrying a long piston rod, and which is supported in such a manner that
the drill clamped to the end of the piston rod alternately strikes and
is withdrawn from the rock as the piston reciprocates back and forth in
the cylinder. Means are devised by which the piston rod and drill turn
slightly on their axis after each stroke, and also by which the drill is
fed forward or advanced as the depth of the drill-hole increases. The
drills of this type which are in most common use in America are the
Ingersoll-Sergeant and the Rand. There are various other makes in common
use, however, which differ from the two named and from each other
chiefly in the methods by which the valve is operated. All of these
drills work either with direct steam pressure or with compressed air.
Workable percussion drills operated by electricity are built, but so far
they do not seem to have been able to compete commercially with the
older forms. No attempt will be made here to make a selection between
the various forms of percussion drills for tunnel work, and for the
differences in construction and the merits claimed for each the reader
is referred to the makers of these machines. All of the leading makes
will give efficient service. It goes almost without saying that a good
percussion drill should operate with little waste of pressure, and
should be composed of but few parts, which can be easily removed and
changed.


_Drill Mountings._--For tunnel work the general European practice is to
mount power drills upon a carriage moving on tracks in order that they
be easily withdrawn during the firing of blasts. Connection is made with
the steam or compressed air pipes by means of flexible hose which can
easily be attached or detached as the drill advances or when it is moved
for repairs or during blasts. Two, four, and sometimes more drills are
mounted and work simultaneously on a single carriage. In America it has
been found that column mountings have been more successful for tunnel
work than any other form. The column mounting made by the
Ingersoll-Sergeant Drill Co. is shown in Fig. 12. In using this form of
mounting no tracks or other special apparatus is required; it is not
necessary, as is the case with the carriage mounting, to remove the
débris before resuming operations, but as soon as the blasting has been
finished and the smoke has sufficiently disappeared the column can be
set up and drilling resumed.

[Illustration: FIG. 12.--Column Mounting for Percussion Drill:
Ingersoll-Sergeant Drill Co.]


_Rotary Drills._--Rotary drilling machines, or more simply rotary
drills, were first used in 1857 in the Mont Cenis tunnel. The advantages
claimed for rotary drills in comparison with percussion drills are: (1)
That less power is required to drive the drill, and the power is better
utilized; (2) once the machines work easily they do not require
continual repairs, and (3) in driving holes of large size the interior
nucleus is taken away intact, thus reducing work and increasing the
speed of drilling. Rotary drills are extensively used for geological,
mining, well-driving, and prospecting purposes; but they are very seldom
employed in tunnels in America, although successfully used for this
purpose in Europe. The reason they have not gained more favor among
American tunnel builders is due to some extent perhaps to prejudice, but
chiefly to the great cost of the machine as compared with percussion
drills, and to the expense of diamonds for repairs. Those who advocate
these machines for tunnel work point out, however, that under ordinary
usage the diamonds have a very long life,--borings of 700 lin. ft. being
recorded without repairs to the diamonds.

[Illustration: FIG. 13.--Sketch of Diamond Drill Bit.]

The form of rotary drill used chiefly for prospecting purposes is the
diamond drill. This machine consists of a hollow cylindrical bit having
a cutting-edge of diamonds, which is revolved at the rate of from two
hundred to four hundred revolutions per minute by suitable machinery
operated by steam or compressed air. The diamonds are set in the
cutting-edge of the bit so as to project outward from its annular face
and also slightly inside and outside of its cylindrical sides (Fig. 13).
When the drill rod with the bit attached is rotated and fed forward the
bit cuts an annular hole into the rock; the drillings being removed from
the hole by a constant stream of water which is forced down through the
hollow drill rod and emerges, carrying the débris with it, up through
the narrow space between the outside of the bit and the walls of the
hole. There are various makes of diamond drills, but they all operate in
essentially the same manner.

The rotary drill principally employed in Europe in tunneling is the
Brandt. The cutting-edge of the Brandt drill consists of hardened steel
teeth. The bit is pressed against the rock by hydraulic pressure, and
usually makes from seven to eight revolutions per minute. Some of the
water when freed goes through the hollow bit, keeping it cool, and
cleaning the hole of débris. A water pressure of from 300 to 450 lbs.
per square inch is required to operate these drills. Rotary rock-drills
may be mounted either on carriages or on columns for tunnel work.
Several machines have recently been constructed for the purpose of
breaking the rock in tunnels without blasting, but they did not meet the
approval of tunnel engineers. One of these machines is provided with
numerous electric torches, which are applied to the rock at the front.
By suddenly chilling the rock with a stream of cold water the stone will
crumble away. Another machine was tested, with little success, in the
excavation for the New Grand Central Depot in New York. On the face of
this machine there is a multitude of chipping drills revolving on four
arms and driven by air pressure. They attack the rock and chip it into
fragments, which are carried away by an endless band.


EXPLOSIVES AND BLASTING.

When the holes are once drilled, either by hand or power drills, they
are charged with explosives. The principal explosives employed in
tunneling are gunpowder, nitroglycerine, and dynamite.


=Gunpowder.=--Gunpowder is composed of charcoal, sulphur, and saltpeter
in proportions varying according to the quality of the powder. For
mining purposes the composition employed is 65% saltpeter, 15% sulphur,
and 20% charcoal. It is a black granulated powder having a specific
gravity of 1.5; the black color is given by the charcoal; and the
grains have an angular form, and vary in size from ¹⁄₈ in. to ³⁄₈ in.
Good blasting powder should contain no fine grains, which may be
detected by pouring some of the powder upon a sheet of white paper. The
force developed by the explosion of gunpowder is not accurately known;
it depends upon the space in which it is confined. Different authorities
estimate the pressure at from 15,000 lbs. per sq. in. in loose blasts to
200,000 lbs. per sq. in. in gunnery. Authorities also differ in opinion
as to the character of the gases developed by the explosion of
gunpowder, a matter of vital concern to the tunnel engineer, since they
are likely to affect the health and comfort of his workmen. It may be
assumed in a general way, however, that the oxygen of the saltpeter
converts nearly all of the carbon of the charcoal into carbon dioxide, a
portion of which combines with the potash of the saltpeter to form
carbonate of potash, the remainder continuing in the form of gas. The
sulphur is converted into sulphuric acid, and forms a sulphate of
potash, which by reaction is decomposed into hyposulphite and sulphide.
The nitrogen of the saltpeter is almost entirely evolved in a free
state; and the carbon not having been wholly burnt into carbonic acid,
there is a proportion of carbonic oxide.


=Nitroglycerine.=--Nitroglycerine is one of the modern explosives used
as a substitute for gunpowder. It is a fluid produced by mixing
glycerine with nitric and sulphuric acids; it freezes at +41° F., and
burns very quietly, developing carbonic acid, nitrogen, oxygen, and
water. By percussion or by the explosion of some substances, such as
capsules of gunpowder or fulminate of mercury, nitroglycerine produces a
sudden explosion in which about 1250 volumes of gases are produced. The
pressure of these gases has been calculated at 26,000 atmospheres, or
324,000 lbs. per sq. in. Nitroglycerine explodes very easily by
percussion in its normal state, but with great difficulty when frozen;
hence, in America, at the beginning of its use, it was transported only
in a frozen state. When dirty, nitroglycerine undergoes a spontaneous
decomposition accompanied by the development of gases and the evolution
of heat, which, reaching 388° F., causes it to explode. Notwithstanding
the enormous pressures which nitroglycerine develops, it is very seldom
used in its liquid state, but is mixed with a granular absorbent earth
composed of the shells of diatoms. The fluid undergoes no chemical
change by being absorbed, and explodes, freezes, and burns under the
same conditions as in the fluid state.


=Dynamite.=--The credit of rendering nitroglycerine available for the
purposes of the engineer by mixing it with a granular absorbent is due
to Albert Nobel of Stockholm, Sweden, who named the new material
dynamite. The nitroglycerine in dynamite loses very little of its
original explosive power, but is very much less easily exploded by
percussion, and can be employed in horizontal as well as vertical holes,
which was, of course, not possible in its liquid state. Dynamite must
contain at least 50% of nitroglycerine. Some manufacturers, instead of
using diatomaceous earth, use other absorbents which develop gases upon
explosion and increase the force of the explosion. These mixtures are
classed under the general name of false dynamites. A great many
varieties of dynamite are manufactured, and each manufacturer usually
makes a number of grades to which he gives special names. Dynamite for
railway work, tunneling, and mining contains about 50% of
nitroglycerine; for quarrying about 35%, and for blasting soft rocks
about 30%. It is sold in cylindrical cartridges covered with paper.


=Storage of Explosives.=--In driving tunnels through rock large
quantities of explosives must be used, and it is necessary to have some
safe place for storing them. In many States there are special laws
governing the transportation and storage of explosives; where there is
no regulation by law the engineer should take suitable precautions of
his own devising. It is best to build a special house or hut in one of
the most concealed portions of the work and away from the tunnel, and
protect it with a lightning-rod and from fire. Strict orders should be
given to the watchman in charge not to allow persons inside with lamps
or fire in any form, and smoking should be prohibited. The use of
hammers for opening the boxes should be prohibited; and dynamite,
gunpowder, and fulminate of mercury should not be stored together in the
same room. A quantity of dynamite for two or three days’ consumption may
be stored near the entrance of the tunnel in a locked box, the keys of
which are kept by the foreman of the work. When dynamite has been frozen
the engineer should provide some arrangement by which it may be heated
to a temperature not exceeding 120° F., and absolutely forbid it being
thawed out on a stove or by an open fire.


=Fuses.=--When gunpowder is used in tunneling it is ignited by the
Blickford match. This match, or fuse as it is more commonly called,
consists of a small rope of yarn or cotton having as a core a small
continuous thread of fine gunpowder. To protect the outside of the fuse
from moisture it is coated with tar or some other impervious substance.
These fuses are so well made that they burn very uniformly at the rate
of about 1 ft. in 20 seconds, hence the moment of explosion can be
pretty accurately fixed beforehand. Blickford matches have the objection
for tunnel work of burning with a bad odor, especially when they are
coated with tar, and to remedy this many others have been invented.
Those of Rzika and Franzl are the best known of these. The former has
many advantages, but it burns too quickly, about 3 ft. per second, and
is expensive; the latter consists of a small hollow rope filled with
dynamite.

Blickford matches cannot be used to explode dynamite, the use of a
cartridge being required. These cartridges are small copper cylinders
containing fulminate of mercury. They may be attached to the end of the
Blickford match, which being ignited the spark travels along its length
until it reaches the copper cylinder, where it explodes the fulminate of
mercury, which in turn explodes the dynamite. Blasts may also be fired
by electricity, which, in fact, is the most common and the preferable
method, because several blasts can be fired simultaneously, and because
the current is turned on at a great distance, thus affording greater
safety to the workmen.

The method of electric firing generally employed in America is known as
the connecting series method, and consists in firing several mines
simultaneously. The ends of the wires are scraped bare, and the wire of
the first hole of the series is twisted together with the wire of the
second hole, and so on; finally the two odd wires of the first and last
holes are connected to two wires of a single cable or to two separate
cables extending to some safe place to which the men can retreat. Here
the two cable wires are connected by binding screws to the poles of a
battery, or sometimes to a frictional electric machine. The current
passes through the wires, making a spark at each break, and so fires the
fulminate of mercury, which explodes the dynamite.

Simultaneous firing by electricity by utilizing the united strength of
the blasts at the same instant secures about 10% greater efficiency from
the explosives. Another advantage of electric firing is that in case of
a missfire of any one of the holes there is slight possibility of
explosion afterwards, and the place can be approached at once to
discover the cause.


=Tamping.=--Tamping is the material placed in the hole above the
explosive to prevent the gases of explosion from escaping into the air.
Tamping generally consists of clay. When gunpowder is used the clay must
be well rammed with a wooden tool, and paper, cotton, or some other dry
material must be placed between the moist clay and the powder. When
dynamite is used it is not necessary to ram the tamping, since the
suddenness of the explosion shatters the rock before the clay can be
driven from the hole.

A few experienced men should be appointed to fire the blasts. These men
should give ample warning previous to the blast in order that all
machinery and tools which might be injured by flying fragments may be
removed out of danger, and so that the workmen may seek safety. When
all is ready they should fire the blasts, keeping accurate count of the
explosions to ensure that no holes have missed fire, and should call the
workmen back when all danger is over. In case any hole has missed fire
it should be marked by a red lamp or flag.


=Nature of Explosions.=--When the explosives are ignited a sudden
development of gases results, producing a sudden and violent increase of
pressure, usually accompanied by a loud report. The energy of the
explosion is exerted in all directions in the form of a sphere having
its center at the point of explosion, and the waves of energy lose their
force as the distance from this central point increases. The energy of
the explosion at any point in the sphere of energy is, therefore,
inversely proportional to the distance of this point from the center of
explosion. In the vicinity of the center of explosion the gases have
sufficient power to destroy the force of cohesion and shatter the rock;
further on, as they lose strength, they only destroy the elasticity of
the material and produce cracks; and still further away they only
produce a shock, and do not affect the material. Within the sphere of
energy there are, therefore, three other concentric spheres: the first
one being where cohesion is destroyed, the second where elasticity is
overcome, and the third where the shock is transmitted by elasticity.
When the latter sphere comes below the surface, the gases remain inside
the rock; but when the surface intersects either of the other two
spheres, the gases blow up the rock, forming a cone or crater, whose
apex is at the point of explosion, and which is called the
blasting-cone. The larger the blasting-cone is, the greater is the
amount of rock broken up; and the object of the engineer should,
therefore, always be so to regulate the depth of the hole and the
quantity of explosive as to secure the largest possible blasting cone in
each case. Experiments are required to determine the most efficient
depth of hole, and quantity of explosive to be employed, since these
differ in different kinds of rock, with the position of the rock
strata, etc.; but in ordinary practice, the depths of the holes are
made from 2 to 3 ft. in the heading and upper portion of the tunnel,
when drilled by hand; and from 6 to 8 ft. when drilled by power drills.
In the lower portion of the profile, the holes are made deeper, from 3
ft. to 4 ft. when drilled by hand, and exceeding 6 ft. when drilled by
power. The distance of the holes apart should be about equal to the
diameter of the blasting-cone; as a general rule it is assumed that the
base of the blasting-cone has a diameter equal to twice the depth of the
hole. The following table gives the average number of holes required in
each part of the excavation for the St. Gothard tunnel in which the
heading was excavated by machine drills while the other parts were
excavated by hand drills:

  NO. OF PART.[5]         NAME OF PART.           NO. OF HOLES.
       1.          Heading                           6 to  9
       2.          Right wing of heading             3 to  5
       3.          Left wing of heading              3 to  5
       4.          Shallow trench with core          2
       5.          Deepening of trench to floor      6 to  9
       6.          Narrow mass of core to left       3
       7.          Greater mass of core to left      6 to  9
       8.          Culvert                           1
                                                    --------
                         Total section              30 to 43

  [5] The location of the parts numbered is shown by Fig. 14, p. 36.

The quantity of explosives required for blasting depends upon the
quality of the rock, since the force of the explosives must overcome the
cohesion of the rock, which varies with its nature, and often differs
greatly in rocks of the same kind and composition. The quantity of
explosives required to secure the greatest efficiency in blasting any
particular rock may be determined experimentally, but in practice it is
usually deduced by the following rules: (1) The blasting force is
directly proportional to the weight of the explosives used, and (2) the
bulk of the blasted rock is proportional to the cube of the depth of the
holes. It is usually assumed, also, that the explosive should fill at
least one-fourth the depth of the hole.

The following table gives the depth of holes and amount of dynamite used
at each advance in the Fort George Tunnel illustrated on page 135.

  +-------------------+------------+--------+-------+----------+
  |  ORDER OF FIRING. |  KINDS OF  | DEPTH. |CHARGE.|  KIND OF |
  |                   |   HOLES.   |        |       | DYNAMITE.|
  +-------------------+------------+--------+-------+----------+
  |Bench   { 1st round|4 grading   |3′ to 5′|50 lbs.|40% climax|
  |Holes   {          |5 bench     |9′ 6″   |45  „  |40%   „   |
  |        { 2nd round|6 trimming  |3′ to 9′|42  „  |40%   „   |
  |                   |            |        |       |          |
  |Heading { 3d round |8 center cut|9′      |56  „  |60%   „   |
  | Holes  { 4th round|8 side      |8′      |48  „  |40%   „   |
  |        { 5th round|6 dry       |8′      |36  „  |40%   „   |
  +-------------------+------------+--------+-------+----------+




CHAPTER IV.

GENERAL METHODS OF EXCAVATION: SHAFTS: CLASSIFICATION OF TUNNELS.


A number of different modes of procedure are followed in excavating
tunnels, and each of the more important of these will be considered in a
separate chapter. There are, however, certain characteristics common to
all of these methods, and these will be noted briefly here.

[Illustration: FIG. 14.--Diagram Showing Sequence of Excavation for St.
Gothard Tunnel.]

[Illustration: FIG. 15.--Diagram Showing Manner of Determining
Correspondence of Excavation to Sectional Profile.]


=Division of Section.=--It may be asserted at the outset that the whole
area of the tunnel section is not ordinarily excavated at one time, but
that it is removed in sections, and as each section is excavated it is
thoroughly timbered or strutted. The order in which these different
sections are excavated varies with the method of excavation, and it is
clearly shown for each method in succeeding chapters. As a single
example to illustrate the proposition just made, the division of the
section and the sequence of excavation adopted at the St. Gothard tunnel
is selected (Fig. 14). The different parts of the section were excavated
in the order numbered; the names given to each part, and the number of
holes employed in breaking it down, are given by the table on page 35.
Whatever method is employed, the work always begins by driving a
heading, which is the most difficult and expensive part of the
excavation. All the other operations required in breaking down the
remainder of the tunnel section are usually designated by the general
term of enlargement of the profile. The various operations of excavation
may, therefore, be classified either as excavation of the heading or
enlargement of the profile.


=Excavation of the Heading.=--There is considerable confusion among the
different authorities regarding the exact definition of the term
“heading” as it is used in tunnel work. Some authorities call a small
passage driven at the top of the profile a heading, and a similar
passage driven at the bottom of the profile a drift; others call any
passage driven parallel to the tunnel axis, whether at the top or at the
bottom of the profile, a drift; and still others give the name “heading”
to all such passages. For the sake of distinctness of terminology it
seems preferable to call the passage a heading when it is located at the
top of the profile, and a drift when it is located near the bottom.

Headings and drifts are driven in advance of the general excavation for
the following purposes: (1) To fix correctly the axis of the tunnel; (2)
to allow the work to go on at different points without the gangs of
laborers interfering with each other; (3) to detect the nature of
material to be dealt with and to be ready in any contingency to overcome
any trouble caused by a change in the soil; and (4) to collect the
water. The dimensions of headings in actual practice vary according to
the nature of the soil through which they are driven. As a general rule
they should not be less than 7 ft. in height, so as to allow the men to
work standing, and have room left for the roof strutting. The width
should not be less than 6 ft., to allow two men to work at the front,
and to give room for the material cars without interfering with the wall
strutting. Usually headings are made 8 ft. wide. The length of headings
in practice varies according to circumstances. In very long tunnels
through hard rock the headings are sometimes excavated from 1000 ft. to
2000 ft. in advance, in order that they may meet as soon as possible and
the ranging of the center line be verified, and so that as great an
area of rock as possible may be attacked at the same time in the work of
enlarging the profile. In short tunnels, where the ranging of the center
line is less liable to error, shorter headings are employed, and in soft
soils they are made shorter and shorter as the cohesion of the soil
decreases. When the material has too little cohesion to stand alone, the
tops and sides of the heading require to be supported by strutting. To
prevent caving at the front of the heading, the face of the excavation
is made inclined, the inclination following as near as may be the
natural slope of the material.


=Enlargement of the Profile.=--The enlargement of the profile is
accomplished by excavating in succession several small prisms parallel
to the heading, and its full length, which are so located that as each
one is taken out the cross-section of the original heading is enlarged.
The number, location, and sequence of these prisms vary in different
methods of excavation, and are explained in succeeding chapters where
these methods are described. To direct the excavation so as to keep it
always within the boundaries of the adopted profile, the engineer first
marks the center line on the roof of the heading by wooden or metal
pegs, or by some other suitable means by which a plumb line may be
suspended. He next draws to a large scale a profile of the proposed
section; and beginning at the top of the vertical axis he draws
horizontal lines at regular intervals, as shown by Fig. 15, until they
intersect the boundary lines of the profile, and designates on each of
these lines the distance between the vertical axis and the point where
it intersects the profile. It is evident that if the foreman of
excavation divides his plumb line in a manner corresponding to the
engineer’s drawing, and then measures horizontally and at right angles
to the vertical center plane of the tunnel the distance designated on
the horizontal lines of the drawing, he will have located points on the
profile of the section, or in other words have established the limits of
excavation.

[Illustration: FIG. 16.--Polar Protractor for Determining Profile of
Excavated Cross-Section.]

In the excavation of the Croton Aqueduct for the water supply of New
York city, an instrument called a polar protractor was used for
determining the location of the sectional profile. It was invented by
Mr. Alfred Craven, division engineer of the work. This instrument
consists of a circular disk graduated to degrees, and mounted on a
tripod in such a manner that it may be leveled up, and also have a
vertical motion and a motion about the vertical axis. The construction
is shown clearly by Fig. 16. In use the device is mounted with its
center at the axis of the tunnel. A light wooden measuring-rod tapering
to a point, shod with brass and graduated to feet and hundredths of a
foot, lies upon the wooden arm or rest, which revolves upon the face of
the disk, and slides out to a contact with the surface of the
excavation at such points as are to be determined. If the only
information desired is whether or not the excavation is sufficient or
beyond the established lines, the rod is set to the proper radius, and
if it swings clear the fact is determined. If a true copy of the actual
cross-section is desired, the rod is brought into contact with the
significant points in the cross-section, and the angles and distances
are recorded.

The general method of directing the excavation in enlarging the profile
by referring all points of the profile to the vertical axis is the one
usually employed in tunneling, and gives good results. It is considered
better in actual practice to have the excavation exceed the profile
somewhat than to have it fall short of it, since the voids can be more
easily filled in with riprap than the encroaching rock can be excavated
during the building of the masonry. In tunnels where strutting is
necessary the excavation must be made enough larger than the finished
section to provide the space for it. In soft-ground tunnels it is also
usual to enlarge the excavation to allow for the probable slight sinking
of the masonry. The proper allowance for strutting is usually left to
the judgment of the foreman of excavation, but the allowance for
settlement must be fixed by the engineer.


SHAFTS.

Shafts are vertical walls or passages sunk along the line of the tunnel
at one or more points between the entrances, to permit the tunnel
excavation to be attacked at several different points at once, thus
greatly reducing the time required for excavation. Shafts may be located
directly over the center of the tunnel or to one side of it, and, while
usually vertical, are sometimes inclined. During the construction of the
tunnel the shafts serve the same purpose as the entrances; hence they
must afford a passageway for the excavated materials, which have to be
hoisted out, and also for the construction tools and materials which
have to be lowered down them. They must also afford a passageway for
workmen, draft animals, and for pipes for ventilation, water, compressed
air, etc. The character of this traffic indicates the dimensions
required, but these depend also upon the method of hoisting employed.
Thus, when a windlass or horse gin is used, and the materials are
hoisted in buckets of small dimensions, the dimensions of the shaft may
also be small; but when steam elevators are employed, and the material
is carried on cars run on to the platform of the elevator, large
dimensions must be given to the shaft. Generally the parts of the shaft
used for different purposes are separated by partitions. The elevator
for workmen and the various pipes are placed in one compartment, while
the elevator for hoisting the excavated material and lowering
construction material is placed in another.

Shafts may be either temporary or permanent. They are temporary when
they are filled in after the tunnel is completed, and permanent when
they are left open to supply ventilation to the tunnel. Permanent shafts
are usually made circular, and lined with brick, unless excavated in
very hard and durable rock. When sunk for temporary use only, shafts are
usually made rectangular with the greater dimension transverse to the
tunnel. They are strutted with timber. A pump is generally located at
the bottom of the shaft to collect the water which seeps in from the
sides of the shaft and from the tunnel excavation. The dimensions of
this pump will of course vary with the amount of water encountered, as
will also the capacity of the pump for forcing it up and out of the
shaft, which has always to be kept dry.

The majority of engineers prefer to sink shafts directly over the center
line of the tunnel. Side shafts are employed chiefly by French
engineers. The chief advantage of the former method is the great
facility which it affords for hoisting out the materials, while in favor
of the latter method is the non-interference of the shaft with the
operations inside the tunnel. Were it not that the side shaft requires
the introduction of a transverse gallery connecting it with the tunnel,
it would be on the whole superior to the center shaft; but the side
gallery necessitates turning the cars at right angles, and consequently
the use of a very sharp curve or a turntable to reach the shaft bottom,
which is a disadvantage that may outweigh its advantages in some other
respects. It is impossible to state absolutely which of these methods of
locating shafts is the best; both present advantages and disadvantages,
and the use of one or the other is usually determined more by the local
conditions than by any general superiority of either.

When side shafts are employed they are sometimes made inclined instead
of vertical. This form is used when the depth of the shaft is small. By
it the hauling is greatly simplified, since the cars loaded at the front
with excavated material can be hauled directly out of the shaft and to
the dumping-place, surmounting the inclined shaft by means of continuous
cables. The short galleries connecting the side shafts with the tunnel
proper usually have a smaller section than the tunnel, but are excavated
in exactly the same manner. Another form of side shaft sometimes used is
one reaching to the surface when the tunnel runs close to the side of
cliff, as is the case with some of the Alpine railway tunnels.


CLASSIFICATION OF TUNNELS.

Tunnels are classified in various ways, but the most logical method
would appear to be a grouping according to the quality of the material
through which they are driven; and this method will be adopted here. By
this method we have first the following general classification: (1)
Tunnels in hard rock; (2) tunnels in ordinary loose soil; (3) tunnels in
quicksand; (4) open-cut tunnels; and (5) submarine tunnels. It is hardly
necessary to say that this classification, like all others, is simply
an arbitrary arrangement adopted for the sake of order and convenience
in treating the subject.


=Tunnels in Hard Rock.=--With the numerous labor-saving methods and
machines now available, hard rock is perhaps the safest and easiest of
all materials through which to drive a tunnel. Tunnels through hard rock
may be excavated, either by a drift or by a heading. The difference
depends upon whether the advance gallery is located close to the floor
or near the soffit of the section.


=Tunnels in Loose Soils.=--In driving tunnels through loose soils many
different methods have been devised, which may be grouped as follows:
(1) Tunnels excavated at the soffit--Belgian method; (2) tunnels
excavated along the perimeter--German method; (3) tunnels excavated in
the whole section--English, Austrian and American methods; (4) tunnels
excavated in two halves independent of each other--Italian method.

(1) Excavating the tunnel by beginning at the soffit of the section, or
by the Belgian method, is the method of tunneling in loose soils most
commonly employed in Europe at the present time. It consists in
excavating the soffit of the section first; then building the arch,
which is supported upon the unexcavated ground; and finally in
excavating the lower portion of the section, and building the side walls
and invert.

(2) In excavating tunnels along the perimeter an annular excavation is
made, following closely the outline of the sectional profile in which
the lining masonry is built, after which the center core is excavated.
In the German method two drifts are opened at each side of the tunnel
near the bottom. Other drifts are excavated, one above the other, on
each side to extend or heighten the first two until all the perimeter is
open except across the bottom. The masonry lining is then built from the
bottom upwards on each side to the crown of the arch, and then the
center core is removed and the invert is built.

(3) This method, as its name implies, consists in taking out short
lengths of the whole sectional profile before beginning the building of
the masonry. In the English method the invert is built first, then the
side walls, and finally the arch. The excavators and masons work
alternately. The Austrian method differs in two particulars from the
English: the length of section opened is made great enough to allow the
excavators to continue work ahead of the masons, and the side walls and
roof are built before the invert. In the American method the whole
section of the tunnel is open at once: excavators and masons work
simultaneously, but a very large quantity of timbering is required.

(4) The Italian method is very seldom employed on account of its
expensiveness, but it can often be used where the other methods fail. It
consists in excavating the lower half of the section, and building the
invert and side walls, and then filling the space between the walls in
again except for a narrow passageway for the cars; next the upper part
of the section is excavated, as in the Belgian method, and the arch is
built; and finally the soil in the lower part is permanently removed.


=Tunnels in Quicksand.=--Tunnels through quicksand are driven by one of
the ordinary soft-ground methods after draining away the water, or else
as submarine tunnels.


=Open-Cut Tunnels.=--Open-cut tunnels are those driven at such a small
depth under the surface that it is more convenient to excavate an open
cut, build the tunnel masonry inside it, and then refill the open
spaces, than it is to carry on the work entirely underground. In firm
soils the usual mode of operation is to excavate first two parallel
trenches for the side walls, then remove the core, and build the arch
and the invert. In unstable soils, since the invert must be built first,
it is usual to open up a single wide trench. In infrequent cases where a
tunnel is desired in a place which is to be filled in, the masonry is
built as a surface structure, which in due time is covered.


=Submarine Tunnels.=--The mode of procedure followed in excavating
submarine tunnels depends upon whether the material penetrated is
pervious or impervious to water. In impervious material any of the
ordinary methods of tunneling found suitable may be employed. In
pervious material the excavation may be accomplished either by means of
compressed air to keep the water out of the excavation, or by means of a
shield closing the front of the excavation, or by a combination of these
two methods. Tunnels on the river bed are built by means of coffer dams
which inclose alternate portions of the work, by sinking a continuous
series of pneumatic caissons and opening communication between them, and
by sinking the tunnel in sections constructed on land.

            {_In hard rock._ {By drifts.
            {                {By a heading.
            {                {_By upper half:_        }
            {                { the arch is built      }Belgian method.
            {                { before the side walls. }
            {                {
            {                {_By the perimeter:_     }
            {                { excavated and lined    }
            {                { before the central     }German method.
            {                { nucleus is removed.    }
            {                {
            {_In loose soil._{_By whole section:_     {English method.
            {                { the lining begins after{Austrian method.
            {                { the whole section is   {American method.
            {                { excavated.             {
            {                {
            {                {_By halves:_            }
            {                { the lower half is      }
            {                { excavated and lined,   }Italian method.
            {                { followed by the work   }
            {                { of the upper half.     }
  METHODS OF{
  EXCAVATING{_In quicksand._
  TUNNELS.  {
            {                {In resistant soils.     {By two lateral
            {                {                        {narrow trenches.
            {_Open-cut_      {
            {_tunnels._      {In loose soils.         {By one very large
            {                {                        {trench.
            {                {
            {                {Built up.                By slices.
            {
            {                {At great depths under   }By any method.
            {                {the river bed.          }
            {                {
            {                {                        {By shield.
            {                {At small depths         {By compressed
            {                {under the river         {air.
            {_Submarine_     {bed.                    {By shield and
            {_tunnels._      {                        {compressed air.
            {                {
            {                {                        {By coffer dams.
            {                {                        {By pneumatic
            {                {On the river bed.       {caissons.
            {                {                        {By built-up
            {                {                        {sections.

The above diagram gives in compact form the classification of tunnels
according to materials penetrated and methods of excavation adopted,
which have been described more fully in the succeeding paragraphs. It
may be noted here again that this is a purely arbitrary classification,
and serves mostly as a convenience in discussing the different classes
of tunnels without confusion.




CHAPTER V.

METHODS OF TIMBERING OR STRUTTING TUNNELS.


The purpose of timbering or strutting in tunnel work is to prevent the
caving-in of the roof and side walls of the excavation previous to the
construction of the lining. As the strutting has to resist all the
pressures developed in the roof and side walls, which may be exceedingly
troublesome and of great intensity in loose soils, its design and
erection call for particular care. The method of strutting adopted
depends upon the method of excavation employed; but in every case the
problem is not only to build it strong enough to withstand the pressures
developed, but to do this as economically as possible, and with as
little hindrance as may be to the work which is going on simultaneously
and which will come later. Only the latter general problems of strutting
peculiar to all methods of tunnel work will be considered here. For this
consideration strutting may be classified according to the material of
which it is built, under the heads of timber structures and iron
structures.

[Illustration: FIG. 17.--Joining Tunnel Struts by Halving.]

[Illustration: FIG. 18.--Round Timber Post and Cap Bearing.]


TIMBER STRUTTING.

Timber is nearly always employed for strutting in tunnel work. So long
as it has the requisite strength, any kind of timber is suitable for
strutting, since, it being only temporarily employed, its durability is
a matter of slight importance. Timber with good elastic properties, like
pine or spruce, is preferably chosen, since it yields gradually under
stress, thus warning the engineer of the approach of danger; while oak
and other strong timbers resist until the last moment, and then yield
suddenly under the breaking load. Soft woods, moreover, are usually
lighter in weight than hard woods, which is a considerable advantage
where so much handling is required in a restricted space. Round timbers
are generally employed, since they are less expensive, and quite as
satisfactory in other respects as sawed timbers. In the English and
Austrian methods of strutting, which are described further on, a few of
the principal struts are of sawed timbers.

The various timbers of the strutting are seldom attached by framed
joints, but wedges are used to give them the necessary bearing against
each other. Where framed joints are employed they are made of the
simplest form usually by halving the joining timbers, as shown by Fig.
17. Fig. 18 shows a form of joint used where round posts carry beams of
similar shape. The reason why it is possible to do away with jointed
connections to such a great extent, is that the strains which the
timbers have to resist are either compressive or bending strains, and
because the timbers are so short that they do not require to be spliced.


=Strutting of Headings.=--The method of strutting the heading that is
employed depends upon the material through which the heading is driven.
In solid rock strutting may not be required at all, or only for the
purpose of preventing the fall of loose blocks from the roof, then
vertical props are erected where required, or horizontal beams are
inserted into the side walls, as shown by Fig. 19. These horizontal
beams may be used singly at dangerous places, or they may be placed from
2 ft. to 3 ft. apart all along the heading. In the latter case they
usually carry a lagging of planks, which may be placed at intervals or
close together, and filled above with stone in case the roof of the
excavation is very unstable. Planks used in this manner are usually
called poling-boards. Where the side walls as well as the roof require
support, vertical side posts are employed to carry the roof beams, as
shown by Fig. 20; and, when necessary, poling-boards are inserted
between these posts and the walls of the excavation.

[Illustration: FIG. 19.--Ceiling Strutting for Tunnel Roofs.]

[Illustration: FIG. 20.--Ceiling Strutting with Side Post Supports.]

[Illustration: FIG. 21.--Sill, Side Post and Cap Cross Frame Strutting.]

[Illustration: FIG. 22.--Reinforced Cross Frame Strutting for
Treacherous Materials.]


_Frame Strutting._--In very loose soils not only the roof and side
walls, but also the floor of the heading require strutting. In these
cases frame strutting is employed, as shown by Fig. 21. It consists
simply of a rectangular frame; at the top there is a crown bar supported
by two vertical side posts setting on a sill laid across the bottom of
the heading. These frames are spaced at close intervals, and carry
longitudinal planks or poling-boards. The sill of the frame is sometimes
omitted when the soil is stable enough to permit it, and in its place
wooden footing blocks are substituted to carry the side posts. In soils
where the pressures are great enough to bend the crown bar, a secondary
frame is employed, as shown by Fig. 22, the two inclined roof members,
or rafters, of which support the crown bar at the center.

[Illustration: FIG. 23.--Longitudinal Poling-Board System of Roof
Strutting.]

[Illustration: FIG. 24.--Transverse Poling-Board System of Roof
Strutting.]

It is the more common practice in driving headings through soft soils to
use inclined poling-boards to support the roof. Fig. 23 shows one method
of doing this. The method of operation is as follows: Assuming the
poling-boards _a_ and _b_ to be in place, and supported by the frames
_A_, _B_, _C_, as shown, the first step in continuation of the work is
to insert the poling-board _c_ over the crown bar of frame _C_, and
under the block _m_. Excavation is then begun at the top, and as fast as
the soil is removed ahead of it the poling-board _c_ is driven ahead
until its rear end only slightly overhangs the crown bar of frame _C_.
The remainder of the face of the heading is then excavated nearly to the
front end of the poling-board _c_, and another frame is set up. By a
succession of these operations the heading is advanced. The
poling-boards at the sides of the heading are placed in a similar manner
to the roof poling-boards. A second method of using inclined
poling-boards is shown by Fig. 24. Here the poling-boards run
transversely, and are supported by the arrangement of timbering shown.
The chief advantage of using these inclined poling-boards, particularly
in the manner shown by Fig. 23, is that the excavators work under cover
at all times, and are thus safe from falling fragments or sudden
cavings.


_Box Strutting._--In very treacherous soils, such as quicksand, peat,
and laminated clay, box strutting is commonly employed. The method of
building this strutting is to set up at the face of the work a
rectangular frame, and use it as a guide in driving a lagging or boxing
of horizontal planks into the soft soil ahead. These planks have sharp
edges, and are driven to a distance of 2 ft. or 3 ft. into the face of
the heading, so as to inclose a rectangular body of earth. This earth is
excavated nearly to the ends of the planks, and then another frame is
inserted close up against the new face of the excavation, which supports
the planks so that the remainder of the earth included by them may be
removed. These two frames, with their plank lagging, constitute a “box;”
and a series of these boxes, one succeeding another, form the strutting
of the heading.


=Strutting the Face.=--In some cases it is found necessary to strut the
face of the heading in order to prevent it from caving in. This is
generally done by setting plank vertically, and bracing them up by means
of inclined props whose feet abut against the sill of the nearest cross
frame. This strutting is erected while the workmen are placing the side
and roof strutting, and is removed to permit excavation.


=Full Section Timber Strutting.=--For strutting the full section two
forms of timbering are employed, known as the polygonal system and the
longitudinal system.

Longitudinal strutting consists of a timber structure so arranged as to
have all the principal members supporting the poling-boards parallel to
the axis of the tunnel. This system of strutting is peculiar to the
English method of tunneling. The longitudinal timbers rest on this
finished masonry at one end, and are carried on a cross frame or by
props at the other end. At intermediate points the longitudinals are
braced apart by struts in planes transverse to the tunnel axis. This
construction makes a very strong strutting framework, since the
transverse struts act as arch ribs to stiffen the longitudinals; but the
use of transverse poling-boards requires the excavation of a larger
cross-section than is necessary when longitudinal poling-boards are
employed, and this increases the cost both for the amount of earth
excavated and the greater quantity of filling required.

In polygonal strutting the main members are in a plane normal to the
axis of the tunnel. They form a polygon whose sides follow closely the
sectional profile of the excavation. These polygonal frames are placed
at more or less short intervals apart, and are braced together by short
longitudinal struts lying close to the sides of the excavation, and
running from one frame to the next, and also by longer longitudinal
members which extend over several frames. The polygonal system of
strutting is peculiar to the Austrian method of tunneling, and is fully
described in a succeeding chapter. One of its distinctive
characteristics is that the poling-boards are inserted parallel to the
tunnel axis. Polygonal strutting is generally held to be stronger than
longitudinal strutting under uniform loads, but it is more liable to
distortion when the loads are unsymmetrical.

[Illustration: FIG. 25.--Shaft with Single Transverse Strutting.]

[Illustration: FIG. 26.--Rectangular Frame Strutting for Shafts.]

[Illustration: FIG. 27.--Reinforced Rectangular Frame Strutting for
Shafts in Treacherous Materials.]


=Strutting of Shafts.=--Tunnel shafts are strutted both to prevent the
caving-in of the sides and to divide them into compartments. When the
material penetrated is very compact, and caving is not likely, a single
series of transverse struts, one above the other, running from the top
to the bottom of the shaft, as shown by Fig. 25, is used to divide it
into two compartments. In softer material, where the sides of the shaft
require support, Fig. 26 shows a form of strutting commonly employed. It
consists of vertical corner posts braced apart at intervals by four
horizontal struts placed close to the walls of the shaft. The longer
side struts are also braced apart at the center by a middle strut which
divides the shaft into two compartments. A lagging of vertical plank is
placed between the walls of the shaft and the horizontal side struts. In
very loose soils the form of strutting shown by Fig. 27 is employed.
This is practically the same construction as is shown by Fig. 26, with
the addition of an interior polygonal horizontal bracing in each half of
the shaft. Referring to Fig. 27, the timbers _a_, _a_, etc., are
vertical and continuous from the top to the bottom of the shaft; and the
horizontal timbers, _b_, _b_, etc., are spaced at more or less close
intervals vertically. The lagging planks may be laid with spaces between
them, or close together, or, in case of very loose material, with their
edges overlapping. The manner of constructing the strutting is also
governed by the stability of the soil. In firm soils it is possible to
sink the shaft quite a depth without timbering, and the timbering can
be erected in sections of considerable length, which is always an
advantage, but in loose soils the timbering has to follow closely the
excavation.

The solid wall shaft struttings which have been described are
discontinued at the point where the shaft intersects the tunnel
excavation; and from this point to the floor of the tunnel an open
timbering is employed, whose only duty is to support the weight of the
solid strutting above. This timbering is made in various forms, but the
most common is a timber truss or arch construction which spans the
tunnel section.


=Quantity of Timber.=--The quantity of timber employed in strutting a
tunnel varies with the character of the material through which the
tunnel is excavated: it is small for solid-rock tunnels, and large for
soft-ground tunnels. In the Belgian method of excavation a smaller
quantity of timber is used than in any of the other ordinary methods.
For single-track tunnels excavated by this method there will be needed
on an average about 3 to 3¹⁄₃ cu. yds. of timber per lineal foot of
tunnel. Practical experience shows that about four-fifths of the timber
once used can be employed for the second time. In any of the methods in
which the whole tunnel section is excavated at once, the average amount
of timber required per lineal foot is about 8.7 cu. yds. Of this amount
about two-thirds can be used a second time. In the Italian method, in
which the upper half and the lower half are excavated separately, about
5 cu. yds. of timber are required per lineal foot of tunnel, about
one-half of which can be employed a second time. For quicksand tunnels
the amount of timbering required per lineal foot varies from 3 to 5
cubic yds. Shaft strutting requires from 1 to 1¹⁄₂ cu. yds. of timber
per lineal foot.


=Dimensions of Timber.=--The dimensions of the principal members
composing the strutting of headings, full section, and shafts, are given
in Table I. The planks used for lagging or the poling-boards are usually
from 4 ins. to 6 ins. wide, with a length depending upon the method of
strutting employed.

TABLE I.

Showing Sizes of Various Timbers Used in Strutting Tunnels Driven
Through Different Materials.

  +---------------------------------+-----------+----------------------+
  |                                 |   ROCK.   |     SOFT SOILS.      |
  |                                 +-----+-----+--------+------+------+
  |                                 |Hard.|Soft.|Compact.|Loose.| Very |
  |                                 |     |     |        |      |loose.|
  |                                 +-----+-----+--------+------+------+
  |                                 | ins.| ins.|  ins.  | ins. | ins. |
  |Headings:                        |     |     |        |      |      |
  | Cap-pieces and vertical struts  |   6 |   8 |   10   |  12  |  14  |
  | Sills                           |     |     |    8   |  10  |  12  |
  | Struts                          |   5 |   5 |    6   |   7  |   8  |
  | Distance apart of the frames in |     |     |        |      |      |
  | feet                            |   6 |  4.5|    3   |  2.6 |  2.6 |
  |                                 |     |     |        |      |      |
  |Strutting of the tunnel,         |     |     |        |      |      |
  |longitudinal strutting:          |     |     |        |      |      |
  | Crown bars                      |  12 |  14 |   14   |      |      |
  | Props vertical or inclined      |     |     |        |      |      |
  | supporting the crown bars       |  10 |  12 |   14   |      |      |
  | Sills                           |   8 |   8 |   10   |      |      |
  | Cap-pieces or saddles           |  10 |  12 |   14   |      |      |
  | Struts to stiffen the structure |   6 |   8 |   10   |      |      |
  | Distance apart of the frames (in|     |     |        |      |      |
  | feet)                           | 4.5 |   4 |    3   |      |      |
  |                                 |     |     |        |      |      |
  |Polygonal strutting:             |     |     |        |      |      |
  | Cap-pieces and contour pieces   |   8 |  10 |   12   |  14  |  16  |
  | Vertical struts on top          |  10 |  12 |   14   |  16  |  18  |
  | Vertical struts below           |  12 |  14 |   16   |  20  |  24  |
  | Intermediate sills              |  12 |  14 |   16   |  20  |  24  |
  | Lower sills                     |     |     |   12   |  16  |  18  |
  | Raking props                    |  10 |  10 |   10   |  12  |  12  |
  | Distance apart of the frames (in|     |     |        |      |      |
  | feet)                           |   6 | 4.5 |    4   |   3  |   3  |
  |                                 |     |     |        |      |      |
  |Shafts:                          |     |     |        |      |      |
  | Horizontal beams forming the    |     |     |        |      |      |
  | frame                           |   8 |   8 |   10   |  12  |  14  |
  | Transverse beams                |   8 |   8 |    8   |  10  |  12  |
  | Vertical struts between the     |     |     |        |      |      |
  | frames                          |   8 |   8 |   10   |  12  |  12  |
  | Struts to reënforce the frame   |     |   6 |    8   |   8  |   8  |
  | Distance apart of the strutting |     |     |        |      |      |
  | (in feet)                       |   6 | 4.5 |    4   |   3  |  2.6 |
  +---------------------------------+-----+-----+--------+------+------+


IRON STRUTTING.

In 1862 Mr. Rziha employed old iron railway rails for strutting the
Naensen tunnel, and his example was successfully followed in several
tunnels built later where timber was scarce and expensive. The
advantages which iron strutting is claimed to possess over the more
common wooden structure are: its greater strength; the smaller amount of
space which it takes up; and the fact that it does not wear out, and
may, therefore, be used over and over again.

[Illustration: FIG. 28.--Strutting of Timber Posts and Railway Rail
Caps.]

[Illustration: FIG. 29.--Strutting made entirely of Railway Rails.]


=Iron Strutting in Headings.=--In strutting the headings the cross
frames have a crown bar consisting of a section of old railway rail
carried either by wood or iron side posts. When wooden side posts are
used their upper ends have a dovetail mortise, and are bound with an
iron band, as shown by Fig. 28. The base of the rail crown bar is set
into the dovetail mortise and fastened by wedges. When iron side posts
are employed they usually consist of sections of railway rails, and the
crown bar is attached to them by fish-plate connections, as shown by
Fig. 29. The iron cross frames are set up as the heading advances, and
carry the plank lagging or poling-boards, exactly in the same manner as
the timber cross frames previously described.

[Illustration: FIG. 30.--Rziha’s Combined Strutting and Centering of
Cast Iron.]

[Illustration: FIG. 31.--Cast-Iron Segment of Rziha’s Strutting and
Centering.]


=Full Section Iron Strutting.=--The iron strutting devised by Mr. Rziha
for full section work is shown by Fig. 30. Briefly described, it
consists of voussoir-shaped cast-iron segments, which are built up in
arch form. Fig. 31 shows the construction of one of the segments, all of
which are alike, with the exception of the crown segment, which has a
mortise and tenon joint which is kept open by filling the mortise with
sand. The segments are bolted together by means of suitable bolt-holes
in the vertical flanges, and when fully connected form an arch rib of
cast iron. This arch rib, A, Fig. 30, carries a series of angle or
T-iron frames bent into approximately voussoir shape, as shown at B,
Fig. 30. Above these frames are inserted the poling-boards, running
longitudinally, and spanning the distance between consecutive arch ribs.
By removing the bent iron frames the cast-iron rib forms a center upon
which to construct the masonry. Finally, to remove the cast-iron rib
itself, the sand is drawn out of the mortise and tenon joint in the
crown segment, which allows the joint to close, and loosen the segments
so that they are easily unbutted.

The illustration, Fig. 30, shows longitudinal poling-boards; more often
longitudinal crown bars of railway rails span the space between
connective arch ribs, and support transverse poling-boards. In building
the masonry, work is begun at the bottom on each side, the bent iron
frames being removed one after another to give room for the masonry. As
each frame is removed, it is replaced with a sort of screw-jack to
support the poling-boards until the masonry is sufficiently completed to
allow their removal. The interior bracing of the arch rib shown at _a a_
and _b b_ consists of railway rails carried by brackets cast on to the
segments. A similar bracing of rails connects the successive arch ribs.
These lines of bracing serve to carry the scaffolding upon which the
masons work in building the lining.

[Illustration: FIG. 32.--Cast-Iron Segmental Strutting for Shafts.]


=Iron Shaft Strutting.=--In soft-ground shaft work, the use of an iron
strutting, consisting of consecutive cast-iron rings, has sometimes
been employed to advantage. Fig. 32 shows the construction of one of
these rings, which, it will be seen, is composed of four segments
connected to each other by means of bolted flanges. The holes shown in
the circumferential web of the ring are to allow for the seepage from
the earth side walls. The method of placing this cylindrical strutting
is to start with a ring having a cutting-edge. By means of excavation
inside the ring, and by ramming, the ring is sunk into the ground a
distance equal to its height. Another ring is then fastened by special
hooks on top of the first one, and the sinking continued until the
second ring is down flush with the surface. A third ring is then added,
and so on until the entire shaft is excavated and strutted. As in timber
shaft strutting, the solid iron ring strutting is carried down only to
the top of the tunnel section, and below this point there is an open
timber or iron supporting framework.




CHAPTER VI.

METHODS OF HAULING IN TUNNELS.


The transportation from one point to another within the tunnel and its
shafts of any material, whether it is excavated spoil or construction
material, is defined as hauling. In all engineering construction, the
transportation of excavated materials, and materials for construction,
constitutes a very important part of the expense of the work; but
hauling in tunnels where the room is very limited, and where work is
constantly in progress over and at the sides of the track, is a
particularly expensive process. Hauling in tunnels may be done either by
way of the entrances, or by way of the shafts, or by way of both the
entrances and shafts.

[Illustration: FIG. 33.--Platform Car for Tunnel Work.]


=Hauling by Way of Entrances.=--When the hauling is done by the way of
the entrances, the materials to be hauled are taken directly from the
point of construction to the entrances, or in the opposite direction,
by means of special cars of different patterns. For general purposes,
these different patterns of cars may be grouped into three
classes,--platform-cars, dump-cars, and box-cars. Representative
examples of these several classes of cars are shown in Figs. 33 to 36[6]
inclusive, but it will be readily understood that there are many other
forms.

  [6] Reproduced from catalogue of Arthur Koppel, New York.

Briefly described, platform-cars (Fig. 33) consist of a wooden platform
mounted on tracks, and they are usually employed for the transportation
of timber, ties, etc. Dump-cars are used in greater numbers in tunnel
work than any other form. Fig. 34 shows a dump-car of metal
construction, and Fig. 35 one constructed with a metal under-frame and
wooden box. These cars are made to run on narrow-gauge tracks, and
usually have a capacity of about one to one and one-half cubic yards.
Box-cars are more extensively employed in Europe for tunnel work than in
America. Fig. 36 shows a typical European box-car for tunnel work. It is
made either to run on narrow-gauge or standard-gauge tracks.

[Illustration: FIG. 34.--Iron Dump-Car for Tunnel Work.]

[Illustration: FIG. 35.--Wooden Dump-Car for Tunnel Work.]

[Illustration: FIG. 36.--Box-Car for Tunnel Work.]

It is usually desirable in tunnel work to employ cars of different forms
for different parts of the work. In rock tunnels it is a common practice
to use narrow-gauge cars of small size in the headings, and larger,
broad-gauge cars for the enlargement of the profile. Where narrow-gauge
cars are employed for all purposes, it will also be found more
convenient to use platform-cars for handling the construction material,
and dump-cars for removing the spoil. The extent to which it is
desirable to use cars of different forms will depend upon the character
and conditions of the work, and particularly upon how far it is possible
to install the permanent track.

As a general ride, it is considered preferable to lay the permanent
tracks at once, and do all the hauling upon them, so that as soon as the
tunnel is completed, trains may pass through without delay. To what
extent this may be done, or whether it can be done at all or not,
depends upon the method of excavation and other local conditions. In
soft-ground tunnels excavated by the English or Austrian methods, it is
quite possible to lay the permanent tracks at first, since the whole
section is excavated at once, and the excavation is kept but a little
ahead of the completed tunnel. In rock tunnels, where the heading is
driven far ahead of the completed section, it is, of course, impossible
to keep the permanent track close to the advance work, and narrow-gauge
tracks must be laid in the heading. The same thing is true in
soft-ground tunnels driven by successive headings and drifts. In these
cases, therefore, where narrow-gauge tracks have to be used for some
portions of the work anyway, the question comes up whether it is
preferable to use temporary narrow-gauge tracks throughout, or to lay
the permanent track as far ahead as possible, and then extend
narrow-gauge tracks to the advance excavation. In the latter case it
will, of course, be necessary to trans-ship each load from the
narrow-gauge to the standard-gauge cars, or _vice versa_, which means
extra cost and trouble. To avoid this, the method is sometimes adopted
of laying a third rail between the standard-gauge rails, so that either
standard- or narrow-gauge cars may be transported over the line.
Whatever form the local conditions may require the system of
construction tracks to assume, it may be set down as a general rule that
the permanent tracks should be kept as far advanced as possible, and
temporary tracks employed only where the permanent tracks are
impracticable.

The motive power employed for hauling in tunnels may be furnished by
animals or by mechanical motors. Animal power is generally employed in
short tunnels and in the advance headings and galleries. In long
tunnels, or where the excavated material has to be transported some
distance away from the tunnel, mechanical power is preferable, for
obvious reasons. The motors most used are small steam locomotives,
special compressed-air locomotives, and electric motors. Compressed air
and electric locomotives are built in various forms, and are
particularly well adapted for tunnel work because of their small
dimensions, and freedom from smoke and heat.


=Hauling by Way of Shafts.=--When the excavated material and materials
of construction are handled through shafts, the operation of hauling may
be divided into three processes: the transportation of the materials
along the floor of the tunnel, the hoisting of them through the shaft,
and the surface transportation from and to the mouth of the shaft. These
three operations should be arranged to work in harmony with each other,
so as to avoid waste of time and unnecessary handling of the materials.
An endeavor should be made to avoid, if possible, breaking or
trans-shipping the load from the time it starts at the heading until it
is dumped at the spoil bank. This can be accomplished in two ways. One
way is to hoist the boxes of the cars from their trucks at the bottom of
the shaft, and place them on similar trucks running on the surface
tracks. The other way is to run the loaded cars on to the elevator
platform at the bottom, hoist them, and then run them on to the surface
tracks. If the first method is employed, the car box is usually made of
metal, and is provided at its top edges with hooks or ears to which to
attach the hoisting cables. When the second method is used, the elevator
platform has tracks laid on it which connect with the tracks on the
tunnel floor, and also with those on the surface.


=Hoisting Machinery.=--The machines most commonly employed for hoisting
purposes in tunnel shafts are steam hoisting engines, horse gins, and
windlasses operated by hand. Windlasses and horse gins are rather crude
machines for hoisting loads, and are used only in special
circumstances, where the shaft is of small depth, when the amount of
material to be hoisted is small, or where for any reason the use of
hoisting engines is precluded. The steam hoisting engine is the standard
machine for the rapid lifting of heavy vertical loads. Recently oil
engines and electric hoists have also come to be used to some extent,
and under certain conditions these machines possess notable advantages.

The construction of hand windlasses is familiar to every one. In tunnel
work this device is located directly over the shaft, with its axis a
little more than half a man’s height, so that the crank handle does not
rise above the shoulder line. To develop its greatest efficiency the
hoisting rope is passed around the windlass drum so that the two ends
hang down the shaft, and as one end descends the other ascends. A skip,
or bucket, is attached to each of the rope ends; and by loading the
descending skip with construction materials and the ascending skip with
spoil, the two skip loads tend to balance each other, thus increasing
the capacity of the windlass, and decreasing the manual labor required
to operate it. Skips varying from 0.3 cu. yd. to 0.5 cu. yd. are used.
The horse gin consists of a vertical cylinder or drum provided with
radial arms to which the horses are hitched, which revolve the cylinder
by walking around it in a circle. The hoisting rope is rove around the
drum so that the two ends extend down the shaft with skips attached, as
described in speaking of the hand windlass. The power of the horse gin
is, of course, much greater than that of a windlass operated by hand,
skips of 1 cu. yd. capacity being commonly used. Horse gins are no
longer economical hoisting machines, according to one prominent
authority, when V(H + 20) > 5000, where V equals the volume of material
to be hoisted, and H equals the height of the hoist, the weight of the
excavated material being 2100 lbs. per cu. yd. As a general rule,
however, it is assumed that it is not economical to employ horse gins
with a depth of shaft exceeding 150 ft.

As already stated, the most efficient and most commonly used device for
hoisting at tunnel shafts is the steam hoisting engine. There are
numerous builders of hoisting engines, each of which manufactures
several patterns and sizes of engines. In each case, however, the
apparatus consists of a boiler supplying steam to a horizontal engine
which operates one or more rope drums. The engines are always
reversible. They may be employed to hoist the skips directly, or to
operate elevators upon which the skips or cars are loaded. In either
case the hoisting ropes pass from the engine drum to and around vertical
sheaves situated directly over the shaft so as to secure the necessary
vertical travel of the ropes down the shaft. Where the shaft is divided
into two compartments, each having an elevator or hoist, double-drum
engines are employed, one drum being used for the operations in one
compartment, and the other for the operations in the other compartment.
Where the work is to be of considerable duration, or when it is done in
cold weather, more or less elaborate shelters or engine houses are built
to cover and protect the machinery.

Choice between the method of hoisting the skips directly, and the method
of using elevators, depends upon the extent and character of the work.
Where large quantities of material are to be hoisted rapidly, it is
generally considered preferable to employ elevators instead of hoisting
the skips directly. In direct hoisting at high speed, oscillations are
likely to be produced which may dash the skips against the sides of the
shaft and cause accidents. The loads which can be carried in single
skips are also smaller than those possible where elevators are used; and
this, combined with the slower hoisting speed required, reduces the
capacity of this method, as compared with the use of elevators. Where
elevators are employed, however, the plant required is much more
extensive and costly; it comprising not only the elevator cars with
their safety devices, etc., but the construction of a guiding framework
for these cars in the tunnel shaft. For these various reasons the
elevator becomes the preferable hoisting device where the quantity of
material to be handled is large, where the shafts are deep, and where
the work will extend over a long period of time; but when the contrary
conditions are the case, direct hoisting of the skips is generally the
cheaper. The engineer has to integrate the various factors in each
individual case, and determine which method will best fulfill his
purpose, which is to handle the material at the least cost within the
given time and conditions.

[Illustration: FIG. 37.--Elevator Car for Tunnel Shafts.]

The construction of elevators for tunnel work is simple. The elevator
car consists usually of an open framework box of timber and iron, having
a plank floor on which car tracks are laid, and its roof arranged for
connecting the hoisting cable (Fig. 37[7]). Rigid construction is
necessary to resist the hoisting strains. The sides of the car are
usually designed to slide against timber guides on the shaft walls. Some
form of safety device, of which there are several kinds, should be
employed to prevent the fall of the elevator, in case the hoisting rope
breaks, or some mishap occurs to the hoisting machinery, which endangers
the fall of the car. Speaking tubes and electric-bell signals should
also be provided to secure communication between the top and bottom of
the shaft.

  [7] Reproduced from the catalogue of the Ledgerwood Manufacturing
  Company, New York.




CHAPTER VII.

TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL LININGS OF
MASONRY.


The masonry lining of a tunnel may be described as consisting of two or
more segments of circular arches combined so as to form a continuous
solid ring of masonry. To direct the operations of the masons in
constructing this masonry ring, templates or patterns are provided which
show the exact dimensions and form of the sectional profile which it is
desired to secure. These patterns or templates will vary in number and
construction with the form of lining and the method of excavation
adopted. Where the excavation is fully lined on all four sides, the
masonry work is usually divided into three parts,--the invert or floor
masonry, the side-wall masonry, and the roof-arch masonry. At least one
separate pattern has to be employed in constructing each of these parts
of the lining; and they are known respectively as ground molds, leading
frames, and arch centers, or simply centers. In the following paragraphs
the form and construction usually employed for each of these three kinds
of patterns is described.


=Ground Molds.=--Ground molds are employed in building the tunnel
invert. They are generally constructed of 3-inch plank cut exactly to
the form and dimensions of the invert masonry as shown in Fig. 38. To
permit of convenience of handling in a restricted space, they are
generally made in two parts, which are joined at the middle by means of
iron fish-plates and bolts. Either one or two ground molds may be
employed. Where two molds are used they are set up a short distance
apart, and cords are stretched from one to the other parallel to the
axis of the tunnel, by which the masons are guided in their work.
Extreme care has to be taken in setting the molds to ensure that they
are fixed at the proper grade, and are in a plane normal to the axis of
the tunnel. Where only one ground mold is employed, the finished masonry
is depended upon to supply the place of the second mold, cords being
stretched from it to the single mold placed the requisite distance
ahead. The leveling and centering of the molds is accomplished by means
of transit and level.

[Illustration: FIG. 38.--Ground Mold for Constructing Tunnel Invert
Masonry.]

[Illustration: FIG. 39.--Combined Ground Mold and Leading Frame for
Invert and Side Wall Masonry.]

Two modifications of the form of ground mold shown by Fig. 39 are
employed. The first modification is peculiar to the English method of
excavation, and consists in combining the ground mold with the leading
frame for the lower part of the side walls, as shown by Fig. 39. The
second modification is employed where the two halves or sides of the
invert are built separately, and it consists simply in using one-half of
the mold shown by Fig. 38. When the last method of constructing the
invert masonry is resorted to, extreme care has to be observed in
setting the half-mold in order to avoid error.

[Illustration: FIG. 40.--Leading Frame for Constructing Side Wall
Masonry.]


=Leading Frames.=--As already stated, leading frames are the patterns,
or molds, used in building the side walls of the lining. Like the ground
mold they are usually built of plank; one side being cut to the curve of
the profile, and the other being made parallel to the vertical axis of
the tunnel section. The vertical side usually has some arrangement by
which a plumb bob can be attached, as shown by Fig. 40, to guide the
workmen in erecting the frame. The combined leading frame and ground
mold shown in Fig. 39 has already been described. The use of this frame
is possible only where the masonry is begun at the invert and carried up
on each side at the same time. This mode of construction is peculiar to
the English method of tunneling; in all other methods the form of
separate ground frame shown by Fig. 40 is employed. The ground frames
are lined in and leveled up by transit and level; and, as in setting the
ground frames, care must be taken to secure accuracy in both direction
and elevation.


=Arch Centers.=--The template or form upon which the roof arch is built
is called a center. Unlike the ground molds and leading frames, the arch
centers have to support the weight of the masonry and the roof pressures
during the construction of the lining, and they, therefore, require to
be made strong. Owing to the fact that the pressures are indeterminate,
it is impossible to design a rational center, and resort is had to those
constructions which past experience has shown to work satisfactorily
under similar conditions. In a general way it can always be assumed that
the construction should be as simple as possible, that the center should
be so designed that it can be set up and removed with the least possible
labor, and that the different pieces of the framework and lagging should
be as short as possible, for convenience in handling.

Tunnel centers are usually composed of two parts,--a mold or curved
surface upon which the masonry rests, and a framework which supports the
mold. The curved surface or mold consists of a lagging of narrow boards
running parallel to the tunnel axis, which rests upon the arched top
members of two or more consecutive supporting frames. The supporting
frame is built in the form of a truss, and must be made strong enough to
withstand the heavy superimposed loads, consisting of the arch masonry
during construction, and of the roof pressures which are transferred to
the center when the strutting is removed to allow the masonry to be
placed. The framework of the center is supported either by posts resting
upon the floor of the excavation, or upon the invert masonry when this
is built first, as in the English and Austrian methods, or it may be
supported directly upon the ground where the arch masonry is built
first, as in the Belgian method of tunneling.

In describing the various methods of tunneling in succeeding chapters,
the center construction and method of supporting the center peculiar to
each will be fully explained, and only a few general remarks are
necessary here. Centers may be classified according to their
construction and composition into plank centers, truss centers, and iron
centers.

[Illustration: FIG. 41.--Plank Center for Constructing the Roof Arch.]

One of the most common forms of plank centers is shown by Fig. 41. It
consists of two half-polygons whose sides consist of 15 in. × 4 ft.
planks having radial end-joints. These two half-polygons are laid one
upon the other so that they break joints, as shown by the figure, and
the extrados of the frame is cut to the true curve of the roof arch. The
planks commonly used for making these centers are 4 ins. thick, making
the total thickness of the center 8 ins. Plank centers of the
construction described are suitable only for work where the pressures to
be resisted are small, as in tunnels through a fairly firm rock,
although there have been instances of their successful use in
soft-ground tunnels.

Where heavy loads have to be carried, trussed centers are generally
employed, the trusses being composed of heavy square beams with scarfed
and tenoned joints, reinforced by iron plates. Different forms of
trusses are employed in each of the different methods of tunneling, and
each of these is described in succeeding chapters; but they are
generally either of the king-post or queen-post type, or some
modification of them. The king-post truss may be used alone, with or
without the tie-beam, as shown by Fig. 42; but generally a queen-post
truss is made to form the base of support for a smaller king-post truss
mounted on its top. This arrangement gives a trapezoidal form to the
center, which approaches closely to the arch profile. Owing to the
character of the pressures transmitted to the center, the usual tension
members can be made very light.

[Illustration: FIG. 42.--Trussed Center for Constructing the Roof Arch.]

The combined center and strutting system devised by Mr. Rziha has
already been described in a previous chapter. In recent European tunnel
work quite extensive use has also been made of iron centers consisting
of several segments of curved I-beams, connected by fish-plate joints so
as to form a semi-circular arch rib. The ends or feet of these I-beam
ribs have bearing-plates or shoes made by riveting angles to the
I-beams. Centers constructed in a similar manner, but made of sections
of old railway rail, were used in carrying out the tunnel work on the
Rhine River Railroad in Germany. The advantages claimed for iron centers
are that they take up less room, and that they can be used over and over
again.


_Setting Up Centers._--According to the method of excavation followed in
building the tunnel, the centers for building the roof arch may have to
be supported by posts resting on the tunnel floor; or where the arch is
built first, as in the Belgian and Italian methods, they may be carried
on blocking resting on the unexcavated earth below. Whichever method is
employed, an unyielding support is essential, and care must be taken
that the centers are erected and maintained in a plane normal to the
tunnel axis. To prevent deflection and twisting, the consecutive centers
are usually braced together by longitudinal struts or by braces running
to the adjacent strutting. Only skilled and experienced workmen should
be employed in erecting the centers; and they should work under the
immediate direction of the engineer, who must establish the axis and
level of each center by transit and level.


_Lagging._--By the lagging is meant the covering of narrow longitudinal
boards resting upon the upper curved chords of the centers, and spanning
the opening between consecutive centers. This lagging forms the curved
surface or mold upon which the arch masonry is laid. When the roof arch
is of ashlar masonry the strips of lagging are seldom placed nearer
together than the joints of the consecutive ring stones, but in brick
arches they are laid close together. Besides the weight of the arch
masonry, the lagging timbers support the short props which keep the
poling-boards in place after the strutting is removed and until the arch
masonry is completed.


_Striking the Centers._--The centers are usually brought to the proper
elevation by means of wooden wedges inserted between the sill of the
center and its support, or between the bottom of the posts carrying the
center and the tunnel floor. These wedges are usually made of hard wood,
and are about 6 ins. wide by 4 ins. thick by 18 ins. long. To strike the
center after the arch masonry is completed, these wedges are withdrawn,
thus allowing the center to fall clear of the masonry. Usually the
center is not removed immediately after striking, so that if the arch
masonry fails the ruins will remain upon the center. The method of
striking the iron center devised by Mr. Rziha has been described in the
previous chapter on strutting.




CHAPTER VIII.

METHODS OF LINING TUNNELS.


Tunnels in soft soils and in loose rock, and rock liable to
disintegration, are always provided with a lining to hold the walls and
roof in place. This lining may cover the entire sectional profile of the
tunnel, or only a part of it, and it may be constructed of timber, iron,
iron and masonry, or, more commonly, of masonry alone.


=Timber Lining.=--Timber is seldom employed in lining tunnels except as
a temporary expedient, and is replaced by masonry as soon as
circumstances will permit. In the first construction of many American
railways, the necessity for extreme economy in construction, and of
getting the line open for traffic as soon as possible, caused the
engineers to line many tunnels with timber, which was plentiful and
cheap. Except for their small cost and the ease and rapidity with which
they can be constructed, however, these timber linings possess few
advantages. It is only the matter of a few years when the decay of the
timber makes it necessary to rebuild them, and there is always the
serious danger of fire. In several instances timber-lined tunnels in
America have been burned out, causing serious delays in traffic, and
necessitating complete reconstruction. Usually this reconstruction has
consisted in substituting masonry in place of the original timber
lining. In a succeeding chapter a description will be given of some of
the methods employed in replacing timber tunnel linings with masonry.
Various forms of timber lining are employed, of which Fig. 44 and the
illustrations in the chapter discussing the methods of relining
timber-lined tunnels with masonry are typical examples.

[Illustration: ~Cross Section.~

~Longitudinal Section.~

FIGS. 43 and 44.--A Typical Form of Timber Lining for Tunnels.]


=Iron Lining.=--The use of iron lining for tunnels was introduced first
on a large scale by Mr. Peter William Barlow in 1869, for the second
tunnel under the River Thames at London, England, and it has greatly
extended since that time. The lining of the second Thames tunnel
consisted of cylindrical cast-iron rings 8 ft. in diameter, the abutting
edges of the successive rings being flanged and provided with holes for
bolt fastenings. Each ring was made up of four segments, three of which
were longer than quadrants, and one much smaller forming the “key-stone”
or closing piece. These segments were connected to each other by flanges
and bolts. To make the joints tight, strips of pine or cement and hemp
yarn were inserted between the flanges. Since the construction of the
second Thames tunnel, iron lining has been employed for a great many
submarine tunnels in England, Continental Europe, and America, some of
them having a section over 28 ft. in diameter. Where circular iron
lining is employed, the bottom part of the section is leveled up with
concrete or brick masonry to carry the tracks, and the whole interior
of the ring is covered with a cement plaster lining deep enough
thoroughly to embed the interior joint flanges. In the succeeding
chapter describing the methods of driving tunnels by shields several
forms of iron tunnel lining are fully described.


=Iron and Masonry Lining.=--During recent years a form of combined
masonry and iron lining has been extensively employed in constructing
city underground railways in both Europe and America. Generally this
form of lining is built with a rectangular section. Two types of
construction are employed. In the first, masonry side walls carry a flat
roof of girders and beams, which carry a trough flooring filled with
concrete, or between which are sprung concrete or brick arches.
Sometimes the roof framing consists of a series of parallel I-beams laid
transversely across the tunnel, and in other cases transverse plate
girders carry longitudinal I-beams. In the second type of construction
the roof girders are supported by columns embedded in the side walls.
Where the tunnel provides for two or four tracks, intermediate column
supports are in some cases introduced between the side columns. In this
construction the roofing consists of concrete filled troughs or of
concrete or brick arches, as in the construction first described.
Examples of combined masonry and iron tunnel lining are illustrated in
the succeeding chapter on tunneling under city streets.


=Masonry Lining.=--The form of tunnel lining most commonly employed is
brick or stone masonry. Concrete and reinforced concrete masonry lining
has been employed in several tunnels built in recent years. The masonry
lining may inclose the whole section or only a part of it. The floor or
invert is the part most commonly omitted; but sometimes also the side
walls and invert are both omitted, and the lining is confined simply to
an arch supporting the roof. The roof arch, the side walls, and the
invert compose the tunnel lining; and all three may consist of stone or
brick alone, or stone side walls may be employed with brick invert and
roof arch. Rubble-stone masonry is usually employed, except at the
entrances, where the masonry is exposed to view. Here ashlar masonry is
usually used. The stone selected for tunnel lining should be of a
durable quality which weathers well. Where bricks are used they should
be of good quality. Owing to the comparative ease with which brick
arches can be built, they are generally used to form the roof arch, even
where the side walls are of stone masonry. Masonry lining may be built
in the form of a series of separate rings, or in the form of a
continuous structure extending from one end of the tunnel to the other.
The latter method of construction produces a stronger structure; but in
case of failure by crushing, the damage done is likely to be more
widespread than where separate rings are employed, one or two of which
may fail without injury to the others adjacent to them. The construction
is also somewhat simpler where separate rings are employed, since no
provision has to be made for bonding the whole lining into a continuous
structure. Where a series of separate rings is employed, the length of
each ring runs from 5 ft. up to 20 ft., it depending upon the character
of the material penetrated, and the method of construction employed. For
the purpose of detailed discussion the construction of masonry lining
may be divided into four parts,--the side-wall foundations, the side
walls themselves, the roof arch, and the invert.

Concrete and reinforced concrete linings are now extensively used on
account of cheapness and facility of handling, but they have the great
disadvantage of resisting pressure after they become hard, which is some
time after being placed. The strutting should, therefore, be left to
support the roof so as to prevent direct pressure on the fresh material.
The roof, as a rule, is supported by longitudinal planks held in
position by five or seven segments of arched frames placed across the
tunnel. A large quantity of timber and carpenter work is thus entirely
wasted and these costly items, in many cases, make the concrete lining
of a tunnel more expensive than the one built of brick and stone. To
avoid these inconveniences tunnels have been successfully lined with
concrete on the side walls and concrete blocks in the arches. These
blocks have been built by hand and molded in the shape of the arch
voussoirs.


=Foundations.=--In tunnels through rock of a hard and durable character
the foundations for the side walls are usually laid directly on the
rock. In loose rock, or rock liable to disintegration, this method of
construction is not generally a safe one, and the foundation excavation
should be sunk to a depth at which the atmospheric influences cannot
affect the foundation bed. In either case the foundation masonry is made
thicker than that of the side walls proper, so as to distribute the
pressure over a greater area, and to afford more room for adjusting the
side-wall masonry to the proper profile. In yielding soils a special
foundation bed has to be prepared for the foundation masonry. In some
instances it is found sufficient to lay a course of planks upon which
the masonry is constructed, but a more solid construction is usually
preferred.

This is obtained by placing a concrete footing from 1 ft. to 2 ft. deep
all along the bottom of the foundation trench, or in some cases by
sinking wells at intervals along the trench and filling them with
concrete, so as to form a series of supporting pillars.

[Illustration: FIG. 45.--Diagram Showing Forms Adopted for Side-Wall
Foundations.]

The form given to the foundation courses and lower portions of the side
walls varies. Where a large bearing area is required, the back of the
wall is carried up vertically as shown by the line _AB_, Fig. 45,
otherwise the rear face of the wall follows the line of excavation _AC_.
For similar reasons the front face of the wall may be made vertical, as
at _FG_, or inclined, as at _FH_. The line _FE_ indicates the shelf
construction designed to support the feet of the posts used to carry the
arch centers during the construction of the roof arch.


=Side Walls.=--The construction of the side walls above the foundation
courses is carried out as any similar piece of masonry elsewhere would
be built. To direct the work and insure that the inner faces of the
walls follow accurately the curve of the chosen profile, leading frames
previously described are employed.


=Roof Arch.=--For the construction of the roof arch, the centers
previously described are employed. Beginning at the edges of the center
on each side, the masonry is carried up a course at a time, care being
taken to have it progress at the same rate on both sides, so that the
load brought onto the centering is symmetrical. As soon as the centers
are erected, the roof strutting is removed, and replaced by short props
which rest on the lagging of the centers and support the poling-boards.
These props are removed in succession as the arch masonry rises along
the curve of the center, and the space between the top of the arch
masonry and the ceiling of the excavation is filled with small stones
packed closely. The keystone section of the arch is built last, by
inserting the stones or bricks from the front edge of the arch ring,
there being no room to set them in from the top, as is the practice in
ordinary open-arch construction. The keying of the arch is an especially
difficult operation, and only experienced men skilled in the work should
be employed to perform it. The task becomes one of unusual difficulty
when it becomes necessary to join the arches coming from opposite
directions.


=Invert.=--In all but one or two methods of tunneling, the invert is the
last portion of the lining to be built. In the English method of
tunneling, the invert is the first portion of the lining to be built,
and the same practice is sometimes necessary in soft soils where there
is danger of the bottoms of the side walls being squeezed together by
the lateral pressures unless the invert masonry is in place to hold them
apart. The ground molds previously described are employed to direct the
construction of the invert masonry.


=General Observations.=--In describing the construction of the roof
arch, mention was made of the stone filling employed between the back of
the masonry ring and the ceiling of the excavation. The spaces behind
the side walls are filled in a similar manner. The object of this stone
filling, which should be closely packed, is to distribute the vertical
and lateral pressures in the walls of the excavation uniformly over the
lining masonry. As the masonry work progresses, the strutting employed
previously to support the walls of the excavation has to be removed.
This work requires care to prevent accident, and should be placed in
charge of experienced mechanics who are familiar with its construction,
and can remove it with the least damage to the timbers, so that they may
be used again, and without causing the fall of the roof or the caving of
the sides by removing too great a portion of the timbers at one time.


=Thickness of Lining Masonry.=--It is obvious, of course, that the
masonry lining must be thick enough to support the pressure of the earth
which it sustains; but, as it is impossible to estimate these pressures
at all accurately, it is difficult to say definitely just what thickness
is required in any individual case. Rankine gives the following formulas
for determining the depths of keystone required in different soils:

For firm soils

         (    _r_²)
  _d_ = √(0.12----),
         (    _s_ )

and for soft soils,

         (    _r_²)
  _d_ = √(0.48----),
         (    _s_ )

where _d_ = the depth of the crown in feet, _r_ = the rise of the arch
in feet, and _s_ = the span of the arch in feet. Other writers, among
them Professor Curioni, attempt to give rational methods for calculating
the thickness of tunnel lining; but they are all open to objection
because of the amount of hypothesis required concerning pressures which
are of necessity indeterminate. Therefore, to avoid tedious and
uncertain calculations, the engineer adopts dimensions which experience
has proven to be ample under similar conditions in the past. Thus we
have all gradations in thickness, from hard-rock tunnels requiring no
lining, and tunnels through rocks which simply require a thin shell to
protect them from the atmosphere, to soft-ground tunnels where a masonry
lining 3 ft. or more in thickness is employed. Table II. shows the
thickness of masonry lining used in tunnels through soft soils of
various kinds.

The thickness of the masonry lining is seldom uniform at all points, as
is indicated by Table II. Figs. 46 and 47 show common methods of varying
the thickness of lining at different points, and are self-explanatory.

[Illustration: FIGS. 46 and 47.--Transverse Sections of Tunnels Showing
Methods of Increasing the Thickness of the Lining at Different Points.]


=Side Tunnels.=--When tunnels are excavated by shafts located at one
side of the center line, short side tunnels or galleries are built to
connect the bottoms of the shafts with the tunnel proper. These side
tunnels are usually from 30 ft. to 40 ft. long, and are generally made
from 12 ft. to 14 ft. high, and about 10 ft. wide. The excavation,
strutting, and lining of these side tunnels are carried on exactly as
they are in the main tunnel, with such exceptions as these short
lengths make possible. Table III. gives the thickness of lining used for
side tunnels, the figures being taken from European practice.


=Culverts.=--The purpose of culverts in tunnels is to collect the water
which seeps into the tunnel from the walls and shafts. The culvert is
usually located along the center line of the tunnel at the bottom. In
soft-ground tunnels it is built of masonry, and forms a part of the
invert, but in rock tunnels it is the common practice to cut a channel
in the rock floor of the excavation. Both box and arch sections are
employed for culverts. The dimensions of the section vary, of course,
with the amount of water which has to be carried away. The following are
the dimensions commonly employed:

  +----------------+--------+--------+----------+-----------+
  |KIND OF CULVERT.| HEIGHT |  WIDTH |THICKNESS | THICKNESS |
  |                |IN FEET.|IN FEET.|OF WALLS  |OF COVERING|
  |                |        |        | IN FEET. |  IN FEET. |
  +----------------+--------+--------+----------+-----------+
  |Box culvert     |1 to 1.5|1 to 1.5|0.8 to 1.2|    0.3    |
  |Arch culvert    |1 to 1.5|1 to 1.5|0.8 to 1.2|    0.4    |
  +----------------+--------+--------+----------+-----------+

It should be understood that the dimensions given in the table are those
for ordinary conditions of leakage; where larger quantities of water are
met with, the size of the culverts has, of course, to be enlarged. To
permit the water to enter the culvert, openings are provided at
intervals along its side; and these openings are usually provided with
screens of loose stones which check the current, and cause the suspended
material to be deposited before it enters the culvert. In cases where
springs are encountered in excavating the tunnel, it is necessary to
make special provisions for confining their outflow and conducting it to
the culvert. In all cases the culverts should be provided with catch
basins at intervals of from 150 ft. to 300 ft., in which such suspended
matter as enters the culverts is deposited, and removed through covered
openings over each basin. At the ends of the tunnel the culvert is
usually divided into two branches, one running to the drain on each
side of the track.

[Illustration: FIG. 48.--Refuge Niche in St. Gothard Tunnel.]


=Niches.=--In short tunnels niches are employed simply as places of
refuge for trackmen and others during the passing of trains, and are of
small size. In long tunnels they are made larger, and are also employed
as places for storing small tools and supplies employed in the
maintenance of the tunnel. Niches are simply arched recesses built into
the sides of the tunnel, and lined with masonry; Fig. 48 shows this
construction quite clearly. Small refuge niches are usually built from 6
ft. to 9 ft. high, from 3 ft. to 6 ft. wide, and from 2 ft. to 3 ft.
deep. Large niches designed for storing tools and supplies are made from
10 ft. to 12 ft. high, from 8 ft. to 10 ft. wide, and from 18 ft. to 24
ft. deep, and are provided with doors. Refuge niches are usually spaced
from 60 ft. to 100 ft. apart, while the larger storage niches may be
located as far as 3000 ft. apart. The niche construction shown by Fig.
48 is that employed on the St. Gothard tunnel.


=Entrances.=--The entrances, or portals, of tunnels usually consist of
more or less elaborate masonry structures, depending upon the nature of
the material penetrated. In soft-ground tunnels extensive wing walls are
often required to support the earth above and at the sides of the
entrance; while in tunnels through rock, only a masonry portal is
required, to give a finish to the work. Often the engineer indulges
himself in an elaborate architectural design for the portal masonry.
There is danger of carrying such designs too far for good taste unless
care is employed; and on this matter the writer can do no better than to
quote the remarks of the late Mr. Frederick W. Simms in his well-known
“Practical Tunneling”:

  “The designs for such constructions should be massive to be suitable
  as approaches to works presenting the appearance of gloom, solidity,
  and strength. A light and highly decorated structure, however elegant
  and well adapted for other purposes, would be very unsuitable in such
  a situation; it is plainness combined with boldness, and massiveness
  without heaviness, that in a tunnel entrance constitutes elegance,
  and, at the same time, is the most economical.”

[Illustration: FIG. 49.--East Portal of Hoosac Tunnel.]

Fig. 49 is an engraving from a photograph of the east portal of the
Hoosac tunnel, which is an especially good design. The portals of the
Mount Cenis tunnel were built of samples of stone encountered all along
the line of excavation. The stones were cut and dressed and utilized for
walls and voussoirs. The only ornament that is usually allowed on the
portals is the date of the opening of the tunnel prominently cut in the
stone above the arch.

TABLE II.

Showing Thickness of Masonry Lining for Tunnels through Soft Ground.

  +------------------------------+------------+-----------+------------+
  |    CHARACTER OF MATERIAL.    |  KEYSTONE. | SPRINGERS.|   INVERT.  |
  +------------------------------+------------+-----------+------------+
  |                              |     Ft.    |    Ft.    |      Ft.   |
  |Laminated clay, first variety |2.15 to 3   |2.75 to 3.5| 1.6  to 2.5|
  |Laminated clay, second variety|3    to 4.5 |3.5  to 5.5| 2.5  to 4  |
  |Laminated clay, third variety |4.5  to 6.5 |5.5  to 8.1| 4    to 4.5|
  |Quicksand                     |2    to 3.28|2    to 4.1| 1.33 to 2.5|
  +------------------------------+------------+-----------+------------+

TABLE III.

Showing Thickness of Masonry Lining for Side Tunnels through Soft
Ground.

  +------------------------------+----------+----------+-----------+
  |    CHARACTER OF MATERIAL.    | KEYSTONE.|SPRINGERS.|  INVERT.  |
  +------------------------------+----------+----------+-----------+
  |                              |    Ft.   |    Ft.   |    Ft.    |
  |Laminated clay, first variety |1.6 to 2.3|1.8 to 3  |1.5 to 2   |
  |Laminated clay, second variety|2.3 to 3  |3   to 4.1|2   to 2.6 |
  |Laminated clay, third variety |3   to 4  |4.1 to 5  |2.6 to 3.29|
  |Quicksand                     |1.6 to 2.5|1.3 to 2  |1.3 to 2   |
  +------------------------------+----------+----------+-----------+




CHAPTER IX.

TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION; REPRESENTATIVE MECHANICAL
INSTALLATIONS FOR TUNNEL WORK.


The present high development of labor-saving machinery for excavating
rock makes this material one of the safest and easiest to tunnel of any
with which the engineer ordinarily has to deal. To operate this
machinery requires, however, the development of a large amount of power,
its transmission to considerable distances, and, finally, its economical
application to the excavating tools. The standard rock excavating
machine is the power drill, which requires either air or hydraulic
pressure for its operation according to the special type employed. Under
present conditions, therefore, the engineer is limited either to air or
water under compression for the transmission of his power. Steam-power
may be employed directly to operate percussion rock drills; but owing to
the heat and humidity which it generates in the confined space where the
drills work, and because of other reasons, it is seldom employed
directly. Electric transmission, which offers so many advantages to the
tunnel builder, in most respects is largely excluded from use by the
failure which has so far followed all attempts to apply it to the
operation of rock drills. As matters stand, therefore, the tunnel
engineer is practically limited to steam and falling water for the
generation of power, and to compressed air and hydraulic pressure for
its transmission.

Whether the engineer should adopt water-power or steam to generate the
power required for his excavating machinery depends upon their relative
availability, cost, and suitability to the conditions of work in each
particular case. Where fuel is plentiful and cheap, and where
water-power is not available at a comparatively reasonable cost,
steam-power will nearly always prove the more economical; where,
however, the reverse conditions exist, which is usually the case in a
mountainous country far from the coal regions, and inadequately supplied
with transportation facilities, but rich in mountain torrents,
water-power will generally be the more economical. In a succeeding
chapter the power generating and transmission plants for a number of
rock tunnels are described, and here only a general consideration of the
subject will be presented.


=Steam-Power Plant.=--A steam-power plant for tunnel work should be much
the same as a similar plant elsewhere, except that in designing it the
temporary character of its work must be taken into consideration. This
circumstance of its temporary employment prompts the omission of all
construction except that necessary to the economical working of the
plant during the period when its operation is required. The power-house,
the foundations for the machinery, and the general construction and
arrangement, should be the least expensive which will satisfy the
requirements of economical and safe operation for the time required. It
will often be found more economical as a whole to operate the machinery
with some loss of economy during the short time that it is in use than
to go to much greater expense to secure better economy from the
machinery by design and construction, which will be of no further use
after the tunnel is completed. The longer the plant is to be required,
the nearer the construction may economically approach that of a
permanent plant. As regards the machinery itself, whose further
usefulness is not limited by the duration of any single piece of work,
true economy always dictates the purchase of the best quality. Speaking
in a general way, a steam-power plant for tunnel work comprises a boiler
plant, a plant of air compressors with their receivers, and an electric
light dynamo. When hydraulic transmission of power is employed, the air
compressors are replaced by high-pressure pumps; and when electric
hauling is employed, one or more dynamos may be required to generate
electricity for power purposes, as well as for lighting. In addition to
the power generating machines proper, there must be the necessary piping
and wiring for transmitting this power, and, of course, the equipment of
drills and other machines for doing the actual excavating, hauling, etc.


=Reservoirs.=--When water-power is employed, a reservoir has to be
formed by damming some near-by mountain stream at a point as high as
practicable above the tunnel. The provision of a reservoir, instead of
drawing the water directly from the stream, serves two important
purposes. It insures a continuous supply and constant head of water in
case of drought, and also permits the water to deposit its sediment
before it is delivered to the turbines. The construction of these
reservoirs may be of a temporary character, or they may be made
permanent structures, and utilized after construction is completed to
supply power for ventilation and other necessary purposes. In the first
case they are usually destroyed after construction is finished. In
either case, it is almost unnecessary to say, they should be built amply
safe and strong according to good engineering practice in such works,
for the duration of time which they are expected to exist.


=Canals and Pipe Lines.=--For conveying the water from the reservoirs to
the turbines, canals or pipe lines are employed. The latter form of
conduit is generally preferable, it being both less expensive and more
easily constructed than the former. It is advisable also to have
duplicate lines of pipe to reduce the possibility of delay by accident
or while necessary repairs are being made to one of the pipes. The pipe
lines terminate in a penstock leading into the turbine chamber, and
provided with the necessary valves for controlling the admission of
water to the turbines.


=Turbines.=--There are numerous forms of turbines on the market, but
they may all be classed either as impulse turbines or as reaction
turbines. Impulse turbines are those in which the whole available energy
of the water is converted into kinetic energy before the water acts on
the moving part of the turbine. Reaction turbines are those in which
only a part of the available energy of the water is converted into
kinetic energy before the water acts on the moving vanes. Impulse
turbines give efficient results with any head and quantity of water, but
they give better results when the quantity of water varies and the head
remains constant. Reaction turbines, on the contrary, give better
results when the quantity of water remains constant and the head varies.
These observations indicate in a general way the form of turbine which
will best meet the particular conditions in each case. The number of
turbines required, and their dimensions, will be determined in each case
by the number of horse-power required and the quantity of water
available. The power of the turbines is transmitted to the air
compressors or pumps by shafting and gearing.


=Air Compressors.=--An air compressor is a machine--usually driven by
steam, although any other power may be used--by which air is compressed
into a receiver from which it may be piped for use. For a detailed
description of the various forms of air compressors the reader should
consult the catalogues of the several makers and the various text-books
relating to air compression and compressed air. Air compressors, like
other machines, suffer a loss of power by friction. The greatest loss of
power, however, results from the heat of compression. When air is
compressed, it is heated, and its relative volume is increased.
Therefore, a cubic foot of hot air in the compressor cylinder, at say,
60 lbs. pressure, does not make a cubic foot of air at 60 lbs. pressure
after cooling in the receiver. In other words, assuming pressure to be
constant, a loss of volume results due to the extraction of the heat of
compression after the air leaves the compressor cylinder. To reduce the
amount of this loss, air compressors are designed with means to extract
the heat from the air before it leaves the compressor cylinder. Air
compressors may first be divided into two classes, according to the
means employed for cooling the air, as follows: (1) Wet compressors, and
(2) dry compressors. A wet compressor is one which introduces water
directly into the cylinder during compression, and a dry compressor is
one which admits no water to the air during compression. Wet compressors
may be subdivided into two classes: (1) Those which inject water in the
form of spray into the cylinder during compression, and (2) those which
use a water piston for forcing the air into confinement.

The following brief discussion of these various types of compressors is
based on the concise practical discussion of Mr. W. L. Saunders, M. Am.
Soc. C. E., in “Compressed Air Production.” The highest isothermal
results are obtained by the injection of water into the cylinders, since
it is plain that the injection of cold water, in the shape of a finely
divided spray, directly into the air during compression will lower the
temperature to a greater degree than simply to surround the cylinder and
parts by water jackets which is the means of cooling adopted with dry
compressors. A serious obstacle to water injection, and that which
condemns this type of compressor, is the influence of the injected water
upon the air cylinder and parts. Even when pure water is used, the
cylinders wear to such an extent as to produce leakage and to require
reboring. The limitation to the speed of a compressor is also an
important objection. The chief claim for the water piston compressor is
that its piston is also its cooling device, and that the heat of
compression is absorbed by the water. Water is so poor a conductor of
heat, however, that without the addition of sprays it is safe to say
that this compressor has scarcely any cooling advantages at all so far
as the cooling of the air during compression is concerned. The water
piston compressor operates at slow speed and is expensive. Its only
advantage is that it has no dead spaces. In the dry compressor a
sacrifice is made in the efficiency of the cooling device to obtain low
first cost, economy in space, light weight, higher speed, greater
durability, and greater general availability.

Air compressors are also distinguished as double acting and simple
acting. They are simple acting when the cylinder is arranged to take in
air at one stroke and force it out at the next, and they are double
acting when they take in and force out air at each stroke. In form
compressors may be simple or duplex. They are simple when they have but
one cylinder, and duplex when they have two cylinders. A straight line
or direct acting compressor is one in which the steam and air cylinders
are set tandem. An indirect acting compressor is one in which the power
is applied indirectly to the piston rod of the air cylinder through the
medium of a crank. Mr. W. L. Saunders writes in regard to direct and
indirect compression as follows:--

  “The experience of American manufacturers, which has been more
  extensive than that of others, has proved the value of direct
  compression as distinguished from indirect. By direct compression is
  meant the application of power to resistance through a single straight
  rod. The steam and air cylinders are placed tandem. Such machines
  naturally show a low friction loss because of the direct application
  of power to resistance. This friction loss has been recorded as low as
  5%, while the best practice is about 10% with the type which conveys
  the power through the angle of a crank shaft to a cylinder connected
  to the shaft through an additional rod.”


=Receivers.=--Compressed air is stored in receivers which are simply
iron tanks capable of withstanding a high internal pressure. The purpose
of these tanks is to provide a reservoir of compressed air, and also to
allow the air to deposit its moisture. From the receivers the air is
conveyed to the workings through iron pipes, which decrease gradually in
diameter from the receivers to the front.


=Rock Drills.=--The various forms of rock drills used in tunneling have
been described in Chapter III., and need not be considered in detail
here except to say that American engineers usually employ percussion
drills, while European engineers also use rotary drills extensively. A
comparison between these two types of drills was made in excavating the
Aarlberg tunnel in Austria, where the Brandt hydraulic rotary drill was
used at one end, and the Ferroux percussion drill was used at the other
end. The rock was a mica-schist. The average monthly progress was 412
ft., with a maximum of 646 ft., with the rotary drills, and an average
of 454 ft. with the percussion drill.


=Excavation.=--Since considerable time is required to get the power
plant established, the excavation of rock tunnels is often begun by
hand, but hand work is usually continued for no longer a period than is
necessary to get the power plant in operation. Generally speaking, the
greatest difficulty is encountered in excavating the advanced drift or
heading. Based on the mode of blasting employed, there are two methods
of driving the advanced gallery, known as the circular cut and the
center cut methods. In the first method a set of holes is first drilled
near the center of the front in such a manner that they inclose a cone
of rock; the holes, starting at the perimeter of the base of the cone,
converge toward a junction at its apex. Seldom more than four to six
holes are comprised in this first set. Around these first holes are
driven a ring of holes which inclose a cylinder of rock, and if
necessary succeeding rings of holes are driven outside of the first
ring. These holes are blasted in the order in which they are driven, the
first set taking out a cone of rock, the second set enlarging this cone
to a cylinder, and the other sets enlarging this cylinder to the
required dimensions of the heading. The number of holes, however, varies
with the quality of rock and they are seldom driven deeper than 4 or 5
ft. This method of excavating the heading, which is commonly followed by
European engineers, is illustrated in Figs. 50 to 52. In these figures
are indicated the number of holes in each round and the sequence of
rounds for the soft, medium and hard rock, as used in the Turchino
tunnel of the Genova Ovada Asti line of the Mediterranean Railway of
Italy. The heading was about 9 ft. square, and five sets of holes were
used in blasting, the depths being 3.91, 4.26 and 4.6 ft. for soft,
medium and hard rock, respectively, and the amount of dynamite consumed
was 2.38, 3.91 and 5.1 pounds per cubic yard for the three classes of
rock.

[Illustration: ~in Soft Rock~

~in Medium Rock~

~in Hard Rock~

FIGS. 50 to 52.--Arrangement of Drill Holes in the Heading of Turchino
Tunnel.]

[Illustration: FIGS. 53 and 54.--Arrangement of Drill Holes in the
Heading of the Fort George Tunnel.]

In the center-cut method, which is the one commonly employed in America,
the holes are arranged in vertical rows, and are driven from 8 to 10 ft.
deep. Fig. 53 shows the arrangement of the holes, and the method of
blasting them, as used in the excavation of the heading for the Fort
George tunnel of the New York rapid transit. The two center rows of
holes converge toward each other so as to take out a wedge of rock;
others are bored straight, or parallel, with the vertical plane of the
tunnel. Those bored around the perimeter are driven either outward or
upward, according as they are located, close to the sides or roof of the
tunnel. In this case, the holes of the center cut were driven 9 ft.
deep, while all the other holes were bored to a depth of 8 ft.

The width of the advanced gallery or heading depends upon the quality of
the rock. In hard rock American engineers give it the full width of the
tunnel section; but this cannot be done in loose or fissured rock, which
has to be supported, the headings here being usually made about 8 × 8
ft. The wider heading is always preferable, where it is possible, since
more room is available for removing the rock, and deeper holes can be
bored and blasted.

The important rôle played by the power plant and other mechanical
installations in constructing tunnels through rock has already been
mentioned. In some methods of soft-ground tunneling, and particularly in
soft-ground subaqueous tunneling, it is also often necessary to employ a
mechanical installation but slightly inferior in size and cost to those
used in tunneling rock. It is proposed to describe very briefly here a
few typical individual plants of this character, which will in some
respects give a better idea of this phase of tunnel work than the more
general descriptions.


=Rock Tunnels.=--The tunnels selected to illustrate the mechanical
installations employed in tunneling through rock are: The Mont Cenis,
Hoosac Tunnel, the Cascade Tunnel, the Niagara Falls Power Tunnel, the
Palisades Tunnel, the Croton Aqueduct Tunnel, the Strickler Tunnel in
America, and the Graveholz Tunnel and the Sonnstein Tunnel in Europe. In
addition there will be found in another chapter of this book a
description of the mechanical installations at the St. Gothard,
Pennsylvania and other tunnels.


_Mont Cenis Power Plant._--The mechanical installation consisted of the
Sommeilier air compressors built near the portals. The Sommeilier
compressors, Mr. W. L. Saunders says, were operated as a ram, utilizing
a natural head of water to force air at 80 lbs. pressure into a
receiver. The column of water contained in the long pipe on the side of
the hill was started and stopped automatically by valves controlled by
engines. The weight and momentum of the water forced a volume of air
with such a shock against the discharge valve that it was opened, and
the air was discharged into the tank; the valve was then closed, the
water checked; a portion of it was allowed to discharge, and the space
was filled with air, which was in turn forced into the tank. Only 73% of
the power of the water was available, 27% being lost by the friction of
the water in the pipes, valves, bends, etc. Of the 73% of net work, 49.4
was consumed in the perforators, and 23.6 in a dummy engine for working
the valves of the compressors and for special ventilation.

The compressed air was conveyed from each end through a cast-iron pipe
7⁵⁄₈ in. in diameter, up to the front of the excavation. The joints of
the pipes were made with turned faces, grooved to receive a ring of
oakum which was tightly screwed and compressed into the joint. To
ascertain the amount of leakage of the pipes, they and the tanks were
filled with air compressed to 6 atmospheres, and the machines stopped;
after 12 hours the pressure was reduced to 5.7 atmospheres, or to 95% of
the original pressure.

Sommeilier’s percussion drilling machines were used in the excavation of
this tunnel. They were provided with 8 or 10 drills acting at the same
time, and mounted on carriages running on tracks. These were withdrawn
to a safe place during the blasting, and advanced again after the broken
rock was removed from the front and the new tracks laid.

Machine shops were built at both ends of the tunnel for building and
repairing the drilling machines, bits, tools, etc. A gas factory was
built at each end for lighting purpose.


_Hoosac Tunnel._--The Hoosac tunnel on the Fitchburg R.R. in
Massachusetts is 25,000 ft. long, and the longest tunnel in America. The
material through which the tunnel was driven was chiefly hard granitic
gneiss, conglomerate, and mica-schist rock. The excavation was conducted
from the entrances and one shaft, the wide heading and single-bench
method being employed, with the center-cut system of blasting which was
here used for the first time. The tunnel was begun in 1854, and
continued by hand until 1866, when the mechanical plant was installed.
Most of the particular machines employed have now become obsolete, but
as they were the first machines used for rock tunneling in America they
deserve mention. The drills used were Burleigh percussion drills,
operated by compressed air. Six of these drills were mounted on a single
carriage, and two carriages were used at each front. The air to operate
these drills was supplied by air compressors operated by water-power at
the portals and steam-power at the shaft. The air compressors consisted
of four horizontal single-acting air cylinders with poppet valves and
water injection. The compressors were designed by Mr. Thomas Deane, the
chief engineer of the tunnel.


_Palisades Tunnel._--The Palisades tunnel was constructed to carry a
double track railway line through the ridge of rocks bordering the west
bank of the Hudson River and known as the Palisades. It was located
about opposite 116th St. in New York City. The material penetrated was a
hard trap rock very full of seams in places, which caused large
fragments to fall from the roof. The excavation was made by a single
wide heading and bench, employing the center-cut method of blasting with
eight center holes and 16 side holes for the 7 × 18 ft. heading.
Ingersoll-Sergeant 2¹⁄₂ in. drills were used, four in each heading and
six on each bench, and 30 ft. per 10 hours was considered good work for
one drill.

The power-plant was situated at the west portal of the tunnel, and the
power was transmitted by electricity and compressed air to the middle
shaft and east portal workings. The plant consisted of eight 100 H. P.
boilers, furnishing steam to four Rand duplex 18 × 22 in. air
compressors, and an engine running a 30 arc light dynamo. The compressed
air was carried over the ridge by pipes, varying from 10 ins. to 5 ins.
in diameter, to the shaft and to the east portal, and was used for
operating the hoisting engines as well as the drills at these workings.
Inside the tunnel, specially designed derrick cars were employed to
handle large stones, they being also operated by compressed air. This
car ran on a center track, while the mucking cars ran on side tracks,
and it was employed to lift the bodies of the cars from the trucks,
place them close to the front, being worked where large stone could be
rolled into them, and return them to the trucks for removal. In addition
to handling the car bodies the derrick was used to lift heavy stones.
The hauling was done first by horse-power, and later by dummy
locomotives.


_Croton Aqueduct Tunnel._--In the construction of the Croton Aqueduct
for the water supply of New York City, a tunnel 31 miles long was built,
running from the Croton Dam to the Gate House at 135th St. in New York
City. The section of the tunnel varies in form, but is generally either
a circular or a horseshoe section. In all cases the section was designed
to have a capacity for the flow of water equal to a cylinder 14 ft. in
diameter. To drive the tunnel, 40 shafts were employed. The material
penetrated was of almost every character, from quicksand to granitic
rock, but the bulk of the work was in rock of some character. The
excavation in rock was conducted by the wide heading and bench method,
employing the center-cut method of blasting. Four air drills, mounted on
two double-arm columns were employed in the heading. The drills for the
bench work were mounted on tripods. Steam-power was used exclusively for
operating the compressors, hoisting engines, ventilating fans and pumps;
but the size and kind of boilers used, as well as the kind and capacity
of the machines which they operated, varied greatly, since a separate
power-plant was employed for each shaft with a few exceptions. A
description of the plant at one of the shafts will give an indication of
the size and character of those at the other shafts, and for this
purpose the plant at shaft 10 has been selected.

At shaft 10 steam was provided by two Ingersoll boilers of 80 H. P.
each, and by a small upright boiler of 8 H. P. There were two 18 × 30
in. Ingersoll air compressors pumping into two 42 in. × 10 ft. and two
42 in. × 12 ft. Ingersoll receivers. In the excavation there were twelve
3¹⁄₂ in. and six 3¹⁄₈ in. Ingersoll drills, four drills mounted on two
double arm columns being used on each heading, and the remainder mounted
on tripods being used on the bench. Two Dickson cages operated by one
12 × 12 in. Dickson reversible double hoisting engine provided
transportation for material and supplies up and down the shaft. A
Thomson-Houston ten-light dynamo operated by a Lidgerwood engine
provided light. Drainage was effected by means of two No. 9 and one No.
6 Cameron pumps. At this particular shaft the air exhausted from the
drills gave ample ventilation, especially when after each blast the
smoke was cleared away by a jet of compressed air. In other workings,
however, where this means of ventilation was not sufficient, Baker
blowers were generally employed.


_Strickler Tunnel._--The Strickler tunnel for the water supply of
Colorado Springs, Col., is 6441 ft. long with a section of 4 ft. × 7 ft.
It penetrates the ridge connecting Pike’s Peak and the Big Horn
Mountains, at an elevation of 11,540 ft. above sea level. The material
penetrated is a coarse porphyritic granite and morainal débris, the
portion through the latter material being lined. The mechanical
installation consisted of a water-power electric plant operating air
compressors. The water from Buxton Creek having a fall of 2400 ft. was
utilized to operate a 36 in. 220 H. P. Pelton water-wheel, which
operated a 150 K. W. three-phase generator. From this generator a 3500
volt current was transmitted to the east portal of the tunnel, where a
step-down transformer reduced it to a 220 volt current to the motor. The
transmission line consisted of three No. 5 wires carried on cross-arm
poles and provided with lightning arresters at intervals. The plant at
the east portal of the tunnel consisted of a 75 H. P. electric motor,
driving a 75 H. P. air compressor, and of small motors to drive a
Sturtevant blower for ventilation, to run the blacksmith shop, and to
light the tunnel, shop, and yards. From the compressor air was piped
into the tunnel at the east end, and also over the mountain to the west
portal workings. Two drills were used at each end, and the air was also
used for operating derricks and other machinery. For removing the spoil
a trolley carrier system was employed. A longitudinal timber was
fastened to the tunnel roof, directly in the apex of the roof arch. This
timber carried by means of hangers a steel bar trolley rail on which the
carriages ran. Outside of the portal this rail formed a loop, so that
the carriage could pass around the loop and be taken back to the working
face. Each carriage carried a steel span of 1¹⁄₂ cu. ft. capacity, so
suspended that by means of a tripping device it was automatically dumped
when the proper point on the loop was reached.


_Niagara Falls Power Tunnel._--The tail-race tunnel built to carry away
the water discharged from the turbines of the Niagara Falls Power Co.,
has a horse-shoe section 19 × 21 ft. and a length of 6700 ft. It was
driven through rock from three shafts by the center-cut method of
blasting. In sinking shaft No. 0 very little water was encountered, but
at shafts Nos. 1 and 2 an inflow of 800 gallons and 600 gallons per
minute, respectively, was encountered. The principal plant was located
at shaft No. 2, and consisted of eight 100 H.P. boilers, three 18 × 30
in. Rand duplex air compressors, a Thomson-Houston electric-light plant,
and a sawmill with a capacity of 20,000 ft. B. M. per day. The shafts
were fitted with Otis automatic hoisting engines, with double cages at
shafts Nos. 1 and 2, and a single cage at shaft No. 0. The drills used
were 25 Rand drills and three Ingersoll-Sergeant drills. The pumping
plant at shaft No. 2 consisted of four No. 7 and one No. 9 Cameron
pumps, and that at shaft No. 2 consisted of two No. 7 and two No. 9
Cameron pumps and three Snow pumps. An auxiliary boiler plant consisting
of two 60 H. P. boilers was located at shaft No. 1, and another,
consisting of one 75 H. P. boiler, was located at shaft No. 0.


_Cascade Tunnel._--The Cascade tunnel was built in 1886-88 to carry the
double tracks of the Northern Pacific Ry. through the Cascade Mountains
in Washington. It is 9850 ft. long with a cross-section 16¹⁄₂ ft. wide
and 22 ft. high, and is lined with masonry. The material penetrated was
a basaltic rock, with a dip of the strata of about 5°. The rock was
excavated by a wide heading and one bench, using the center-cut system
of blasting. A strutting consisting of five-segment timber arches
carried on side posts, spaced from 2 ft. to 4 ft. apart, and having a
roof lagging of 4 × 6 in. timbers packed above with cord-wood. The
mechanical plant of the tunnel is of particular interest, because of the
fact that all the machinery and supplies had to be hauled from 82 to 87
miles by teams, over a road cut through the forests covering the
mountain slopes. This work required from Feb. 22 to July 15, 1886, to
perform. In many places the grades were so steep that the wagons had to
be hauled by block and tackle. The plant consisted of five engines, two
water-wheels, five air compressors, eight 70 H. P. steam-boilers, four
large exhaust fans, two complete electric arc-lighting plants, two fully
equipped machine-shop outfits, 36 air drills, two locomotives, 60 dump
cars, and two sawmill outfits, with the necessary accessories for these
various machines. This plant was divided about equally between the two
ends of the tunnel. The cost of the plant and of the work of getting it
into position was $125,000.


_Graveholz Tunnel._--The Graveholz tunnel on the Bergen Railway in
Norway is notable as being the longest tunnel in northern Europe, and
also as being built for a single-track narrow-gauge railway. This tunnel
is 17,400 feet long, and is located at an elevation of 2900 feet above
sea-level. Only about 3% of the length of the tunnel is lined. The
mechanical installation consists of a turbine plant operating the
various machines. There are two turbines of 100 H. P. and 120 H. P.
taking water from a reservoir on the mountain slope, and furnishing 220
H. P., which is distributed about as follows: Boring-machines, 60 H. P.;
ventilation, 30 to 40 H. P.; electric locomotives, 15 H. P.; machine
shop, 15 H. P.; electric-lighting dynamo, 25 H. P.; electric drills, the
surplus, or some 40 H. P. The boring-machines and electric drills will
be operated by the smaller 100 H. P. turbine.


_Sonnstein Tunnel._--The Sonnstein tunnel in Germany is particularly
interesting because of the exclusive use of Brandt rotary drills. The
tunnel was driven through dolomite and hard limestone by means of a
drift and two side galleries. The dimensions of the drift were 7¹⁄₂ ×
7¹⁄₂ ft. The power plant consisted of two steam pressure pumps, one
accumulator, and four drills. The steam-boiler plant, in addition to
operating the pumps, also supplied power for operating a rotary pump for
drainage and a blower for ventilation. The hydraulic pressure required
was 75 atmospheres in the dolomite, and from 85 to 100 atmospheres in
the limestone. The drift was excavated with five 3¹⁄₂ in. holes, one
being placed at the center and driven parallel to the axis of the
tunnel, and four being placed at the corners of a rectangle
corresponding to the sides of the drift, and driven at an angle
diverging from the center hole. The average depths of the holes were 4.3
ft., and the efficiency of the drills was 1 in. per minute. One drill
was employed at each front, and was operated by a machinist and two
helpers, who worked eight-hour shifts, with a blast between shifts at
first, and later twelve-hour shifts, with a blast between shifts. The 24
hours of the two shifts were divided as follows: boring the holes, 10.7
hours; charging the holes, 1.1 hours; removing the spoil, 11.7 hours;
changing shifts, 0.5 hour. The average progress per day for each machine
was 6.7 ft. The total cost of the plant was $17,450.


_St. Clair River Tunnel._--The submarine double-track railway tunnel
under the St. Clair River for the Grand Trunk Ry. is 8500 ft. long, and
was driven through clay by means of a shield, as described in the
succeeding chapter on the shield system of tunneling. The mechanical
plant installed for prosecuting the work was very complete. To furnish
steam to the air compressors, pumps, electric-light engines,
hoisting-engines, etc., a steam-plant was provided on each side of the
river, consisting of three 70 H. P. and four 80 H. P. Scotch portable
boilers. The air-compressor plant at each end consisted of two 20 × 24
in. Ingersoll air compressors. To furnish light to the workings, two 100
candle-power Edison dynamos were installed on the American side, and two
Ball dynamos of the same size were installed on the Canadian side. The
dynamos on both sides were driven by Armington & Sims engines. These
dynamos furnished light to the tunnel workings and to the machine-shops
and power-plant at each end. Root blowers of 10,000 cu. ft. per minute
capacity provided ventilation. The pumping plant consisted of one set of
pumps installed for permanent drainage, and another set installed for
drainage during construction, and also to remain in place as a part of
the permanent plant. The latter set consisted of two 500 gallon
Worthington duplex pumps set first outside of each air lock, closing the
ends of the river portion of the tunnel. For permanent drainage, a
drainage shaft was sunk on the Canadian side of the river, and connected
with a pump at the bottom of the open-cut approach. In this shaft were
placed a vertical, direct-acting, compound-condensing pumping engine
with two 19¹⁄₂ in. high-pressure and two 33³⁄₈ in. low-pressure
cylinders of 24 in. stroke, connected to double-acting pumps with a
capacity of 3000 gallons per minute, and also two duplex pumps of 500
gallons capacity per minute. For permanent drainage on the American
side, four Worthington pumps of 3000 gallons’ capacity were installed in
a pump-house set back into the slope of the open-cut approach. For the
permanent drainage of the tunnel proper two 400 gallon pumps were placed
at the lowest point of the tunnel grade. Spoil coming from the tunnel
proper was hoisted to the top of the open cut by derricks operated by
two 50 H. P. Lidgerwood hoisting-engines. The pressure pumping plant
for supplying water to the hydraulic shield-jacks at each end of the
tunnel consisted of duplex direct-acting engines with 12 in. steam
cylinders and 1 in. water cylinders, supplying water at a pressure of
2000 lbs. per sq. in.




CHAPTER X.

TUNNELS THROUGH HARD ROCK (Continued).


EXCAVATION BY DRIFTS: THE SIMPLON AND MURRAY HILL TUNNELS.

[Illustration: FIG. 55.--Diagram Showing Sequence of Excavations in
Drift Method of Tunneling Rock.]


=General Description.=--The method of tunneling through hard rock by
drifts is preferred by European engineers. All the great Alpine tunnels,
from the Mont Cenis tunnel to the Simplon, are examples of tunneling by
drifts. In this method the sequence of excavation is shown
diagrammatically by Fig. 55. The work begins by excavating a drift close
to the floor of the proposed tunnel (as shown in the center of the
figure) and far in advance of the excavation of any other part. The
section marked 2 is next removed and still later the portions marked 3.
Then with the removal of the parts marked 4 the whole section of the
tunnel will be open.

The drift is usually strutted by means of side posts carrying a
cap-piece placed at intervals, and having a ceiling of longitudinal
planks resting on the successive caps. In hard rock the roof of the
section does not, as a rule, require regular strutting, occasional
supports being placed at intervals to prevent the fall of isolated
fragments: When the rock is disintegrated or full of seams, a regular
strutting may be necessary, and this may be either longitudinal or
polygonal in type. When longitudinal strutting is employed, a sill is
laid across the roof of the drift, and upon this are set up two struts
converging toward the top and supporting a cap-piece close to the roof.
On this cap-piece are placed the first longitudinal crown bars carrying
transverse poling-boards. Additional props standing on the sill and
radiating outward are inserted as parts No. 3 are excavated. These
radial props carry longitudinal bars which in turn support transverse
poling-boards. When polygonal strutting is used, it may take the form of
three or five segment arches of heavy timbers.

In hard rock tunnels, as a rule, there is no danger of caving in because
of heavy pressures, and the whole section is left open for some time
before it is lined. The lining may be of concrete masonry, but in many
long tunnels, excavated through hard rock, the side walls are lined with
rubble masonry and the arch with brick, and, in some instances, even the
arch has been lined with rubble masonry. With skilful laborers at hand
the rubble masonry lining has proved most efficient and economical,
because the rock is utilized as it is excavated without any further
operation. Concrete, however, is more extensively employed for lining
tunnels than any other material.

Tunnels excavated by drifts enable simple means of hauling to be
employed, and this is one of the reasons why the method finds so much
favor with European engineers. The tracks are laid along the floor of
the drift, and carry all the spoil from parts Nos. 2, 3, and 4, as well
as from the front of the drift itself. As fast as the full section is
completed, this single track in the drift is replaced by two tracks
running close to the sides of the tunnel, or by a broad-gauge track with
a third rail.


THE SIMPLON TUNNEL.[8]

Before entering upon a description of the constructive details of this,
the longest railway tunnel in the world, it may be well to give a
general idea of the undertaking. Many schemes for the connection of
Italy and Switzerland by a railway near the Simplon Road Pass have been
devised, including one involving no great length of underground work,
the line mounting by steep gradients and sharp curves. The present
scheme, put forward in 1881 by the Jura-Simplon Ry. Co., consists
broadly of piercing the Alps between Brigue, the present railway
terminus in the Rhone Valley, and Iselle, in the gorge of the Diveria,
on the Italian side, from which village the railway will descend to the
existing southern terminus at Domo d’Ossola, a distance of about 11
miles.

  [8] Abstract from a paper read before the Institution of Civil
  Engineers by Charles B. Fox, Jan. 26, 1900.

In conjunction with this scheme a second tunnel is proposed, to pierce
the Bernese Alps under the Lötschen Pass from Mittholz to a point near
Turtman in the Rhone Valley; and thus, instead of the long détour by
Lausanne and the Lake of Geneva, there will be an almost direct line
from Berne to Milan _via_ Thun, Brigue, and Domo d’Ossola.

Starting from Brigue, the new line, running gently up the valley for
1¹⁄₄ miles, will, on account of the proximity of the Rhone, which has
already been slightly diverted, enter the tunnels on a curve to the
right of 1050 ft. radius. At a distance of 153 yards from the entrance,
the straight portion of the tunnel commences, and extends for 12 miles.
The line then curves to the left with a radius of 1311 ft. before
emerging on the left bank of the Diveria. Commencing at the northern
entrance, a gradient of 1 in 500 (the minimum for efficient drainage)
rises for a length of 5¹⁄₂ miles to a level length of 550 yards in the
center, and then a gradient of 1 in 143 descends to the Italian side. On
the way to Domo d’Ossola one helical tunnel will be necessary, as has
been carried out on the St. Gothard. There will be eventually two
parallel tunnels having their centers 56 ft. apart, each carrying one
line of way; but at the present time only one heading, that known as No.
1, is being excavated to full size, No. 2 being left, masonry lined
where necessary, for future developments. By means of cross headings
every 220 yds. the problems of transport and ventilation are greatly
facilitated, as will be seen later. As both entrances are on curves, a
small “gallery of direction” is necessary, to allow corrections of
alinement to be made direct from the two observatories on the axis of
the tunnel.

The outside installations are as nearly in duplicate as circumstances
will allow, and consist of the necessary offices, workshops,
engine-sheds, power-houses, smithies, and the numerous buildings
entailed by an important engineering scheme. Great care is taken that
the miners and men working in the tunnel shall not suffer from the
sudden change from the warm headings to the cold Alpine air outside; and
for this purpose a large building is in course of erection, where they
will be able to take off their damp working clothes, have a hot and cold
douche, put on a warm dry suit, and obtain refreshments at a moderate
cost before returning to their homes. Instead of each man having a
locker in which to stow his clothes, a perfect forest of cords hangs
down from the wooden ceiling, 25 ft. above floor-level, each cord
passing over its own pulleys and down the wall to a numbered
belaying-pin. Each cord supports three hooks and a soap-dish, which,
when loaded with their owner’s property, are hauled up to the ceiling
out of the way. There are 2000 of these cords, spaced 1 ft. 6 ins.
apart, one to each man. The engineers and foremen are more privileged,
being provided with dressing-rooms and baths, partitioned off from the
two main halls. An extensive clothes washing and drying plant has been
laid down, and also a large restaurant and canteen. At Iselle, a
magazine holding 2200 lbs. of dynamite is surrounded and divided into
two separate parts by earth-banks, 16 ft. high. The two wooden houses,
in which the explosive is stored, are warmed by hot-water pipes to a
temperature between 61° F. and 77° F., and are watched by a military
patrol; but at Brigue a dynamite manufactory, started by an enterprising
company at the time of the commencement of the works, supplies this
commodity at frequent intervals, thereby avoiding the necessity of
storing in such large quantities. This dynamite factory has been
largely increased, and supplies dynamite to nearly all the mining and
tunneling enterprises in Switzerland.


=Geological Conditions.=--Before the Simplon tunnel was authorized,
expert evidence was taken as to the feasibility of the project. The
forecasts of the three engineers chosen, in reference to the rock to be
encountered and its probable temperature, have, as far as the galleries
have gone (an aggregate distance of nearly 2¹⁄₂ miles), generally been
found correct. At the north end, a dark argillaceous schist veined with
quartz was met with, and from time to time beds of gypsum and dolomite
have been traversed, the dip of the strata being on the whole favorable
to progress, though timbering is resorted to at dangerous places. Water
was plentiful at the commencement; in fact, one inrush has not been
stopped, and is still flowing down the heading. The total quantity of
water flowing from the tunnel mouth is 16 gallons per second, of which 2
gallons per second are accounted for by the drilling machines. At
Iselle, however, a very hard antigorio gneiss obtains, and is likely to
extend for 4 miles. Very dry and very compact, it requires no timbering,
and represents no great difficulty to the powerful Brandt rock-drills,
which work under a head of 3280 ft. of water.

The temperature of the rock depends not only on the depth from the
surface, but largely upon the general form of that surface combined with
the conductivity of the rock. Taking these points into consideration
with the experience gained from the construction of the St. Gothard
tunnel, 95° F. was estimated as the probable maximum temperature, owing
to the height of Monte Leone (11,660 ft.), which lies almost directly
over the tunnel axis.


=Survey.=--After having determined upon the general position of the
tunnels, taking into consideration the necessary gradients, the
temperature of the rock, and a large bed of troublesome gypsum on the
north side, two fixed points on the proposed center line were taken,
one at each entrance of tunnel No. 1, and the bearings of these two
points, with reference to a triangulation survey made in 1876, were
calculated sufficiently accurately to determine, for the time being, the
direction of the tunnel. In 1898, a new triangulation survey was made,
taking in eleven summits, Monte Leone holding the central position. This
survey was tied into that of the Wasenhorn and Faulhorn, made by the
Swiss Government, and the accuracy was such that the probable error in
the meeting of the two headings is only 6 cms. or 2¹⁄₂ ins.

On the top of each summit is placed a signal, consisting of a small
pillar of masonry founded on rock, and capped with a sharp pointed cone
of zinc, 1 ft. 6 ins. high. An observatory was built at each end of the
tunnel in such a position that three of the summits could be seen, a
condition very difficult to fulfill on the south side owing to the depth
of the gorge, the mountains on either side being over 7000 ft. high.
Having taken the angles to and from each visible signal, and therefrom
having calculated the direction of the tunnel, it was necessary to fix,
with extreme accuracy, sighting-points on the axis of the tunnel, in
order to avoid sighting on to the surrounding peaks for each subsequent
correction of the alinement of the galleries. To do this, a theodolite
24 ins. long and 2³⁄₈ ins. in diameter, with a magnifying power of 40
times, was set up in the observatory, and about 100 readings were taken
of the angles between the surrounding signals and the required
sighting-points. In this manner the error likely to occur was diminished
to less than 1′. Thus at the north end two points were found about 550
yds. before and behind the observatory, while on the south side, owing
to the narrowness of the gorge, the points could only be placed at 82
yds. and 126 yds. in front. One of these sighting-points consists of a
fine scratch ruled on a piece of glass fixed in an iron frame, behind
which is placed an acetylene lamp,--corrections of alinement are always
done by night,--the whole being rigidly fixed into a niche cut in the
rock and protected from climatic and other disturbing agencies by an
iron plate.


=Method of Checking Alinement.=--The direction of heading No. 1 is
checked by experts from the Government Survey Department at Lausanne
about three times a year, and for this purpose a transit instrument is
set up in the observatory. A number of three-legged iron tables are
placed at intervals of 1 mile or 2 miles along the axis of tunnel No. 1,
and upon each of these is placed a horizontal plane, movable by means of
an adjusting screw, in a direction at right angles to the axis, along a
graduated scale. On this plane are small sockets, into which the legs of
an acetylene lamp and screen, or of the transit instrument, can be
quickly and accurately placed. The screen has a vertical slit, 3 ins. in
height, and variable between ¹³⁄₁₆ in. and ³⁄₁₆ in. in breadth,
according to the state of the atmosphere, and at a distance shows a fine
thread of light. The instrument, having first been sighted on to the
illuminated scratch of the sighting-point, is directed up the tunnel,
where a thread of light is shown from the first table. With the aid of a
telephone this light is adjusted so that its image is exactly coincident
with the cross hairs, and the reading on the graduated scale is noted.
This is done four or five times, the average of these readings being
taken as correct, and the plane is clamped to that average. The
instrument is then taken to the first table and is placed quickly and
accurately over the point just found (by means of the sockets), and the
lamp is carried to the observatory. After first sighting back, a second
point is given on the second table, and so on. These points are marked
either temporarily in the roof of the heading by a short piece of cord
hanging down, or permanently by a brass point held by a small steel
cylinder, 8 ins. long and 3 ins. in diameter, embedded in concrete in
the rock floor, and protected by a circular casting, also sunk in cement
concrete, holding an iron cover resembling that of a small manhole. From
time to time the alinement is checked from these points by the
engineers, and after each blast the general direction is given by the
hand from the temporary points. To check the results of the
triangulation survey, astronomical observations have been taken
simultaneously at each end. With regard to the levels, those given on
the excellent Government surveys have been taken as correct, but they
have also been checked over the pass.


=Details of Tunnels.=--In cross-section, tunnel No. 1 is 13 ft. 7 ins.
wide at formation level, increasing to 16 ft. 5 ins., with a total
height of 18 ft. above rail-level, and a cross-sectional area of about
250 sq. ft. This large section will allow of small repairs being
executed in the roof without interruption of the traffic, and will also
allow of strengthening the walls by additional masonry on the inside.
The thickness of the lining, never wholly absent, and the material of
which it is composed, depend upon the pressure to be resisted, and only
in the worst case is an invert resorted to. The side drain, to which the
rock floor is made to slope, will be composed of half-pipes of 7 to 1
cement concrete. The roof is constructed of radial stones.

Tunnel No. 2, being left as a heading, is driven on that side nearest to
No. 1, to minimize the length of the cross-headings, and measures 10 ft.
2 ins. wide by 6 ft. 7 ins. high. Masonry is used only where necessary,
and in that case is so built as to form part of the lining of the tunnel
when eventually completed. Concrete is put in to form a foundation for
the side wall, and a water channel. The cross-headings, connecting the
two parallel headings, occur every 220 yds., and are placed at an angle
of 56° to the axis of the tunnel, to avoid sharp curves in the
contractors’ railway lines. They will eventually be used as much as
possible for refuges, chambers for storing the tools and equipment of
the platelayers, and signal-cabins. The refuges, 6 ft. 7 ins. wide by 6
ft. 7 ins. high and 3 ft. 3 ins. deep, occur every 110 yards, every
tenth being enlarged to 9 ft. 10 ins. wide by 9 ft. 10 ins. deep and 10
ft. 2 ins. high, still larger chambers being constructed at greater
intervals.


=Method of Excavation.=--The work at each end of the tunnel is carried
on quite independently, consequently, though similar in principle, the
methods vary in detail, apart from the fact that different geological
strata require different treatment. Broadly speaking, the two parallel
headings, each 59 sq. ft. in section, are first driven by means of
drilling-machines and the use of dynamite, this work being carried on
day and night, seven days in the week; No. 1 heading is then enlarged to
full size by hand-drilling and dynamite. On the Italian side, where the
rock is hard and compact, breakups are made at intervals of 50 yds., and
a top gallery is driven in both directions, but, for ventilation
reasons, is never allowed to get more than 4 yds. ahead of the break-up,
which is gradually lengthened and widened to the required section. No
timbering is required, except to facilitate the excavation and the
construction of the side walls. Steel centers are employed for the arch;
they entail fewer supports, give more room, and are capable of being
used over again more frequently without damage. They consist of two
I-beams bent to a template and riveted together at the crown, resting at
either side on scaffolding at intervals of 6 ft.; longitudinals 12 ft.
by 4 ins. by 4 ins. support the roof. Hand rock-drilling is carried out
in the ordinary way, one man holding the tool and a second striking;
measurements of excavation are taken every 2 or 3 yds., a plumb-line is
suspended from the center of the roof, and at every half-meter (20 ins.)
of height horizontal measurements are taken to each side.

At the Brigue end a softer rock is encountered, necessitating at times
heavy timbering in the heading, and especially in the final excavation
to full size, Fig. 56. The bottom heading, 6 ft. 6 in. high, is driven
in the center, and the heading is then widened to the full extent and
timbered; the concrete forming the water channel and the foundation for
one side wall is put in; the side walls are built to a height of 6 ft. 6
ins., and the tunnel is fully excavated to a further height of 6 ft. 6
ins. from the first staging. The side walls are then continued up for
the second 6 ft. 6 ins., and from the second floor a third height of 6
ft. 6 ins. is excavated and timbered. Finally the crown is cleared out,
heavy wooden centers are put in, the arch is turned and all timbers are
withdrawn except the top poling-boards, supporting the loose rock.

[Illustration: FIG. 56.--Sketches Showing Sequence of Work in Excavating
and Lining the Simplon Tunnel.

1

2

3

4

5

6

7

8]

The masonry for the side walls is obtained either from the tunnel itself
or from a neighboring quarry, and varies in character according to the
pressure; but the face of the arch is always of cut or artificial
stones, the latter being 7 to 1 cement concrete. Where the alinement
heading, or the “gallery of direction,” joins the curving portion of
tunnel No. 1, the section is very much greater, and necessitates special
timbering.


=Transport (Italian Side).=--A small line of railway, 2 ft. 7¹⁄₂ ins.
gauge, with 40-lb. rails, enters all three portals; but since the
construction of a wooden bridge over the Diveria, the route through the
“gallery of direction,” across heading No. 2, to tunnel No. 1, is used
exclusively; this railway leads to the face in both headings, and, where
convenient, from one heading to the other by the cross-galleries.
Different types of wagons are in use; but in general they are
four-wheeled, non-tipping box wagons, supplied with brakes and holding 2
cu. yds. of débris. A special type of locomotive is used, designed to
pass round curves of 50 ft. radius, and supplied with a specially large
boiler to avoid firing in the tunnel.

[Illustration: FIG. 57.--General Details of the Brandt Rotary Drills
Employed at the Simplon Tunnel.]


=Method of Working.=--The drilling-machines employed are of the Brandt
type, Fig. 57, and are mounted in the following manner: A small
four-wheeled carriage supports at its center a beam, the shorter arm of
which carries the boring mechanism and the longer a counterpoise; near
its center is the distributor. In the short arm is a clamp holding the
rack-bar or butting column, which is a wrought-iron cylinder with a
plunger constituting a ram, and is jammed by hydraulic pressure between
the walls of the heading, thus forming a rigid support for the
boring-machine, and an efficient abutment against the reaction of the
drill. This rack-bar can be rotated on its clamp in a plane parallel to
the axis of the beam. Three or four separate boring-machines can be
mounted on the rack-bar, and can be adjusted in any reasonable position.

The boring-machine performs the double function of continually pressing
the drill into the rock by means of a hollow ram (_I_) and of imparting
to the drill and ram a uniform rotary motion. This rotary motion is
given by a twin cylinder single-acting hydraulic motor (_E_), the two
pistons, of 2⁷⁄₈ ins. stroke, acting reciprocally as valves. The cranks
are fixed at an angle of 90° to each other on the shaft, which carries a
worm, gearing with a worm-wheel (_Q_) mounted upon the shell (_R_) of
the hollow ram (_I_), and this shell in turn engages the ram by a long
feather, leaving it free to slide axially to or from the face of the
rock. The average speed of the motor is 150 revolutions to 200
revolutions per minute, the maximum speed being 300 revolutions per
minute. The loss of power between the worm and worm-wheel is only 15% at
the most; the worm being of hardened steel and the wheel of gun-metal,
the two surfaces in contact acquire a high degree of polish, resulting
in little wearing or heating. Taking into consideration all other
sources of loss, 70% of the total power is utilized. The pressure on the
drill is exerted by a cylinder and hollow ram (_I_), which revolves
about the differential piston (_S_), which is fixed to the envelope
holding the shell (_R_). This envelope is rigidly connected to the
bed-plate of the motor, and, by means of the vertical hinge and pin
(_T_), is held by the clamp (_V_) embracing the rack-bar. When water is
admitted to the space in front of the differential piston the ram
carrying the drilling-tool is thrust forward, and when admitted to the
annular space behind the piston, the ram recedes, withdrawing the tool
from the blast-hole. The drill proper is a hollow tube of tough steel
2³⁄₄ ins. in external diameter, armed with three or four sharp and
hardened teeth, and makes from five to ten revolutions per minute,
according to the nature of the rock. When the ram has reached the end of
its stroke of 2 ft. 2¹⁄₂ ins., the tool is quickly withdrawn from the
hole and unscrewed from the ram; an extension rod is then screwed into
the tool and into the ram, and the boring is continued, additional
lengths being added as the tool grinds forward; each change of tool or
rod takes about 15 secs. to 25 secs. to perform. The extension rods are
forged steel tubes, fitted with four-threaded screws, and having the
same external diameter as the drill. They are made in standard lengths
of 2 ft. 8 ins., 1 ft. 10 ins., and 11³⁄₄ ins. The total weight of the
drilling-machine is 264 lbs., and that of the rack-bar when full of
water is 308 lbs. The exhaust water from the two motor cylinders escapes
through a tube in the center of the ram and along the bore of the
extension rods and drill, thereby scouring away the débris and keeping
the drill cool; any superfluous water finds an exit through a hose below
the motors and thence away down the heading. The distributor, already
mentioned, supplies each boring-machine and the rack-bar with hydraulic
pressure from the mains, with which connection is effected by means of
flexible or articulated pipe connections, allowing freedom in all
directions. The area of the piston for advancing the tool is 15¹⁄₂ sq.
ins., which, under a pressure of 1470 lbs. per sq. in., gives a pressure
of over 10 tons on the tool, while for withdrawing the tool 2¹⁄₂ tons is
available. In the rock found at Iselle, namely, antigorio gneiss, a hole
2³⁄₄ ins. in diameter and 3 ft. 3 ins. in length is drilled, normally,
in 12 mins. to 25 mins.; a daily rate of advance of 18 ft. to 19 ft. 6
ins. is made in a heading having a minimum cross-section of 59 sq. ft.;
the time taken to drill ten to twelve holes, 4 ft. 7 ins. deep, is 2¹⁄₂
hrs.

When the débris resulting from one operation has been sufficiently
cleared away, a steel flooring, which is provided near the face to
enable shoveling to be more easily done, and to give an even floor for
the wheels of the drilling-carriage, is laid bare at the head of the
line of rails, and the drilling-machines are brought up on their
carriage by eight or ten men. When advanced sufficiently close to the
face, the rack-bar is slewed round across the gallery and is wedged up
against the rock sides; connection is made between the distributor and
the hydraulic main, by means of the flexible pipe, and pressure is
supplied by a small copper tube to the rack-bar ram, thereby rigidly
holding the machine. Next, connections are made between the three
drilling-machines and the distributor, and in 20 mins. from the time the
machine was brought up all three drills are hard at work, water pouring
from the holes.

The noise of the motors and grinding-tools is sufficient to drown all
but shouts; and where the extension rods do not fit tightly, small jets
of water play in all directions, necessitating the wearing of tarpaulins
by the men directing the tools. Lighting is done wholly by small
oil-lamps, provided with a hook to facilitate fixing in any crack in the
rock; electricity will probably be used to light that portion of the
tunnel which is completed.

Two men are allotted to each drill, one to drive the motor, the other to
direct and replenish the tool, one foreman and two men in reserve
completing the gang. A small hammer is freely used to loosen the screw
joints of the extension rods and drill. A hole is usually commenced by a
two-edged flat-pointed tool, until a sufficient depth is reached to
prevent the circular tool from wandering over the face of the rock, but
in many instances the hole is commenced with a circular tool. The
exhaust water during this period flows away by the hose underneath the
motor. In the antigorio gneiss, ten to twelve holes are drilled for each
attack, three to four in the center to a depth of 3 ft. 3 ins., the
remainder, disposed round the outside of the face, having a depth of 4
ft. 7 ins. The average time taken to complete the holes is 1³⁄₄ hr. to
2¹⁄₂ hrs. Instead of pulverizing the rock, as do the diamond drills, it
is found that the rock is crushed, and that headway is gained somewhat
in the manner of a circular saw through wood. The core of rock inside
the tool breaks up into small pieces, and can be taken out if necessary
when the drill requires lengthening.

The lowest holes, inclined downwards, are full of water; consequently
two detonators and two fuses are inserted, but apart from this, water
has little effect on the charge. The fuses of the central holes are
brought together and cut off shorter than those of the outer holes, in
order that they may explode first to increase the effect of the outer
charges. All portable objects, such as drills, pipe connections, tools,
etc., have meanwhile been carried back; the steel flooring is covered
over with a layer of débris to prevent injury from falling rock, and to
the end of the hydraulic main is screwed a brass plug pierced by five
holes; and immediately the explosions occur a valve is opened in the
tunnel, and five jets of water play upon the rock, laying the dust and
clearing the air. The necessity for this was shown on one occasion when
this nozzle was broken by the explosion and the water had to be turned
off immediately to avoid useless waste; on reaching the face, the
atmosphere was found to be so highly charged with dust and smoke that it
was impossible to distinguish the stones at the feet, although a lamp
had been placed on the ground; and despite the fact that the air tube
was in full blast, the men experienced great difficulty in breathing. A
truck is now brought up, and four men clear a passage in front, through
the heap of débris, two with picks and two with shovels, while on either
side and behind are as many men as space will permit. The stone is
thrown either to the sides of the heading or into the wagon, shoveling
being greatly aided by the steel flooring, which, before the explosion,
had been laid over the rails for nearly 10 yds. down the tunnel to
receive the falling rock. These steel plates are taken up when cleared,
and the wagon is pushed forward until the drilling-machine can be
brought up again, leaving the remaining débris at the sides to be
handled at leisure during the next attack. The roof and side walls are,
of course, carefully examined with the pick, to discover and detach any
loose or hanging rock. The times taken for each portion of the attack in
this particular antigorio gneiss are as follows: Bringing up and
adjustment of drills, 20 mins.; drilling, between 1³⁄₄ hr. and 2¹⁄₂
hrs.; charging and firing, 15 mins.; clearing away débris, 2 hrs.; or
for one whole attack, between 4¹⁄₂ hrs. and 5¹⁄₂ hrs., resulting in an
advance of 3 ft. 9 in., or a daily advance of nearly 18 ft.

From this it appears that the time spent in clearing away the débris
equals that taken up in drilling, and it is in this clearing that a
saving of time is likely to be effected rather than in the process of
drilling. Many schemes have been tried, such as a mechanical plow for
making a passage; at Brigue, “marinage,” or clearing by means of
powerful high-pressure water-jets, directed down the tunnel, was tried,
but the idea is not yet sufficiently developed.

Another series of experiments has been tried at Brigue with regard to
the utilization of liquid air as an explosive agent instead of dynamite;
and for this purpose a plant has been laid down, consisting of one
ammonia-compressor, two air-compressors, and two refrigerators,
furnishing ¹⁄₁₀ gallon of liquid air per hour at an expenditure of 17 H.
P. The system used is that of Professor Linde, who himself directs the
experiments. The great difficulty experienced is that of shortening the
interval of time that must elapse between the manufacture of the
cartridge and its explosion. The liquid oxygen, with which the
cartridge, containing kieselguhr (silicious earth) and paraffin, is
saturated, evaporates very readily, losing power every moment; hence the
effect of each cartridge cannot be guaranteed, and though it is an
exceedingly powerful explosive when used immediately after manufacture,
no practical result has yet been obtained.


=Power Station.=--Water is abundant at either end, and therefore
hydraulic power is the motive force employed. On the Italian side, a dam
5 ft. high has been thrown across the Diveria at a point near the Swiss
frontier, about 3 miles above the site of the installations. A portion
of the water thus held back enters, through regulating doors and
gratings, a masonry channel leading to two parallel settling tanks, each
111 ft. by 16 ft., whence, after dropping all its sand and solid matter,
the now pure water passes into the water-house, and, after flowing over
a dam, through a grating and past the admission doors, enters a metallic
conduit of 3-ft. pipes. Each of the settling tanks and the approach
canal are provided with doors at the lower end leading direct to the
river, through which all the sand and solid matter deposited can be
scoured naturally by allowing the river-water to rush freely through.
For this purpose the floor of the basins is on an average gradient of 1
in 30. For a similar reason the river-bed just outside the entrance to
the approach canal is lined with wooden planks, from which the stones
collecting behind the dam can be scoured by allowing an iron flap,
hinged at the bottom, to change its position from the vertical to the
horizontal in a gap left purposely in the dam, so causing a rushing
torrent to sweep it clean.

The chief levels are:

  Level of water at dam  794.00 meters above sea level.
    „   in water-house   793.70   „      „    „    „
    „   at turbines      618.50   „      „    „    „

giving a total fall of 175.20 ms. or 570 ft., and a pressure of 17.52
atmospheres.

The quantity of water capable of being taken from the Diveria in winter,
when the rivers which are dependent upon the mountain snows for their
supply are at their lowest, is calculated to be 352 gallons per second.
Thus, taking the fall to be diminished by friction, etc., to 440 ft.,
and the useful effect at 70%, there is obtained 2000 H. P. on the
turbine shaft.

The metallic conduit varies in material according to the pressure; thus
cast-iron pipes 3 ft. in diameter and ¹³⁄₁₆ in. thick are used up to a
pressure of 2 atmospheres, from which point they are of wrought-iron.
The cast-iron portion has of late caused a good deal of trouble, owing
to settlement of the piers causing occasional bursts, consequently a
masonry pier has been placed under each joint of this portion. The
following table gives the thicknesses and diameters, varying with the
pressure:

  +---------+-------------+-------------+------+
  |  WATER  |  THICKNESS. |  DIAMETER.  |WEIGHT|
  |PRESSURE.|             |             |  PER |
  |         |             |             | YARD.|
  +---------+-------+-----+-----+-------+------+
  |   Head  | Milli-|Inch.|Feet.|Inches.| Lbs. |
  | in Feet.|meters.|     |     |       |      |
  +---------+-------+-----+-----+-------+------+
  |   246   |    6  | ¹⁄₄ |  3  |  0    |  326 |
  |   311   |    7  | ... |  3  |  0    |  383 |
  |   360   |    8  | ... |  3  |  0    |  431 |
  |   393   |    9  | ... |  3  |  0    |  483 |
  |   426   |   10  | ... |  3  |  0    |  556 |
  |   476   |   12  | ... |  3  |  0    |  651 |
  |   590   |   16  | ⁵⁄₈ |  3  |  3¹⁄₃ |  977 |
  +---------+-------+-----+-----+-------+------+

This pipe is supported every 30 ft. on small masonry piers, on the top
of which is placed a block of wood hollowed out to receive the pipe,
thus allowing any movement due to the contraction and expansion of the
conduit. However, to prevent this movement becoming excessive, the pipe
is passed at intervals of 300 yds. to 500 yds. through a cubical block
of masonry of 13 ft. side, strengthened by longitudinal tie-bars. Five
bands of angle-bar riveted round the pipe, with their flanges embedded
in the masonry, constitute a rigid fixed point. Straw mats are thrown
over the pipe where it is exposed to the sun. The temperature of the
conduit is not, however, found to vary greatly, since the pipe is kept
full of water. To supply the rock-drills with water at a maximum
pressure of 100 atmospheres, or 1470 lbs. per sq. in., a plant of four
pairs of high-pressure pumps has been laid down, and a still larger
addition is in course of erection. At present, two Pelton turbines of
250 H.P. each, running at 170 revolutions per minute, drive the pumps,
by means of toothed gearing, at 63 revolutions per minute. These pumps
are of very simple but strong construction, single suction and double
delivery, entailing one suction and one delivery-valve, both heavy and
both of small lift. The larger portion of the plunger has exactly double
the cross-sectional area of the smaller portion, so that in the forward
stroke half of the water taken in at the last admission is pumped into
the high-pressure mains, and at the same time a fresh supply of water is
sucked in. During the backward stroke half of this new supply is pumped
into the mains, and the remainder enters the second chamber, to be
pumped during the next forward stroke. Thus the work done in the two
strokes is practically the same. The pumps are in pairs, and are set at
an angle of 90°, to insure uniform pressure and uniform delivery in the
mains. Their size varies; but at Iselle there are three pairs, with a
stroke of 2 ft. 2¹⁄₂ ins., and the plungers of 2¹¹⁄₁₆ in. and 1⁷⁄₈ ins.
(approximately) in diameter, supplying 1.32 gallons per second.

To avoid injury to the valves, the water to be pumped is taken from a
stream up the mountain side, and is passed through filter screens. The
high-pressure water, after passing an accumulator, enters the tunnel in
solid drawn wrought-iron tubes, 3¹⁄₈ ins. in internal diameter, ³⁄₁₆ in.
thick, and in lengths of 26 ft. The diameter of these mains varies with
their length, so as to avoid loss of pressure. With the 1250 yds. of
tunnel now driven 10 atmospheres are lost.

At Brigue the installations are, as far as possible, identical. The
Rhone water, however, before reaching the water-house, is carried from
the filter basins, a distance of 2 miles, in an armored canal built upon
the Hennebique system,[9] the walls and supporting beams, of cement
concrete, being strengthened by internal tie-bars of steel. The concrete
struts, resembling balks of timber at a distance, are occasionally 35
ft. high and 1 ft. 7¹⁄₂ ins. square. The metallic conduit is 5 ft. in
diameter, with a minimum flow of 176 cu. ft. per second and a total fall
of 185 ft. In case water-power should be unavailable, three
semi-portable steam engines, two of 80 H.P. and one of 60 H.P., are
always kept in readiness at each end of the tunnel, and are geared by
belts to the turbine shaft.

  [9] Network of steel rods embedded in concrete.


=Ventilation.=--In tunneling, one of the most important problems to be
solved is that of ventilation, and it is for this reason that the
Simplon tunnel consists of two parallel headings with cross cuts at
intervals of 220 yds. At Brigue, a shaft 164 ft. deep was sunk through
the overlying rock until the “gallery of direction” was encountered. Up
this chimney the foul air is drawn by wood fires, the fresh air--a
volume of 19,000,000 cu. ft. per day, or 13,200 cu. ft. per
minute--entering by heading No. 2, penetrating up to the last cross
gallery, and returning by tunnel No. 1. The entrances of No. 1 and the
“gallery of direction,” besides those of all the intermediate cross
galleries, are closed by doors. By this arrangement, however, fresh air
does not reach the working faces; therefore a pipe, 8 ins. in diameter,
is led from the fresh air in No. 2 to within 15 yds. of the face of each
heading, and up this pipe a draft of air is induced by means of a jet of
water, the volume to each face being 800 cu. ft. per minute. One single
jet of water from the high-pressure mains, with a diameter of ¹⁄₁₆ in.,
is capable of supplying over 1000 cu. ft. of air per minute at the end
of 160 yds. of pipe, and during the attack the men at the drills are in
a constant breeze with the thermometer standing at 70° F. At Iselle, air
is blown into the entrance of heading No. 2 at the rate of 14,100 cu.
ft. per minute by two fans driven from the turbine shaft. This air
travels from the fans along a pipe 18 ins. in diameter, till a point 15
yds. up the tunnel is reached, where beyond a door the pipe narrows to
form a nozzle 10 ins. in diameter. This door is kept open to allow the
outside air to be induced up the tunnel, as the headings are at present
only 2500 yds. long, giving a resistance of not quite sufficient power
to cause the air to return. The fresh air then travels up No. 2,
crossing over the top of the “gallery of direction,” from which it is
shut off by doors, to the last cross gallery, returning by No. 1, and
finally leaving either by the “gallery of direction” or by No. 1. A
system of cooling the air and driving it on by means of a large number
of water-jets will be installed in No. 2 where that heading crosses over
the “gallery of direction,” but at present there is no need for it.

The average temperature at the face is 73° F. during the drilling
operation, 76° F. after firing the charges, and a maximum of 80° F.,
lately attaining to 86° F. on the south side, with 80° F. and 85° F.
before and after firing. The temperature of the rock is taken at every
110 yds. in holes 5 ft. deep, and shows a gradual increase according to
the depth of over-laying rock, to the conductivity of the rock, and to
the form of the mountain surface. The maximum hitherto reached on the
north side is 68° F., while on the south side, although a smaller
distance has been traversed, it attains to 79° F., due to the more rapid
increase in depth. Moreover, the temperature of the rock is observed at
the permanent stations, 550 yds. from the entrances, in its relation to
that of the tunnel and outside air, and though on the north side that of
the rock varies almost as quickly as that of the tunnel air, on the
south it is influenced very much less.

A few statistics may be of interest with regard to the progress of the
last three months (taken from the trimestrial report of January, 1900).
At Brigue, where there are three drilling-machines in No. 1 and two in
the parallel heading, the total length excavated was 995 yds. or 6409
cu. yds. in 89 working days, the average cross-sectional area being 57
sq. ft. This required 507 attacks and 3066 holes, which had a total
depth of 26,600 ft. and 14,700 re-sharpenings of the drilling-tool, with
44,000 lbs. of dynamite.

The average time occupied in drilling was 2 hrs. 45 mins., while
charging, firing, and clearing away the débris took 6 hrs., 35 mins. At
Brigue 648 men and 29 horses were employed at one time in the tunnel. At
Iselle the numbers were 496 men and 16 horses, working in shifts of 8
hrs. Outside the tunnel, in the shops, forges, etc., the men work 8 hrs.
to 11 hrs. per day, the total being 541 men at Brigue and 346 men at
Iselle. On the Italian side, where the rock is very much harder, there
were three drilling-machines in each heading; the total length
excavated, with a cross-sectional area of 62 sq. ft., was 960 yds. or
6700 cu. yds. in 91 working days. This required 61,293 re-sharpened
tools, 758 attacks, 7940 holes with a total depth of 33,000 ft., and
56,000 lbs. of dynamite. The average time spent in drilling was 2 hrs.
55 mins., and in charging and clearing 2 hrs. 36 mins. Thus, in the hard
gneiss, to excavate 1 cu. yd. of rock required 8¹⁄₂ lbs. of dynamite,
and each tool pierced 6¹⁄₂ ins. of rock before it required
re-sharpening.


THE MURRAY HILL TUNNEL.

The drift method of excavating tunnels was followed in Section IV of the
New York Subway, under Park Avenue between 33rd and 41st Streets. At
this point the four tracks of the subway pass under a rocky elevation,
known as Murray Hill, in two double track parallel tunnels, 43 ft.
apart, center to center. Here already existed a double track tunnel
which was built many years ago by the New York Central and Hudson River
R.R., and is now used by the Madison Avenue surface cars. The two subway
tunnels were driven close below the existing tunnel and also very near
the foundations of expensive residences along Park Avenue, particularly
on Murray Hill, one of the best residential sections of the city.


=Material Penetrated.=--The material penetrated by the excavation
consisted chiefly of a surface outcrop of the mica-schist rock which
underlies Manhattan Island. The rock was for the most part in compact
strata, dipping at about 45° from East to West, but at intervals an
unstable stratum was encountered which when free slid on the underlying
stratum. Troubles from such slides were experienced during the
construction of the tunnel.


=Cross-Section.=--The cross-section selected for the tunnels had
vertical side walls and a three-centered roof arch with the flattest
curve at the crown. The interior dimensions were 25 ft. wide and 16 ft.
high. The selected cross-section was not the best suited for a tunnel to
be driven through rock, where the sharpest curve should be at the top,
but in this case the flattened curve was chosen because of local
conditions; chiefly, the presence of the existing tunnel and the
consequent necessity of leaving a certain thickness of rock between it
and the new tunnel, without depressing very much the grade of the
subway.

[Illustration: FIG. 58.--Sequence of Excavation in the Murray Hill
Tunnel.]


=Excavation.=--The two parallel tunnels were driven exclusively from the
ends reached by shafts; thus the tunnels were attacked at four parts. It
was in these tunnels that a comparative test was made of the different
methods of driving tunnels through rock. The contractor applied the
heading and drift method at the southern ends of the tunnels, the
eastern tunnel being driven by means of a drift while in the western
tunnel the usual heading method was followed. This latter method is
illustrated in the chapter following and the eastern tunnel at 33rd
Street, excavated by means of a drift, is here considered.

Fig. 58 shows the sequence of cuts adopted for this tunnel. It was begun
by a bottom drift, about 10 ft. high, 8 ft. wide and 7 ft. deep, which
was located at one side of the axis of the tunnel, as indicated in the
figure. This drift was immediately widened by removing the portions
marked 2. About 50 ft. in the rear the part marked 3 was taken away,
thus clearing the entire lower portion of the tunnel. Section 4, about
50 ft. to the rear of section 3, was then broken down and removed.

The methods of drilling and blasting were as follows: In taking out the
original drift, a wedge-shaped center cut was made and then enlarged to
the full size of the drift by drilling parallel holes. The succeeding
sections, 2 and 3, were removed by driving parallel holes, while the top
section, 4, was taken away by a center cut and parallel holes. The
drills were mounted on columns, two drills to a column, and the holes
were usually drilled about 7 ft. deep, starting with a diameter of 2³⁄₄
in. and ending with a diameter of 1³⁄₄ in. They were blasted with 40%
dynamite in light charges, only a few holes being fired at a time,
usually not more than three or four.

[Illustration: FIG. 59.--Traveling Platform for the Excavation of the
Upper Side of the Murray Hill Tunnel.]

To remove section 4, a traveling platform 10¹⁄₂ ft. long and 25 ft. wide
was used. This platform, as shown in Fig. 59, consisted of two
longitudinal beams mounted on four double flanged wheels which were
running on tracks laid 23 ft. apart. Resting on top of these beams were
four 12 in. × 12 in. uprights braced in every direction against the
framework of the platform. This frame was built of 12 in. × 12 in. beams
laid longitudinally, the transverse beams being 12 in. × 14 ins. The
platform proper was made of 3 in. planks, and was set 9 ft. above the
tunnel floor. The columns supporting the drills for the excavation of
the upper section 4, were set up above the platform which was then
reinforced by other vertical props, as indicated by the dotted lines in
the figure. These props, however, were placed so as to leave a clearance
beneath the platform for the cars to carry away the débris from the
front. During the blasting the platform was moved back so that the
blasted rock fell to the floor of the tunnel, whence it was loaded into
boxes on the cars.


=Strutting.=--When the rock was seamy and full of fissures, running in
every direction, it was necessary to support the roof of the excavation.
This was done in the following manner: After part 4 was removed the
timbers supporting the roof of the excavation were set up. In this case,
the polygonal strutting was used. This consisted of heavy timber frames
placed transversely to the axis of the tunnel and supporting the planks
or poling-boards which ran longitudinally against the roof of the
excavation. The seven-segment arch frame was used in the Murray Hill
tunnel. At the bottom of part 4 were placed longitudinally 12 × 16 in.
beams and upon them rested the inclined segments which, with a
horizontal one, formed the arch frame as shown in Fig. 60. When the
pressures were too heavy the crown segment was reinforced by a 6 × 12
in. beam, kept in place by two 12 × 12 in. inclined props which rested
on the templates. As the tunnel was lined with concrete, the timbering
was left in place and it was built outside the line of the extrados of
the concrete lining. Timbering was only used for a short distance but it
necessitated a larger amount of rock excavation when it was required.

[Illustration: FIG. 60.--Timbering Used in the Murray Hill Tunnel.]


=Hauling.=--Great efficiency was shown in the method of hauling away the
excavated materials. Three narrow-gauge parallel tracks were laid on the
floor of the tunnel and extended to the faces of the advance drifts.
Small flat cars were run on these tracks. They carried steel boxes, 5
ft. square and 15 ins. deep, fitted with three lifting rings and chains.
When filled, the cars were run to the bottom of the shaft, the boxes
were hoisted by a stiff-legged derrick placed at the shaft head, and the
débris was dumped into storage bins of 300 cu. yds. capacity. These bins
were elevated 8 ft. above the street so that the wagons could be driven
under it to take loads of spoil by means of chutes. The broken rock was
loaded into the boxes by hand.


=Concrete Lining.=--The tunnel was lined with concrete which was
manufactured by a quite elaborate plant. A stone crushing plant,
consisting of bins for raw and crushed stone, was erected at the shaft
head and a mixing plant was suspended from the shaft. On the platform of
the shaft head were two bins side by side, one for crushed stone, the
other for sand; both of which communicated, by means of trap doors, with
a hopper chute. The materials from the hopper were delivered into a
measuring box where cement was laid on top of the other ingredients by
hand. They were then conveyed through a canvas chute into a cubical
mixer operated by an engine. The mixer discharged its contents into
skips set on cars at the bottom of the shaft and the concrete was hauled
inside the tunnel ready for use.

The construction of the lining was accomplished by means of traveling
platforms. The footing courses were laid first. Because these projected
inward about 18 ins. from the faces of the finished sidewalks it was
possible to lay a track rail on their top inner edges on each side of
the tunnel. These track rails carried the traveling platforms. There
were three of these platforms; the forward one was used for building the
side walls; the center one, for carrying a derrick; the last one, for
building the roof arch. The side wall platform was mounted on six
wheels. On each side there was mounted an adjustable lagging which was
curved to conform to the inside profile of the side wall. In operation
this platform was run to the point where the side walls were to be
constructed and the lagging was adjusted to position and fastened. Skips
of concrete were then hoisted on its top, their contents were shoveled
into the space between the lagging and the wall of the excavation and
were there rammed into place until the finished concrete had reached the
top of the lagging. When the concrete had set, the wedges holding the
lagging in place were loosened and the platform was moved ahead and
adjusted for building a new section of wall. The derrick platform was
23¹⁄₂ ft. wide and 18 ft. long. Transversely, it had three bays, two of
which were floored over and one was left without flooring to allow
passage for the concrete skips to and from the cars, on the tunnel floor
beneath. At the center of the floored area was mounted a derrick to
handle the skips. In operation, the derrick platform came between the
side wall platform ahead and the roof platform behind. The construction
of the roof platform was practically the same as the side wall platform
with the addition of roof arch centers at each bent on which lagging
could be placed. The mode of procedure was to erect the form for a small
space between the side walls already built and the haunches of the
center, to shovel concrete from the skips and to run it into place. Then
the roof lagging, a part at a time, was placed upward from the haunches
and the concrete was filled and rammed behind it. The lining was built
from the haunches upward until the two sides approached within a
distance of about 5 ft. from each other at the crown. This 5 ft. crown
strip or key was built by working from the rear toward the front end of
the platform.


=Plant.=--The plant used by the contractors for Section IV. of the
subway comprised a central power plant located about 4000 ft. from the
work. This was on 42nd Street near the East River and furnished power
for the work on both Sections IV. and V. The buildings consisted of an
engine room 63 × 30 ft. and a boiler room, 42 × 28 ft. In the former
room was located one Rand-Corliss air compressor, 22 × 40 × 48 ins.,
having a capacity of 5000 cu. ft. of free air per minute; in the latter
room there were two 200 H.P. water tube boilers. There were also the
necessary equipment of feed water pump, air condenser pump, etc. The
compressors discharged into a 20 × 5¹⁄₂ ft. receiver of riveted steel
through a 7 in. pipe. The air from the receiver was carried by a 10 in.
pipe 3.277 ft. to the corner of Park Avenue and 41st Street, and was
thence run south along Park Avenue in an 8 in. pipe, from which 3 in.
branches led to the four headings of the work.


=Ventilation.=--The ventilation of the tunnel caused very little
trouble. In cool weather the natural draft of the shafts and the air
discharged from the drills served to keep the atmosphere wholesome. In
warm weather, artificial means were necessary to clear the workings of
foul air, particularly after blasting. They comprised at each end a 4
ft. American exhaust fan drawing air from a 12 in. riveted galvanized
iron pipe, which extended to the working faces.


=Illumination.=--The tunnel was lighted by electric lamps which extended
even to the working face. During the blasting, however, all the lamps
and wires within 100 ft. from the front were removed and gasoline
torches were used; they were also employed before the electric lamps and
wires could be replaced, to light the tunnel during the operation of
clearing the débris.




CHAPTER XI.

TUNNELS THROUGH HARD ROCK (Continued).--EXCAVATION BY HEADINGS.


EUROPEAN AND AMERICAN METHODS.

The more common method of tunneling through hard rock is to begin the
work by a heading, instead of by a drift. This heading may be of small
dimensions, and the remainder of the section may also be removed in
successive small parts, or it may be the full width of the section, and
the enlargement of the section be made in one other cut.

[Illustration: FIG. 61.--Diagram Showing Sequence of Excavation in
Heading Method of Tunneling Rock.]


=General Discussion.=--When the tunnel is excavated by means of several
cuts, which is the method usually employed in Europe, the sequence of
work is as indicated by Fig. 61. Work is begun by driving the center top
heading No. 1, whose floor is at the level of the bottom of the roof
arch, and which is usually excavated by the circular cut method. This
heading is widened by removing parts Nos. 2 and 3 until the top part of
the section is removed, then the roof arch is built with its feet
resting on the unexcavated rock below. The lower portion of the section
or bench is removed by first sinking the trench No. 4, after which part
No. 5 is taken out, and then parts Nos. 6 and 7, and the side walls
built. Part No. 8 for the culvert is finally opened. The heading is, as
a rule, driven far in advance, but the excavation of each of the other
parts follows the preceding one at a distance behind of about 300 ft.

The strutting, when any is required, is usually the typical radial
strutting of the Belgian method of tunneling. The masonry lining is
constructed practically the same as in tunnels excavated by a drift. The
hauling is done on a single track laid in the heading No. 1, which
separates into double tracks where the full top section has been
excavated by the removal of parts No. 2. These two tracks are again
combined and form a single track along the top of part No. 5, which has
been left wider than part No. 4 for this particular purpose. When part
No. 3 is excavated a standard-gauge track is laid on its floor; and as
the full section of the tunnel is completed by taking out parts Nos. 4
and 5, this single track is replaced by two standard-gauge tracks, into
which it switches. Spoil is transferred from the narrow-gauge tracks on
the upper level, to the standard-gauge tracks on the tunnel floor, by
means of chutes, and building material is transferred in the opposite
direction by means of hoisting apparatus.

When the excavation is made by a single wide heading, and a single other
cut for removing the bench, which is the method preferred by American
engineers, it is called the Heading and Bench method. The work begins by
removing a top heading the full width of the section; this heading is
usually made 7 ft. or 8 ft. high, and is excavated by the center cut
method. The method of strutting usually employed is to erect successive
three- or five-segment timber arches, whose feet rest on the top of the
bench; when the bench is removed, posts are inserted under the feet of
each arch. These arches are covered with a lagging of plank. In America
it has often been the practice to let this strutting serve as a
temporary lining, and to replace it only after some time, often after
years, with a permanent lining of masonry. In a succeeding chapter, some
of the methods adopted in relining timber-lined arches with masonry are
described. The hauling is done by either narrow or broad gauge tracks
laid on the floor of the completed section below. A device called a
bench carriage is often employed to enable the cars running on the
heading tracks to dump their loads into the cars below, without
interfering with the work on the bench front. This device consists of a
wide platform carried on trucks, running on rails at the sides of the
tunnel floor, so that it is level with the floor of the heading. The
front of this platform carries a hinged leaf which may be raised and
lowered, and which forms a sort of gang-plank reaching to the floor of
the heading. By running the heading cars out on to this traveling
platform, they can be dumped into the cars below entirely clear of the
work in progress on the bench front.

For the purpose of illustrating the two methods of driving tunnels by a
heading, which have been briefly described, the St. Gothard and the Fort
George tunnels have been selected. The St. Gothard tunnel is selected,
as being one of the longest tunnels in the world, and because it was
excavated by a number of small parts; and the Fort George tunnel, as
being a double-track tunnel, driven by a heading, and bench, and having
a concrete lining.


ST. GOTHARD TUNNEL.

The St. Gothard tunnel penetrates the Alps between Italy and France, and
is 9¹⁄₄ miles long. It was constructed in 1872-82.


=Material Penetrated.=--The St. Gothard tunnel was excavated through
rock, consisting chiefly of gneiss, mica-schist, serpentine, and
hornblende, the strata having an inclination of from 45° to 90°. At many
points the rock was fissured, and disintegrated easily, and water was
encountered in large quantities, causing much trouble.


=Excavation.=--The sequence of excavation is shown by Fig. 14, p. 36.
First the top center heading, No. 1, whose dimensions varied from 8.25 ×
8.6 ft. to 8.5 × 9 ft., according to the quality of the rock, was driven
never less than 1000 ft. and sometimes over 3000 ft. in advance of parts
No. 2. The excavation of parts No. 2 opened up the full top section, and
parts Nos. 3, 4, 5, 6, and 7, were removed in the order numbered.


=Strutting.=--Where regular strutting was required, the construction
shown in Fig. 62 was adopted.


=Masonry.=--The St. Gothard tunnel is lined throughout with masonry.
After the upper portion of the section was fully excavated, the roof
arch was built with its feet resting upon short planks on the top of the
bench. Plank centers were used in constructing the arch. For the arch
brick masonry was employed, but the side walls were built of rubble
masonry. Shelter niches, about 3 ft. deep, were built into the side
walls at intervals, and about every 3,000 ft. storage niches about 10
ft. deep, and closed with a door, were constructed. The culvert was of
brick masonry.


=Mechanical Installation.=--Water-power was used exclusively in driving
the St. Gothard tunnel. At the north end, the Reuss, and at the south
end, the Tessin and the Tremola, rivers or torrents were dammed, and
their waters conducted to turbine plants at the opposite ends of the
tunnel. The power thus furnished by the Reuss was about 1,500 H.P., and
the power furnished by the combined supply of the Tessin and Tremola was
1,220 H.P. The turbine plant at both ends at first consisted of four
horizontal impulse turbines, but later, two more turbines were added at
the south end. Each of the two sets of four turbines first installed
drove five groups of three compressors each, and the two supplementary
turbines drove two groups of four compressors each. The compressors were
of the Colladon type with water injection, and four groups of three
compressors each were capable of furnishing 1,000 cu. yds. of air
compressed to between seven and eight atmospheres every hour, or about
100 H.P. per hour, delivered to the drills at the front. This air when
exhausted provided about 8,000 cu. yds. of fresh air per hour for
ventilation.

The compressors at each entrance discharged into a group of four
cylindrical receivers of wrought-iron each 5.3 ft. in diameter by 29.5
ft. long, and having a capacity of 593 cu. ft. The cylinders were placed
horizontally, the first one receiving the air at one end and discharging
it at the other end into the next cylinder, and so on. By this
arrangement the air was drained of its moisture, and the discharge from
the end receiver into the tunnel delivery pipes was not affected by the
pulsations of the compressors. The delivery pipe decreased from 8 in. in
diameter at the receiver to 4 ins. in diameter, and finally to 2¹⁄₂ ins.
in diameter, at the front.

The drills employed were of various patterns. The first one employed was
the Dubois & François “perforator,” in which the drill-bit was fed
forward by hand. This was replaced by Ferroux drills having an automatic
feed. Jules McKean’s “perforator” was employed at the north end of the
tunnel. All of these drills were of the percussion type, and were
mounted on carriages running on tracks. Their comparative efficiency was
officially tested in drilling granitic gneiss with an operating air
pressure of 5.5 atmospheres with the following results:

  NAME OF DRILL.          PENETRATION
                         INS. PER MIN.

  Ferroux                    1.6
  McKean                     1.4
  Dubois & François          1.04
  Soummelier                 0.85

The heading was excavated by the circular cut method, the holes being
driven as follows: Near the center of the heading three holes were first
drilled, converging so as to inclose a pyramid with a triangular base.
Around these center holes from 9 to 13 others were driven parallel to
the tunnel axis. The center holes were blasted first, and then the
surrounding holes. From 3 to 5 hours were required to drill the two sets
of holes, and from three to four hours were required to remove the
blasted rock. The number of holes drilled in removing each of the
various parts was as follows:

  Part No. 1                6 to  9
  Part No. 2                6 to 10
  Part No. 3                2
  Part No. 4                6 to  9
  Part No. 5                3
  Part No. 6                6 to  9
  Part No. 7                1
                           --------
  Total for full section   36 to 40


=Hauling.=--Two different systems were employed for hauling the spoil
and construction material in the St. Gothard tunnel. To remove the spoil
from parts Nos. 1 and 2 a narrow-gauge track was laid on the floor of
the heading, and the cars were hauled by horses, the grade being
descending from the fronts. These narrow-gauge cars were dumped into
larger broad-gauge cars running on the track laid on the floor of the
completed section and hauled by compressed air locomotives (Fig. 63). To
raise the incoming structural material from the broad-gauge cars to the
narrow-gauge cars running on the level above, hoisting devices were
employed.

[Illustration: FIG. 62.--Method of Strutting Roof, St. Gothard Tunnel.]

[Illustration: FIG. 63.--Sketch Showing Arrangement of Car Tracks, St.
Gothard Tunnel.]


FORT GEORGE TUNNEL.[10]

From a point north of 157th Street and Broadway almost to Dyckman
Street, that is, a distance of nearly two miles, the New York Subway
passes under an elevation known as Fort Washington Heights, which almost
bounds Manhattan Island at its upper end near the Harlem Ship Canal.
Under this elevation the rapid transit railroad was constructed in
tunnel. The tunnel was driven from two intermediate shafts over 110 ft.
deep, located one at 169th Street and the other at 181st Street and
Broadway. Both shafts were sunk at one side of the center line of the
tunnel. After these shafts had been utilized for working purposes during
the construction of the tunnel, they were equipped with electric
elevators to carry passengers from the streets to the deep station.

  [10] Condensed from a paper by Stephen W. Hopkins in _Harvard
  Engineering Journal_, April, ’08.


=Material.=--The material encountered in the excavation of the Fort
George tunnel was the usual mica schist met everywhere on Manhattan
Island. It was full of seams with strata running in every direction to
such an extent that at many points the roof of the tunnel had to be
supported by timbers; at other parts along the line the rock was so
disintegrated that it was considered a very loose and treacherous soil.
Two serious accidents, each accompanied by loss of life, occurred during
the construction of this tunnel. Both of them were caused by the sudden
fall of a large ledge of rock which, after the tunnel had been excavated
to the full section, remained hanging on the roof, deprived of any
support and held in place by the little cohesion of the material packing
the seams.


=Excavation.=--The tunnel was excavated by the heading method in only
two cuts, viz., the heading and bench as indicated in the Fig. 65. The
heading, almost as wide as the upper portion of the tunnel section, was
excavated in the manner explained on page 91. After the heading was
removed, the enlargement of the entire upper section of the tunnel was
accomplished by driving three inclined holes at each side of the
heading. They were driven at different depths and inclinations, as shown
in the figure and were called trimming holes. At the same time the bench
was removed by means of five holes--three vertical and two inclined. The
line of subgrade was reached by means of five grading holes driven
almost horizontal with a slight inclination downward. The air drills for
the heading were mounted on columns, all the others on tripods. The
blasting was done in the following order: the grading holes were blasted
in the first round, the bench and trimming in the second, the center cut
of the heading in the third, the sides in the fourth and the dry holes
in the last. Thus each advance of 7 ft. of the whole tunnel section was
made by means of forty holes fired in five rounds which consumed 277
lbs. of dynamite with an average additional quantity of 76 lbs., making
a total of 353 lbs. With the exception of the center cut, where 60%
dynamite was used, all the other holes were discharged with 40%
dynamite.

[Illustration: ~Cross Section.~

~Longitudinal Section.~

FIG. 64.--Arrangement of Drill Holes in the Fort George Tunnel.

FIG. 65.--Longitudinal Section of the Heading and Bench Excavation at
the Fort George Tunnel.]


=Strutting.=--When the rock was of such a character as to be dangerous
and required permanent timber support, until the masonry lining was in
place, the method employed was as follows: a top heading was first
excavated about 10 ft. deep and from 10 ft. to 12 ft. wide for some
distance, 100 ft. to 500 ft., the dangerous rock being supported by 10 ×
10 in. yellow pine plumb or raking posts and sometimes by timber bents
(“caps and legs”). The next process was to widen the heading to the full
width of 30 ft. for a length of about 20 ft., placing timber supports
under the dangerous rock as the widening-out progressed. The excavation
was deepened a little at the sides to 9.5 ft. below the roof grade
(ordered line of excavation) or about 11 ft. below the roof grade, which
was necessary when segmental timbering was to be used, to allow for
placing a 12 × 12 in. “wall plate” (timber sill) along each side. These
wall plates, generally 20 ft. long, were set to the correct elevation
and were leveled by blocking and wedging. As soon as the wall plates
were set, the work of erecting the segmental timber sets, one set at a
time, was begun by starting from the wall plates and supporting the
timber on scaffolding until keyed in, then it was blocked up to the rock
at each joint and at other necessary points. When two or more sets were
erected, lagging, made of boards 2 ins. thick by 6 to 10 ins. wide, was
placed over the segmental timber “sets” and the space above the timber
dry packed with small stone placed by hand. Sometimes there was enough
room between the timber and the rock to do all the dry packing after the
full number of sets, generally six, had been placed on the two wall
plates. The temporary timber posts and braces were taken out as the
segmental timber sets were erected.

The seven timbers that made up a timber set were of yellow pine each
10 × 10 ins., 5 ft. 2 ins. long at the intrados and 5 ft. 6 ins. at the
extrados. The sets were spaced from 3 ft. to 5 ft. apart, but generally
3.5 ft. and braced to each other at each joint of the segmental timbers
by 6 × 8 in. spreaders which were wedged against the joint splices.

When the timbers were all erected on a set of wall plates (20 ft.) and
the lagging and dry packing were completed the work of taking out the
bench, which had been partly drilled as the timber sets were erected,
was resumed. The face of the bench, which had been left about 4 ft. from
the end of the previous set of wall plates, was brought forward slowly
by placing 10 × 10 in. plumb posts which extended below subgrade under
the wall plates. These posts were generally spaced the same as the
timber sets above and directly under them.

When the face of the bench had been brought to within 3 or 4 ft. of the
forward end of the wall plate, the process of widening out and timbering
another 20 ft. length of heading was begun. In some places the rock,
though needing permanent support, was such that the work of taking out
the bench and widening the heading was carried on simultaneously without
increasing the danger; but the greater portion of the work, when
strutting was required, was done as has been described.

[Illustration: FIG. 66.--Diagram Showing the Arrangement of Drill Holes
in the Heading and Bench of the Gallitsin Tunnel.]


=Hauling.=--The excavated material was loaded at the foot of the bench
in dump cars which were run by mule power to the portal or the shaft
according to location, on 36 in. gauge-service tracks. Inclines at 159th
Street were graded from the portal at 158th Street to the street
surface. The cars were formed at this portal into a train and were taken
up the incline to the dump at 162nd Street and the North River by
construction locomotives. At the 168th Street and 181st Street shafts,
the cars were hoisted to the surface in cages (elevators). In the former
case, they were taken to the dump at 165th Street and the North River by
mules and gravity; in the latter case, to various dumps by teams. At
both shafts, stone crushers were located, therefore a great part of the
material did not have to be hauled to the dumps or even taken to the
surface as a great deal of stone was used in dry packing over the
concrete arch. The material from the portal at Fort George was hauled by
mules directly to the dump near by.


=Lining.=--The entire tunnel was lined with concrete, consisting of a
floor 4 ins. thick and vertical side walls 18 ins. thick and 25 ft.
apart, which carried a semicircular arch 18 ins. thick except in the
timbered portions where the thickness was increased to 21 ins. and to 24
and 27 ins. in some places. The springing line of the arch is 6 ft. 2
ins. above the concrete floor (5 ft. 6 ins. above the base of rail),
hence the maximum clearance above the base of rail is 18 ft. The side
walls and arch were built solid of rock to a height of 8 ft. above
springing line and the space above that point between the concrete and
the rock was packed by hand with small stones. The concrete of the arch
was laid on timber centers erected for that purpose.

The heading and bench method of excavating rock tunnels is not always
followed in the manner just described but is employed with slight
modifications. There is a large variety of modifications but only the
two most commonly used in practical works are given here. The heading
and bench method illustrated in Fig. 66 was used, among others, on the
Gallitsin tunnel along the Pennsylvania R.R. at the summit of the
Alleghenies near Altoona, Pa., and more recently in the tunnels
constructed by the same company under Bergen Hill, N. J., for the
entrance to New York City. The shape of the cross-section of these
tunnels was semicircular arch on vertical side walls. The excavation was
made in three consecutive cuts, viz., the heading marked 1 in the
figure, the top bench 2, and the lower bench 3. A heading 7 ft. high and
10 ft. wide was attached near the crown of the arch and the rock was
removed by means of a center cut and parallel side holes, the number of
holes depending upon the consistency of the rock. The part No. 2 was
excavated by drilling holes at each side to different depths and at
different inclinations in order to reach the line of the profile as well
as the springing line of the proposed tunnel. The central part of the
top bench was excavated by means of holes driven vertically from the
floor of the heading. The bottom bench No. 3, included between the
springing line of the arch and subgrade, was removed by means of five
vertical holes driven from the floor of the top bench. The three
different working parts were kept nearly 10 ft. apart. Blasting was
effected in reversed order to the figures marked in the diagram, viz.,
the bottom bench first and the heading last.

[Illustration: FIG. 67.--Diagram Showing a Modification of the Heading
and Bench Method.]

Still another modification of the heading and bench method, commonly
followed by American engineers, is the one shown in Fig. 67. This
consists in dividing the tunnel section in three parts by horizontal
lines. The resultant parts are first the heading excavated close to the
roof, and as wide as the whole section of the tunnel; second, the top
bench in the middle, and lastly the bottom bench excavated to the depth
of the proposed tunnel floor. The excavation proceeds in the numerical
order, beginning at the heading which was excavated, as usual, by means
of a center cut and side holes to the full width of the proposed tunnel.
First the top bench, then the bottom bench, are removed by means of
vertical holes driven from the floor of the heading and the floor of the
top bench, respectively.


COMPARISON OF METHODS.

The differences between the drift and heading methods of excavating
tunnels through rock, consist chiefly in the excavations, strutting, and
hauling. When the drift method is employed an advanced gallery is opened
along the floor of the tunnel before the upper part of the section is
removed, and when the heading method is employed the upper part of the
section is completely excavated before any part of the section below is
excavated. When the drift method of driving is employed polygonal
strutting is usually used, and longitudinal strutting is employed with
the heading method of driving. In the drift method the hauling is done
by one system of tracks at the same level, while in the heading method
two systems of tracks are employed at different levels.

It is, perhaps, impossible to state without qualification which method
is the better. European engineers who have been connected with both the
Mont Cenis and St. Gothard tunnels, driven by the drift and heading
methods respectively, had the opportunity to practically observe the
advantages and disadvantages of these two methods. Their conclusion was
that the drift method was more convenient for tunnels driven through
hard and compact rock, and that the heading method was better for
tunnels of fissured and disintegrated rocks. To prove this opinion,
experiments were made in one of the tunnels approaching the great St.
Gothard tunnel. On a short tunnel the excavation was made by the drift
method from one portal, while at the other, the heading method was
followed. Although the general rule was fully confirmed still the
conditions at the portals were not identical. More conclusive
experiments were made by Mr. Ira A. Shaler, the contractor for Section
IV., of New York Rapid Transit Railway. He had the opportunity of
driving two parallel tunnels under Murray Hill only 17 ft. apart. The
eastern tunnel was driven by the drift method, the western one by the
heading method. After the work had proceeded for a few months, Mr.
Shaler stated that in his case the drift method was more convenient. He
could spare drilling several holes at each advance, thus obtaining
economy in time, labor and material without considering the advantage of
a simpler transportation of the débris. He promised to publish his
results for the benefit of the profession, but, unfortunately, lost his
life in an accident in the tunnel before the completion of the work.

An advantage that the drift method affords in long tunnels is, that the
water, which is usually found in large quantities under high mountains,
is easily collected in the drift and conveyed to the culvert, while in
the heading method the water from the advance gallery, before being
collected into the culvert built on the floor of the tunnel, must pass
through all the workings. This may be a serious inconvenience when water
is found in large quantities, as, for instance, was the case in the St.
Gothard tunnel, where the stream amounted to 57 gallons per second.




CHAPTER XII.

EXCAVATING TUNNELS THROUGH SOFT GROUND; GENERAL DISCUSSION; THE BELGIAN
METHOD.


GENERAL DISCUSSION.

It may be set down as a general truth that the excavation of tunnels
through soft ground is the most difficult task which confronts the
tunnel engineer. Under the general term of soft ground, however, a great
variety of materials is included, beginning with stratified soft rock
and the most stable sands and clays, and ending with laminated clay of
the worst character. From this it is evident that certain kinds of
soft-ground tunneling may be less difficult than the tunneling of rock,
and that other kinds may present almost insurmountable difficulties.
Classing both the easy and the difficult materials together, however,
the accuracy of the statement first made holds good in a general way.
Whatever the opinion may be in regard to this point, however, there is
no chance for dispute in the statement that the difficulty of tunneling
the softer and more treacherous clays, peats, and sands is greater than
that of tunneling firm soils and rock; and if we describe the methods
which are used successfully in tunneling very unstable materials, no
difficulty need be experienced in modifying them to handle stable
materials.


=Characteristics of Soft-Ground Tunneling.=--The principal
characteristics which distinguish soft-ground tunneling are, first, that
the material is excavated without the use of explosives, and second,
that the excavation has to be strutted practically as fast as it is
completed. In treacherous soils the excavation also presents other
characteristic phenomena: The material forming the walls of the
excavation tends to cave and slide. This tendency may develop
immediately upon excavation, or it may be of slower growth, due to
weathering and other natural causes. In either case the roof of the
excavations tends to fall, the sides tend to cave inward and squeeze
together, and the bottom tends to bulge or swell upward. In materials of
very unstable character these movements exert enormous pressures upon
the timbering or strutting, and in especially bad cases may destroy and
crush the strutting completely. Outside the tunnel the surface of the
ground above sinks for a considerable distance on each side of the line
of the tunnel.


=Methods of Soft-Ground Tunneling.=--There are a variety of methods of
tunneling through soft ground. Some of these, like the quicksand method
and the shield method, differ in character entirely, while in others,
like the Belgian, German, English, Austrian, and Italian methods, the
difference consists simply in the different order in which the drifts
and headings are driven, in the difference in the number and size of
these advance galleries, and in the different forms of strutting
framework employed. In this book the shield method is considered
individually; but the description of the Belgian, German, English,
Austrian, Italian, and quicksand methods are grouped together in this
and the three succeeding chapters to permit of easy comparison.


THE BELGIAN METHOD OF TUNNELING THROUGH SOFT GROUND.

[Illustration: FIGS. 68 and 68A.--Diagrams Showing Sequence of
Excavations in the Belgian Method.]

The Belgian method of tunneling through soft ground was first employed
in 1828 in excavating the Charleroy tunnel of the Brussels-Charleroy
Canal in Belgium, and it takes its name from the country in which it
originated. The distinctive characteristic of the method is the
construction of the roof arch before the side walls and invert are
built. The excavation, therefore, begins with the driving of a top
center heading which is enlarged until the whole of the section above
the springing lines of the arch is opened. Various modifications of the
method have been developed, and some of the more important of these will
be described farther on, but we shall begin its consideration here by
describing first the original and usual mode of procedure.


=Excavation.=--Fig. 68 is the excavation diagram of the Belgian method
of tunneling. The excavation is begun by opening the center top heading
No. 1, which is carried ahead a greater or less distance, depending upon
the nature of the soil, and is immediately strutted. This heading is
then deepened by excavating part No. 2, to a depth corresponding to the
springing lines of the roof arch. The next step is to remove the two
side sections No. 3, by attacking them at the two fronts and at the
sides with four gangs of excavators. The regularity and efficiency of
the mode of procedure described consist in adopting such dimensions for
these several parts of the section that each will be excavated at the
same rate of speed. When the upper part of the section has been
excavated as described, the roof arch is built, with its feet supported
by the unexcavated earth below. This portion of the section is excavated
by taking out first the central trench No. 4 to the depth of the bottom
of the tunnel, and then by removing the two side parts No. 5. As these
side parts No. 5 have to support the arch, they have to be excavated in
such a way as not to endanger it. At intervals along the central trench
No. 4, transverse or side trenches about 2 ft. wide are excavated on
both sides, and struts are inserted to support the masonry previously
supported by the earth which has been removed. The next step is to widen
these side trenches, and insert struts until all of the material in
parts No. 5 is taken out.

When the material penetrated is firm enough to permit, the plan of
excavation illustrated by the diagram, Fig. 68A, is substituted for the
more typical one just described. The only difference in the two methods
consists in the plan of excavating the upper part of the profile, which
in the second method consists in driving first the center top heading
No. 1, and then in taking out the remainder of the section above the
springing lines of the arch in one operation, while in the first method
it is done in two operations. The distance ahead of the masonry to which
the various parts can be driven varies from 10 ft. to, in some cases,
100 ft., being very short in treacherous ground, and longer the more
stable the material is.


=Strutting.=--The longitudinal method of strutting, with the
poling-boards running transversely of the tunnel, is always employed in
the Belgian method of tunneling. In driving the first center top
heading, pairs of vertical posts carrying a transverse cap-piece are
erected at intervals. On these cap-pieces are carried two longitudinal
bars, which in turn support the saddle planks. As fast as part No. 2,
Fig. 68, is excavated, the vertical posts are replaced by the batter
posts _A_ and _B_, Fig. 69. The excavation of parts No. 3 is begun at
the top, the poling-boards _a_ and _b_ being inserted as the work
progresses. To support the outer ends of these poling-boards, the
longitudinals _X_ and _Y_ are inserted and supported by the batter posts
_C_ and _D_. In exactly the same way the poling-boards _c_ and _d_, the
longitudinals _V_ and _W_, and the struts _E_ and _F_, are placed in
position; and this procedure is repeated until the whole top part of the
section is strutted, as shown by Fig. 69, the cross struts _x_, _y_,
_z_, etc., being inserted to hold the radial struts firmly in position.
The feet of the various radial props rest on the sill _M N_. These
fan-like timber structures are set up at intervals of from 3 ft. to 6
ft., depending upon the quality of the soil penetrated.

[Illustration: FIG. 69.--Sketch Showing Radial Roof Strutting, Belgian
Method.]

[Illustration: FIG. 70.--Sketch Showing Roof Arch Center, Belgian
Method.]


=Centers.=--Either plank or trussed centers may be employed in laying
the roof arch in the Belgian method, but the form of center commonly
employed is a trussed center constructed as shown by Fig. 70. It may be
said to consist of a king-post truss carried on top of a modified form
of queen-post truss. The collar-beam and the tie-beam of the queen-post
truss are spaced about 7 ft. apart, and the posts themselves are left
far enough apart to allow the passage of workmen and cars between them.
The tie beam of the king-post truss is clamped to the collar-beam of the
queen-post truss by iron bands. On the rafters of the two trusses are
fastened timbers, with their outer edges cut to the curve of the roof
arch. These centers are set up midway between the fan-like strutting
frames previously described. They are usually built of square timbers.
The tie beams are usually 6 × 6 in., and the struts and posts 4 × 4 in.
timbers. The reason for giving the larger sectional dimensions to the
tie beams, contrary to the usual practice in constructing centers, is
that it has to serve as a sill for distributing the pressure to the
foundation of unexcavated soil which supports the center. Sometimes a
sub-sill is used to support the center upon the soil; and in any case
wedges are employed to carry it, which can be removed for the purpose of
striking the center. After the arch is completed, the centers may be
removed immediately, or may be left in position until the masonry has
thoroughly set. In either case the leading center over which the arch
masonry terminates temporarily is left in position until the next
section of the arch is built.


=Masonry.=--The masonry of the roof arch, which is the first part built,
is of necessity begun at the springing lines, and the first course rests
on short lengths of heavy planks. These planks, besides giving an even
surface upon which to begin the masonry, are essential in furnishing a
bearing to the struts inserted to support the arch while the earth below
them, part No. 5, Fig. 68, is being excavated. As the arch masonry
progresses from the springing lines upward, the radial posts of the
strutting are removed, and replaced by short struts resting on the
lagging of the centers, which support the crown bars or longitudinals
until the masonry is in place, when they and the poling-boards are
removed, and the space between the arch masonry and walls of the
excavation is filled with stone or well-rammed earth.

Considering now the side wall masonry, it will be remembered that in
excavating the part No. 5, Fig. 68, of the section, frequent side
trenches were excavated, and struts inserted to take the weight of the
masonry. These struts are inserted on a batter, with their feet near the
center of the tunnel floor, so that the side wall masonry may be carried
up behind them to a height as near as possible to the springing lines of
the arch. When this is done the struts are removed, and the space
remaining between the top of the partly finished side wall and the arch
is filled in. This leaves the arch supported by alternate lengths or
pillars of unexcavated earth and completed side wall. The next step is
to remove the remaining sections of earth between the sections of side
wall, and fill in the space with masonry. Fig. 71 is a cross-section,
showing the masonry completed for one-half and the inclined props in
position for the other half; and Fig. 72 is a longitudinal section
showing the pillars of unexcavated earth between the consecutive sets of
inclined struts and several other details of the lining, strutting, and
excavating work.

[Illustration: FIG. 71.--Sketch Showing Method of Underpinning Roof Arch
with the Side Wall Masonry.]

[Illustration: FIG. 72.--Longitudinal Section Showing Construction by
the Belgian Method.]

The invert masonry is built after the side walls are completed. This is
regarded as a defect of this method of tunneling, since the lateral
pressures may squeeze the side walls together and distort the arch
before the invert is in place to brace them apart. To prevent as much as
possible the distortion of the arch after the centers are removed, it is
considered good practice to shore the masonry with horizontal beams
having their ends abutting against plank, as shown by Fig. 71. These
horizontal beams should be placed at close intervals, and be supported
at intermediate points by vertical posts, as shown by the illustration.
Since the roof arch rests are for some time supported directly by the
unexcavated earth below, settlement is liable, particularly in working
through soft ground. This fact may not be very important so long as the
settlement is uniform, and is not enough to encroach on the space
necessary for the safe passage of travel. To prevent the latter
possibility the centers are placed from 9 ins. to 15 ins. higher than
their true positions, depending upon the nature of the soil, so that
considerable settlement is possible without any danger of the necessary
cross-section being infringed upon. In conclusion it may be noted that
the lining may be constructed in a series of consecutive rings, or as a
single cylindrical mass.


=Hauling.=--Since in this method of tunneling the upper part of the
section is excavated and lined before the excavation of the lower part
is begun, the upper portion is always more advanced than the lower. To
carry away the earth excavated at the front, therefore, an elevation has
to be surmounted; and this is usually done by constructing an inclined
plane rising from the floor of the tunnel to the floor of the heading,
as shown by Fig. 72. This inclined plane has, of course, to be moved
ahead as the work advances, and to permit of this movement with as
little interruption of the other work as possible, two planes are
employed. One is erected at the right-hand side of the section, and
serves to carry the traffic while the left-hand side of the lower
section is being removed some distance ahead and the other plane is
being erected. The inclination given to these planes depends upon the
size of the loads to be hauled, but they should always have as slight a
grade as practicable. Narrow-gauge tracks are laid on these planes and
along the floor of the upper part of the section passing through the
center opening mentioned before as being left in the centers and
strutting.

In excavating the top center heading there is, of course, another rise
to its floor from the floor of the upper part of the section. Where, as
is usually the case in soft soils, this top heading is not driven very
far in advance, the earth from the front is usually conveyed to the rear
in wheelbarrows, and dumped into the cars standing on the tracks below.
In firm soils, where the heading is driven too far in advance to make
this method of conveyance adequate, tracks are also laid on the floor of
the heading, and an inclined plane is built connecting it with the
tracks on the next level below. In place of these inclined planes, and
also in place of those between the floor of the tunnel and the level
above, some form of hoisting device is sometimes employed to lift the
cars from one level to the other. There are some advantages to this
method in point of economy, but the hoisting-machines are not easily
worked in the darkness, and accidents are likely to occur.

[Illustration: FIG. 73.--Diagram Showing Sequence of Excavation in
Modified Belgian Method.]

In the advanced top heading and in the upper part of the section
narrow-gauge tracks are necessarily employed, and these may be continued
along the floor of the finished section, or the permanent broad-gauge
railway tracks may be laid as fast as the full section is completed. In
the former case the permanent tracks are not laid until the entire
tunnel is practically completed; and in the latter case, unless a third
rail is laid, the loads have to be transshipped from the broad- to the
narrow-gauge tracks or _vice versa_. It is the more general practice to
use a third rail rather than to transship every load.


=Modifications.=--Considering the extent to which the Belgian method of
tunneling has been employed, it is not surprising that many
modifications of the standard mode of procedure have been developed. The
modification which differs most from the standard form is, perhaps, that
adopted in excavating the Roosebeck tunnel in Germany. This method
preserves the principal characteristic of the Belgian method, which is
the construction of the upper part of the section first; but instead of
building the side walls from the bottom upward, they are built in small
sections from the top downward. The excavation begins by driving the
center top heading No. 1, Fig. 73, whose floor is at the level of the
springing lines of the roof arch, and then the two side parts No. 2 are
excavated, opening up the entire upper portion of the section in which
the roof arch is built, as in the regular Belgian method. The next step
is to excavate part No. 3, shoring up the arch at frequent intervals.
Between these sets of shoring the side walls are built, resting on
planks on the floor of part No. 3, and then the sets of shores are
removed and replaced by masonry. Next part No. 4 is excavated, shored,
and filled with masonry as was part No. 3. In exactly the same way parts
5, 6, 7, and 8 are constructed in the order numbered. To prevent the
distortion of the arch during the side-wall construction it is braced by
horizontal struts, as indicated above in Fig. 71.


=Advantages.=--The advantages of the Belgian method of tunneling may be
summarized as follows: (1) The excavation progresses simultaneously at
several points without the different gangs of excavators interfering
with each other, thus securing rapidity and efficiency of work; (2) the
excavation is done by driving a number of drifts or parts of small
section, which are immediately strutted, thus causing the minimum
disturbance of the surrounding material; (3) the roof of the tunnel,
which is the part of the lining exposed to the greatest pressures, is
built first.

[Illustration: FIG. 74.--Sketch Showing Failure of Roof Arch by Opening
at Crown.]


=Disadvantages.=--The disadvantages of the Belgian method of tunneling
may be summarized as follows: (1) The roof arch which rests at first on
compressible soil is liable to sink; (2) before the invert is built
there is danger of the arch and side walls being distorted or sliding
under the lateral pressures; (3) the masonry of the side walls has to be
underpinned to the arch masonry.


=Accidents and Repairs.=--One of the most frequent accidents in the
Belgian method of tunneling is the sinking of the roof arch owing to
its unstable foundation on the unexcavated soil of the lower portion of
the section. The amount of settlement may vary from a few inches in firm
soil to over 2 ft. in loose soils. To counteract the effect of this
settlement it is the general practice to build the arch some inches
higher than its normal position. When the settlement is great enough to
infringe seriously upon the tunnel section, repairs have to be made; and
the only way of accomplishing them is to demolish the arch and rebuild
it from the side walls. It is usually considered best not to demolish
the arch until the invert has been placed, so that no further
disturbance is likely to occur once the lining is completed anew.

The rotation of the arch about its keystone, or the opening of the arch
at the crown, by the squeezing inward of the haunches by the lateral
pressures, is another characteristic accident. Fig. 74 shows the nature
of the distortion produced; the segments of the arch move toward each
other by revolving on the intradosal edges of the keystone, which are
broken away and crushed together with the operation, while the
extradosal edges are opened. It is to prevent this occurrence that the
horizontal struts shown in Fig. 71 are employed. The manner of repairing
this accident differs, depending upon the extent of the injury. When the
intradosal edges of the keystone are but slightly crushed, the repairing
is done as directed by Fig. 75. When the keystone is completely crushed,
however, the indications are that the material of the keystone, usually
brick, is not strong enough to resist the pressures coming upon it, and
it is advisable to substitute a stronger material in the repairs, and a
stone keystone is constructed as shown by Fig. 75. The middle stone of
this keystone extends through the depth of the arch ring, and the two
side stones only half-way through, their purpose being merely to resist
the crushing forces which are greatest at the intrados. Sometimes, when
the pressures are unsymmetrical, the arch ring breaks at the haunches as
well as the crown, as shown by Fig. 75, which also indicates the mode of
repairing. This consists in demolishing the original arch, and
rebuilding it with stone voussoirs inserted in place of the brick in
which the rupture occurred.

[Illustration: FIG. 75.--Sketches Showing Methods of Repairing Roof Arch
Failures.]




CHAPTER XIII.

THE GERMAN METHOD--EXCAVATING TUNNELS THROUGH SOFT GROUND (Continued);
BALTIMORE BELT LINE TUNNEL.


The German method of tunneling was first used in 1803 in constructing
the St. Quentin Canal. In 1837 the Königsdorf tunnel of the Cologne and
Aix la Chapelle R.R. was excavated by the same method. The success of
the method in these two difficult pieces of soft-ground tunneling led to
its extensive adoption throughout Germany, and for this reason it
gradually came to be designated as the German method. Briefly explained
the method consists in excavating first an annular gallery in which the
side walls and roof arch are built complete before taking out the center
core and building the invert.

[Illustration: FIG. 76.--Diagrams Showing Sequence of Excavation in
German Method of Tunneling.]


=Excavation.=--The excavation of tunnels by the German method is begun
either by driving two bottom side drifts or by driving a center top
heading. Fig. 76 shows the mode of procedure when bottom side drifts are
used to start the work. The two side drifts No. 1 are made from 7 ft. to
8 ft. wide, and about one-third the total height of the full section;
the width of each heading has to be sufficient for the construction of
the masonry and strutting, and for the passage of narrow spoil cars
alongside them. These drifts are increased in height to the springing
line of the arch by taking out the two drifts No. 2. Next the top center
heading No. 3 is driven, and finally the two haunch headings No. 4 are
excavated. The center core No. 5 is utilized to support the strutting
until the side walls and roof arch are completed, when it is broken down
and removed. In case of very loose material, where the first side drifts
cannot be carried as high as one-third the height of the section, it is
the common practice to make them about one-fourth the height, and to
take out the side portions of the annular gallery in three parts, as
shown by Fig. 76.

[Illustration: FIG. 77.--Diagram Showing Sequence of Excavations in
Water Bearing Material, German Method.]

The top center heading plan of commencing the excavation is usually
employed in firm materials or when a vein of water is encountered in the
upper part of the section. In the latter contingency a small bottom
drift _A_, Fig. 77, is first driven to serve as a drain; but in any case
the excavation proper of the tunnel consists in first driving the center
top heading No. 1, and then by working both ways along the profile
parts, Nos. 2, 3, 4, and 5 are removed. Part No. 6 is left to support
the strutting until the side walls and roof arch are built, when it is
also excavated.


=Strutting.=--When the excavation is begun by bottom side drifts these
drifts are strutted by erecting vertical posts close against the sides
of the drift and placing a cap-piece transversely across the roof of the
drift. The side posts are usually supported by sills placed across the
bottom of the drift. These frameworks of posts, cap, and sill are
erected at short intervals, and the roof, and, if necessary, the sides
of the drift between them, are sustained by means of longitudinal
poling-boards extending from one frame to the next. The cap-pieces of
the strutting for the bottom drifts serve as sills for the exactly
similar strutting of the heading next above. To support the additional
weight, and to allow the construction of the side walls, the strutting
of the bottom drifts is strengthened by inserting an intermediate post
between the original side posts of each frame. These intermediate posts
are not inserted at the center of the frames or bents, but close to the
wall masonry line as shown by Fig. 78. This eccentric position of the
post avoids any interference with the hauling, and also allows the
removal of the adjacent side post when the masonry is constructed.

[Illustration: FIG. 78.--Sketch Showing Work of Excavating and Timbering
Drifts and Headings.]

[Illustration: FIG. 79.--Sketch Showing Method of Roof Strutting.]

Two methods of strutting the soffit of the excavation are employed, one
being a modification of the longitudinal system employed in the English
method of tunneling described in a succeeding chapter, and the other a
modification of the Belgian system previously described. Fig. 79 shows
the method of employing the radial strutting of the Belgian system. At
the beginning the center top heading is strutted with rectangular bents
such as are employed for strutting the drifts. As this heading is
enlarged by taking out the haunch sections, radial posts are inserted,
as shown by Fig. 79, which also indicates the method of strutting the
side trenches when the excavation is carried downward from the center
top heading instead of upward from bottom side drifts.


=Masonry.=--Whatever plan of excavation or strutting is employed, the
construction of the masonry lining in the German method of tunneling
begins at the foundations of the side walls and is carried upward to the
roof arch. The invert, if one is required, is built after the center
core of earth is removed.


=Centering.=--Tunnel centers are generally employed in the German method
of tunneling, a common construction being shown by Fig. 80. It is
essentially a queen-post truss, the tie beam of which rests on a
transverse sill as shown by the illustration. The transverse sill is
supported along its central portion by the unexcavated center core of
earth, and at its ends either directly on the vertical posts or on
longitudinal beams resting on these posts. The diagonal members of the
queen-post truss form the bottom chords of small king-post trusses which
are employed to build out the exterior member of the center to a closer
approximation to the curve of the arch.

[Illustration: FIG. 80.--Sketch Showing Roof Arch Centers and Arch
Construction.]


=Hauling.=--When the bottom side drift plan of excavation is employed,
the spoil from the front of the drift is removed in narrow-gauge cars
running on a track laid as close as practicable to the center core.
These same cars are also employed to take the spoil from the drifts
above, through holes left in the ceiling strutting of the bottom drifts.
The spoil from the soffit sections may be removed by the same car lines
used in excavating the drifts, or a narrow-gauge track may be laid on
the top of the center core for this special purpose. In the latter case
the soffit tracks are usually connected by means of inclined planes
with the tracks on the bottoms of the side drifts. Generally, however,
the separate soffit car line is not used unless the material is of such
a firm character that the headings and drifts can be carried a great
distance ahead of the masonry work. With the center top heading plan of
beginning the excavation, the car track has, of course, to be laid on
the top of the center core. The center core itself is removed by means
of car tracks along the floor of the completed tunnel.


=Advantages and Disadvantages.=--Like the Belgian method of tunneling,
the German method has its advantages and disadvantages. Since the
excavation consists at first of a narrow annular gallery only, the
equilibrium of the earth is not greatly disturbed, and the strutting
does not need to be so heavy as in methods where the opening is much
larger. The undisturbed center core also furnishes an excellent support
for the strutting, and for the centers upon which the roof arches are
built. Another important advantage of the method is that the
construction of the masonry lining is begun logically at the bottom, and
progresses upward, and a more homogeneous and stable construction is
possible. The great disadvantage of the method is the small space in
which the hauling has to be done. The spoil cars practically fill the
narrow drifts in passing to and from the front, and interfere greatly
with the work of the carpenters and masons. Another objection to the
method is that the invert is the very last portion of the lining to be
built. This may not be a serious objection in reasonably compact and
stable materials, but in very loose soils there is always the danger of
the side walls being squeezed together before the invert masonry is in
position to hold them apart. Altogether the difficulties are of a
character which tend to increase the expense of the method, and this is
the reason why to-day it is seldom used even in the country where it was
first developed, and for some time extensively employed. For repairing
accidents, such as the caving in of completed tunnels, the German method
of tunneling is frequently used, because of the ease with which the
timbering is accomplished. In such cases the cost of the method used
cuts a small figure, so long as it is safe and expeditious.


BALTIMORE BELT LINE TUNNEL.

In the last few years a modification of the German method was used in
this country for the construction of several railroad tunnels. The
modification consists in excavating the two-side drifts up to the
springing line of the arch of the proposed tunnel. Then a central
heading, which is afterward enlarged to the whole section of the tunnel,
is excavated close to the crown. At the same time the masonry is
constructed from the foundation up in the side drifts. From the floor of
the upper section already excavated and strutted, the top of the masonry
of the drifts is reached by means of small side cuts; thus the lining is
made continuous up to the keystone. The central nucleus or bench is
removed after the tunnel has been lined.

The most important tunnel excavated by this method was the Baltimore
Belt Line tunnel described as follows:

The Baltimore Belt Ry. Co. was organized in 1890 by officials of the
Baltimore & Ohio, and Western Maryland railways, and Baltimore
Capitalists, to build 7 miles of double track railway, mostly within the
city limits of Baltimore. This railway was partly open cut and
embankment, and partly tunnel, and its object was to afford the
companies named facilities for reaching the center of the city with
their passengers and freight. To carry out the work the Maryland
Construction Co. was organized by the parties interested, and in
September, 1890, this company let the contract for construction to Ryan
& McDonald of Baltimore, Md. The chief difficulties of the work centered
in the construction of the Howard-street tunnel, 8350 ft. long, running
underneath the principal business section of the city.


=Material Penetrated.=--The soil penetrated by the tunnel was of almost
all kinds and consistencies, but was chiefly sand of varying degrees of
fineness penetrated by seams of loam, clay, and gravel. Some of the
clay was so hard and tough that it could not be removed except by
blasting. Rock was also found in a few places. For the most part,
however, the work was through soft ground, furnishing more or less
water, which necessitated unusual precautions to avoid the settling of
the street, and consequent damage to the buildings along the line. A
large quantity of water was encountered. Generally this water could be
removed by drainage and pumps, and the earth be prevented from washing
in by packing the space between the timbering with hay or other
materials. At points where the inflow was greatest, and the earth was
washed in despite the hay packing, the method was adopted of driving
6-in. perforated pipes into the sides of the excavation, and forcing
cement grout through them into the soil to solidify it. These pipes
penetrated the ground about 10 ft., and the method proved very efficient
in preventing the inflow of water.


=Excavation.=--The excavation was carried out according to the German
method of tunneling. Bottom side drifts were first driven, and then
heightened to the springing line of the roof arch. Next a center top
heading was driven, and the haunch sections taken out. The object of
beginning the excavations by bottom side drifts, was to drain the soil
of the upper part of the section. The center core was removed after the
side walls and roof arch were completed, its removal being kept from 50
ft. to 75 ft. to the rear of the advanced heading. The dimensions of the
side drifts proper were about 8 × 8 ft., but they were often carried
down much below the floor level to secure a solid foundation bed for the
side walls.


=Strutting.=--The side drifts were strutted by means of frames composed
of two batter posts resting on boards, and having a cap-piece extending
transversely across the roof of the drift. These frames were spaced
about 4 ft. apart. The excavation was advanced in the usual way by
driving poling-boards at the top and sides, with a slight outward and
upward inclination, so that the next frame could be easily inserted
leaving space enough between it and the sheeting to permit the next set
of poling-boards to be inserted. These poling-boards were driven as
close together as practicable so as to prevent as much as possible the
inflow of water and earth.

[Illustration: FIG. 81.--Sketch Showing Method of Excavating and
Strutting Baltimore Belt Line Tunnel.]

The center top heading was strutted in the same manner as were the side
drifts. The arrangement of the strutting employed in enlarging the
center top heading is shown clearly by Fig. 81, which also shows the
manner of strutting the side drifts and face of the excavation, and of
building the masonry.


=Centers.=--Both wood and iron centers were employed in building the
roof arch. The timber centering was constructed of square timbers, as
shown by Fig. 82. This construction of the iron centers is shown by Fig.
83. Each of the iron centers consisted of two 6 × 6 in. angles butted
together, and bent into the form of an arch rib. Six of these ribs were
set up 4 ft. apart. They were made of two half ribs butted together at
the crown, and were held erect and the proper distance apart by spacing
rods. The rearmost rib was held fast to the completed arch masonry, and
in turn supported the forward ribs while the lagging was being placed.

[Illustration: FIG. 82.--Roof Arch Construction with Timber Centers,
Baltimore Belt Line Tunnel.]


=Masonry.=--The side walls of the lining were built first in the bottom
side drifts, as shown by Fig. 81. They were generally placed on a
foundation of concrete, from 1 ft. to 2 ft. thick. As a rule the side
walls were not built more than 20 ft. in advance of the arch, but
occasionally this distance was increased to as much as 90 ft. The roof
arch consisted ordinarily of five rings of brick, but at some places in
especially unstable soil eight rings of brick were employed. The arch
was built in concentric sections about 18 ft. in length. All the timber
of the strutting above the arch and outside of the side walls was left
in place, and the voids were filled with rubble masonry laid in cement
mortar. It required about 125 mason hours to build an 18-ft. arch
section. Figs. 82 and 83 show various details of the masonry arch work.

[Illustration: FIG. 83.--Roof Arch Construction with Iron Centers,
Baltimore Belt Line Tunnel.]

Owing to the very unstable character of the soil, considerable
difficulty was experienced in building the masonry invert. The process
adopted was as follows: Two parallel 12 × 12 in. timbers were first
placed transversely across the tunnel, abutting against longitudinal
timbers or wedges resting against the side walls. Short sheet piles were
then driven into the tunnel bottom outside of these timbers, forming an
inclosure similar to a cofferdam, from which the earth could be
excavated without disturbing the surrounding ground. The earth being
excavated, a layer of concrete 8 ins. thick was placed, and the brick
masonry invert constructed on it. In less stable ground each of the
above described cofferdams was subdivided by transverse timbers and
sheet piling into three smaller cofferdams. Here the masonry of the
middle section was first constructed, and then the side sections built.
Where the ground was worst, still more care was necessary, and the
bottom had to be covered with a sheeting of 1¹⁄₄-in. plank held down by
struts abutting against the large transverse timbers. The invert masonry
was constructed on this sheeting. Refuge niches 9 ft. high, 3 ft. wide,
and 15 ins. deep were built in the side walls.


=Accidents.=--In this tunnel, owing to the quick striking of the
centers, it was found that the masonry lining flattened at the crown and
bulged at the sides. This was attributed to the insufficient time
allowed for the mortar to set in the rubble filling. Earth packing was
tried, but gave still worse results. Finally dry rubble filling was
adopted, with satisfactory results. There was necessarily some sinking
of the surface. This resulted partly from the necessity of changing and
removing of the timbers, and from the compression and springing of the
timbers under the great pressures. The crown of the arch also settled
from 2 ins. to 6 ins., due to the compression of the mortar in the
joints. The maximum sinking of the surface of the street over the tunnel
was about 18 ins.; it usually ran from 1 to 12 ins. Some damage was done
to the water and gas mains. This damage was not usually serious, but it
of course necessitated immediate repairs, and in some instances it was
found best to reconstruct the mains for some distance. At one point
along the tunnel where very treacherous material was found, the surface
settlement caused the collapse of an adjacent building, and necessitated
its reconstruction.




CHAPTER XIV.

THE FULL SECTION METHOD OF TUNNELING: ENGLISH METHOD; AMERICAN METHOD;
AUSTRIAN METHOD.


ENGLISH METHOD.

The English method of tunneling through soft ground, as its name
implies, originated in England, where, owing to the general prevalence
of comparatively firm chalks, clays, shales, and sandstones, it has
gained unusual popularity. The distinctive characteristics of the method
are the excavation of the full section of the tunnel at once, the use of
longitudinal strutting, and the alternate execution of the masonry work
and excavation. In America the method is generally designated as the
longitudinal bar method, owing to the mode of strutting, which has
gained particular favor in America, and is commonly employed here even
when the mode of excavation is distinctively German or Belgian in other
respects.

[Illustration: FIG. 84.--Diagram Showing Sequence of Excavation in
English Method of Tunneling.]


=Excavation.=--Although, as stated above, the distinctive characteristic
of the English method is the excavation of the full section at once, the
digging is usually started by driving a small heading or drift to locate
and establish the axis of the tunnel, and to facilitate drainage in wet
ground. These advance galleries may be driven either in the upper or in
the lower part of the section, as the local conditions and choice of the
engineer dictate. Whether the advance gallery is located at the top or
at the bottom of the section makes no difference in the mode of
enlarging the profile. This work always begins at the upper part of the
section. A center top heading is driven and strutted by erecting posts
carrying longitudinal bars supporting transverse poling-boards. This
heading is immediately widened by digging away the earth at each side,
and by strutting the opening by temporary posts resting on blocking, and
carrying longitudinal bars supporting poling-boards. This process of
widening is continued in this manner until the full roof section, No. 1,
Fig. 84, is opened, when a heavy transverse sill is laid, and permanent
struts are erected from it to the longitudinal bars, the temporary posts
and blocking being removed. The excavation of part No. 2 then begins by
opening a center trench and widening it on each side, temporary posts
being erected to support the sill above. As soon as part No. 2 is fully
excavated, a second transverse sill is placed below the first, and
struts are placed between them. The excavation of part No. 3 is carried
out in exactly the same manner as was part No. 2. The lengths of the
various sections, Nos. 1, 2, and 3, generally run from 12 ft. to 20 ft.,
depending upon the character of the soil.


=Strutting.=--The strutting in the English method of tunneling consists
of a transverse framework set close to the face of the excavation, which
supports one end of the longitudinal crown bars, the other ends of which
rest on the completed lining. The transverse framework is composed of
three horizontal sills arranged and supported as shown by Fig. 85. The
bottom sill _A_ is carried by vertical posts resting on blocking on the
floor of the excavation. From the bottom sill vertical struts rise to
support the middle sill _B_. The top sill, or miners’ sill _C_, is
carried by vertical posts or struts rising from the middle sill _B_. The
vertical struts are usually round timbers from 6 ins. to 8 ins. in
diameter; and the sills are square timbers of sufficient section to
carry the vertical loads, and generally made up of two posts
scarf-jointed and butted to permit them to be more easily handled. In
firm soils the struts between the sills are all set vertically, but
those at the extreme sides of the roof section are inclined. In loose
soils, however, where the sides of the excavation must be shored, the
V-bracing shown by Fig. 85 is employed between one or more pairs of
sills as the conditions necessitate. The manner of holding the
transverse framework upright is explained quite clearly by Fig. 85;
inclined props extending from the completed masonry to the sills of the
framework being employed. Two props are used to each sill. Sometimes, in
addition to the props shown, another nearly horizontal prop extends from
the crown of the arch masonry to the middle piece of the strutting.

[Illustration: FIG. 85.--Sketches Showing Construction of Strutting,
English Method.]

Referring to Fig. 85, it will be observed that the longitudinal crown
bars are above the extrados of the roof arch. When, therefore, the
lining masonry has been completed close up to the transverse framework,
the latter is removed, leaving the crown bars resting on the arch
masonry; and excavation, which has been stopped while the masonry was
being laid, is continued for another 12 ft. to 20 ft., and the
transverse framework is erected at the face, and braced or propped
against the completed lining as shown by Fig. 85. The next step is to
place the crown bars, and this is done by pulling them ahead from their
original position over the masonry of the completed section of the roof
arch. It will be understood that the crown bars are not pulled ahead
their full length at one operation, but are advanced by successive short
movements as the excavation progresses, their outer ends being supported
by temporary posts until the transverse framework is built at the face
of the excavation.


=Centers.=--Two standard forms of centers are employed in the English
method of tunneling, as shown by Figs. 86 and 87. Both consist of an
outer portion, constructed much like a typical plank center, which is
strengthened against distortion by an interior truss framework. The
elemental members of this truss framework take the form of a queen-post
truss, as is shown more particularly by Fig. 86. In Fig. 87 the
queen-post truss construction is less easily distinguished, owing to the
cutting of the bottom tie-beam and other modifications, but it can still
be observed. The possibility of cutting the tie-beam as shown in Fig.
87, without danger, is due to the fact that the lateral pressures on the
haunches of the center counteract the tendency of the center to flatten
under load, which is usually counteracted by the tie-beam alone. The
object of cutting the tie-beam is to afford room for the props running
from the completed masonry to the transverse framework of the strutting
as shown by Fig. 85.

[Illustration: FIGS. 86 and 87.--Sketches of Typical Timber Roof-Arch
Centers, English Method.]

Generally four or five centers are used for each length of arch built.
They are set up so that the tie-beams rest on double opposite wedges
carried by a transverse beam below. This transverse beam in turn rests
on another transverse beam which is supported by posts carried on
blocking on the invert masonry. It is usually made with a butted joint
at the middle to permit its removal, since it is so long that the
masonry has to be built around its extreme ends. The lagging is of the
usual form, and rests on the exterior edges of the curved upper member
of the centers.


=Masonry.=--In the English method of tunneling, the masonry begins with
the construction of the invert, and proceeds to the crown of the arch.
The lining is built in lengths, or successive rings, corresponding to
the length of excavation, which, as previously stated, is from 12 ft. to
20 ft. Each ring or length of lining terminates close to the transverse
strutting frame erected at the face of the excavation. Work is first
begun on the invert at the point where the preceding ring of masonry
ends, and is continued to the transverse strutting frame at the front of
the excavation. As fast as the invert is completed, work is begun on the
side walls. In very loose soils the longitudinal bars supporting the
sides of the excavation are removed after the side walls are built; but
in firmer soils they may be taken out one by one just ahead of the
masonry, or in very firm soils it may be possible to remove them
entirely before beginning the side walls. In all cases it is necessary
to fill the space between the masonry and the walls of the excavation
with riprap or earth. To build the roof arch the centers are first
erected as described above, and the crown bars are removed as previously
described by pulling them ahead after the arch ring is completed. As
with the side walls, the vacant space between the arch ring and the roof
of the excavation must be filled in. Usually earth or small stones are
used for filling; but in very loose soils it is sometimes the practice
not to remove the poling-boards, but to support them by short brick
pillars resting on the arch ring and then to fill around these pillars.


=Hauling.=--To haul away the material and take in supplies, tracks are
laid on the invert masonry. Generally the permanent tracks are laid as
fast as the lining is completed. A short section of temporary track is
used to extend this permanent track close to the work of the advanced
drift.


=Advantages and Disadvantages.=--The great advantage of the English
method of tunneling is that the masonry lining is built in one piece
from the foundations to the crown, making possible a strong, homogeneous
construction. It also possesses a decided advantage because of the
simple methods of hauling which are possible: there being no differences
of level to surmount, no hoisting of cars nor trans-shipments of loads
are necessary. The chief disadvantage of the method is that the
excavators and masons work alternately, thus making the progress of the
work slower perhaps than in any other method of tunneling commonly
employed under similar conditions. This disadvantage is overcome to a
considerable extent when the tunnel is excavated by shafts, and the work
at the different headings is so arranged that the masons or excavators
when freed from duty at one heading may be transferred to another where
excavation or lining is to be done as the case may be. Another
disadvantage of the English method arises from the excavation of the
full section at once, which in unstable soils necessitates strong and
careful strutting, and increases the danger of caving. The fact also
that the arch ring has to carry the weight of the crown bars, and their
loading at one end while the masonry is green, increases the chances of
the arch being distorted.


=Conclusion.=--The English method of tunneling in its entirety is
confined in actual practice pretty closely to the country from which it
receives its name. A possible extension of its use more generally is
considered by many as likely to follow the development of a successful
excavating machine for soft material. The space afforded by the opening
of the full section at once, especially adapts the method to the use of
excavators like, for example, the endless chain bucket excavator used
on the Central London Ry., and illustrated in Fig. 11. The method also
furnishes an excellent opportunity for electric hauling and lighting
during construction.

The English method of tunneling has been used in building the Hoosac,
Musconetcong, Allegheny, Baltimore and Potomac, and other tunnels in
America. The names of the European tunnels built by this method are too
numerous to mention here.


AMERICAN METHOD.

In this country tunnels through loose soils are excavated according to
the “Crown Bar” or American Method. This consists in opening the whole
section of the tunnel before the construction of the lining as in the
English Method. It differs from the English method, however, in that
many timber structures are erected for the support of the roof, and that
the excavation and construction of the lining are far apart, so allowing
the miners and the masons to work continuously and without interfering
with each other.

[Illustration: FIG. 88.--Sequence of Excavation in the American Method.]

[Illustration: ~Section A-B.~

FIG. 89.--Strutting the Heading in the American Method.]

[Illustration: ~Section C-D.~

FIG. 90.--Temporary Timbering of the Roof in the American Method.]

[Illustration: ~Section E-F.~

FIG. 91.--Showing Crown Bars Supported by Segmental Arches.]


=Excavation.=--The diagram in Fig. 88 shows the sequence of excavation.
The work begins by driving a central heading usually 7 × 8 ft., strutted
by means of vertical or batter posts and cap-piece. Fig. 89,[11] the
props resting on foot blocks. Between the cap-pieces of the consecutive
frames are placed planks driven upward at a slightly inclined angle.
After the heading has been excavated and strutted, the floor is lowered
by removing the part marked 2 in the figure. The two batter posts
supporting the cap-piece are now substituted by two longer ones resting
on the floor of part 2 and abutting against longitudinal beams which
are inserted underneath the cap-pieces. These longitudinal beams are
called crown bars. The new batter posts are resting either on foot
blocks or sills according to the quality of soil and they are strongly
wedged to the crown bars. On each side of these crown bars are inserted
poling-boards or planks close to each other, which are driven downward.
The part marked 3 in the figure is removed by enlarging the cut 1 × 2 on
both sides. The plank, inserted above the crown bar, is driven in either
preceding or following the excavation and another crown bar is inserted
at the end of this plank. This second crown bar is supported by a prop
whose other end abuts against the foot of the rafter strutting the
heading. Between this crown bar and the roof of the excavation, other
planks are placed transversally to the axis of the tunnel and are driven
in until they are supported by a new crown bar, etc. The various props
supporting the crown bars are placed radially or in a fan-like manner,
similar to the characteristic arrangement of the timbering in the
Belgian method. Bracers to strengthen the timbering and the roof of the
excavation are inserted longitudinally between the various posts and
transversally between the crown bars, Fig. 90. As a rule, only three or
four of these radial structures are temporarily erected. A trench is
excavated at the side of the part marked 3 in the figure to receive the
wall plate which is a heavy timber laid on the floor parallel to the
longitudinal axis of the tunnel. On the wall plates are erected the
arched timber sets composed of five or seven segments of hewn timbers so
as to form a polygonal frame which is wedged to the crown bars and
which will support the arch of the roof. After one of these segmental
timber sets is erected the temporary radial structure is removed and the
upper section of the tunnel is cleared of any obstruction as the
pressures are transferred to the wall plates, Fig. 91. The bench marked
4 in the figure is taken away and the vertical props inserted under the
wall plates, Fig. 92.

  [11] Figs. 89 to 91 are taken from a paper by S. W. Hopkins in
  _Harvard Engineering Journal_, April, ’03, on the Fort George tunnel.

[Illustration: ~Section G-H.~

~Longitudinal Section.~

FIG. 92.--Transversal and Longitudinal Section of a Tunnel Excavated and
Strutted According to the American Method.]


=Strutting.=--The longitudinal strutting is used in connection with the
American method of tunneling. In fact, the strutting consists of a
series of longitudinal bars supporting planks laid transversally to the
axis of the tunnel and abutting against the roof of the excavation.
These crown bars during the excavations and immediately after are
temporarily supported by radial timbers forming almost a fan-like
structure, but this is soon substituted by a permanent one composed of a
polygonal timber frame of five or seven segments which are cut to
dimensions. The batter posts of the heading, the radial posts of the
temporary timber structure and the crown bars are all round timbers from
10 to 12 ins. in diameter. All the other timbers are square edged, the
usual dimensions being 10 × 10 ins. or 12 × 12 ins. with the exception
of the wall plates which are 14 × 14 ins. The dimensions of the various
members of the strutting and the distance apart of the different frames
vary with the quality of the soil. For instance, in ordinary loose
soils the frames are placed between 4 to 6 ft., but in very soft soils
they are erected only 3 or 3¹⁄₂ ft. apart.

Chiefly in the southwest, in tunnels excavated according to the American
method, the timbering has been left as regular lining and it was only
after many years when this temporary structure had decayed or was burned
down, that the tunnels were lined with masonry. But in many instances
the whole timber structure was left in place even when the tunnel was
lined with masonry immediately after the excavation had been made. This
was usually done when the tunnel was lined with concrete masonry. In
such a case the timbering was left to support the pressures of the roof
while the concrete was plastic and before it hardened.


=Centers.=--In the American method the whole section of the tunnel is
open before the construction of the lining, thus the masonry can be
built from the foundations up. The centers are designed so as to support
only the weight of the masonry during its construction and not the
pressures of the tunnel as in the other methods and consequently they
are of light construction. The centers described in the Murray Hill
tunnel, page 123, may be advantageously used in building the concrete
lining in tunnels through loose soils excavated by the American method.


=Hauling.=--The excavation of the heading and the upper section of the
tunnel is usually far ahead of the bench, consequently the hauling of
both the débris and the building materials is made at two different
levels, viz., on the bench and on the floor of the tunnel. When the face
of the heading and the excavation of the bench are not more than 50 ft.
apart, the hauling can be conveniently done on the tunnel floor, while
the materials and débris on the upper section of the tunnel are hauled
by wheelbarrows or light cars propelled by handpower. For a greater
distance, however, it is more convenient to use light cars running on
narrow-gauge tracks all through the tunnel. In this case the tracks on
the tunnel floor and on top of the bench are connected by means of an
inclined platform where the cars may ascend and descend without
interfering with the excavation of the bench. Here, as a rule, tunnels
have been excavated in soils considered good, generally through rock,
while loose soils have been encountered only in small sections. The same
method of excavation for whatever material is encountered is certainly
very convenient, as it affords a great regularity in the work; hence its
extensive use. A great disadvantage of this method is the double
strutting, viz., the polygonal and the longitudinal strutting succeeding
each other, whereas one of them could be easily spared. Another defect
is that it requires a larger amount of excavation, in case the strutting
is left in place.


AUSTRIAN METHOD.

The Austrian full-section method of tunneling through soft ground was
first used in constructing the Oberau tunnel on the Leipsic and Dresden
R.R., in Austria in 1837. It consists in excavating the full section and
building up the lining masonry from the foundations as in the English,
but with the important exception that the invert is built last instead
of first in all cases except where the presence of very loose soil
requires its construction first. A still more important difference in
the two methods is that the excavation is carried out in smaller
sections and is continuous in the Austrian method instead of alternating
with the mason work as it does in the English method.


=Excavation.=--The excavation in the Austrian method begins by driving
the bottom center drift No. 1, Fig. 93, rising from the floor of the
tunnel section nearly to the height of the springing lines of the roof
arch. When this drift has been driven ahead a distance varying from 12
ft. to 20 ft. or sometimes more, the excavation of the center top
heading No. 2 is driven for the same distance. The next operation is to
remove part No. 3, thus forming a central passage the full depth of the
tunnel section at the center. This trench is enlarged by removing parts
Nos. 4, 5, 6, 7, and 8 in the order named until the full section is
opened. A modification of this plan of excavation is shown by Fig. 94
which is used in firm soils.

[Illustration: FIGS. 93 and 94.--Diagrams Showing Sequence of Excavation
in Austrian Method of Tunneling.]


=Strutting.=--Each part of the section is strutted as fast as it is
excavated. The center bottom drift first excavated is strutted by laying
a transverse sill across the floor, raising two side posts from it, and
capping them with a transverse timber having its ends projecting beyond
the side posts and halved as shown by Fig. 95. The top center heading
No. 2, which is next excavated, is strutted by means of two side posts
resting on blocking and carrying a transverse cap as also shown by Fig.
95. Sometimes the side posts in the heading strutting-frames are also
carried on a transverse sill as are those of the bottom drift. This
construction is usually adopted in loose soils. When the sill is
employed, the middle part, No. 3, is strutted by inserting side posts
between the bottom of the top sill and the cap of the frame in the drift
below. When, however, the posts of the top heading frame are carried on
blocking, it is the practice to replace them with long posts rising from
the cap of the bottom drift frame to the cap of the top heading frame.
Further, when the intermediate sill is employed at the bottom level of
the top heading it projects beyond the side posts and has its ends
halved.

[Illustration: FIGS. 95 to 97.--Sketches Showing Construction of
Strutting, Austrian Method.]

After the completion of the center trench strutting the next task is to
strut parts Nos. 4 and 5. This is done by continuing the upper sill by
means of a timber having one end halved to join with the projecting end
of the sill in position. This extension timber is shown at _a_, Fig. 96.
The next operation is to place the timber _b_, having one end resting on
the cap-piece of the top heading frame and the other beveled and resting
on the top of the sill _a_ near the end. The timber _b_ is laid tangent
to the curve of the roof arch, and to support it against flexure the
strut _c_ is inserted as shown. To support the thrust of this strut the
additional post _d_ is inserted and the original bottom heading frame is
reinforced as shown. The next step is to insert the strut _e_, and when
this and the previous construction are duplicated on the opposite side
of the tunnel section we have the strutting of the parts Nos. 1 to 5;
inclusive, complete. Part No. 6 is then removed and strutted by
extending the bottom drift cap-piece by a timber similar to timber _a_
above, and then by inserting a side strut between the outer ends of
these two timbers, as indicated by Fig. 97. As the final parts. Nos. 7
and 8, are removed, the inclined prop _a_, Fig. 97, is inserted as
shown. When the soil is loose some of the members of the framework are
doubled and additional bracing is introduced as shown by Fig. 97.

The frames just described are placed at intervals of about 4 ft. along
the excavation, and are braced apart by horizontal struts. Some of the
longitudinal bearing beams, as at _b_, Fig. 97, also extend through two
or three frames, and help to tie them together. Finally, the
longitudinal poling-boards extending from one frame to the next along
the walls of the excavation serve to connect them together. The short
transverse beam _c_, Fig. 90, located just above the floor of the
invert, serves to carry the planking upon which the train car tracks are
laid. Besides the timber strutting peculiar to the Austrian method, the
Rziha iron strutting described in a previous chapter is frequently used
in tunneling by the Austrian process.

[Illustration: FIG. 98.--Sketch Showing Manner of Constructing the
Lining Masonry, Austrian Method.]


=Centers.=--The two forms of centers used in the English method of
tunneling are also used in the Austrian method. One of the methods of
supporting these centers is shown by Fig. 98. The tie-beam of the center
rests on longitudinal timbers carried by the strutting frames and
intermediate props. In single-track tunnels it is the frequent practice
also to carry the ends of the tie-beams in recesses left in the side
wall masonry, with intermediate props inserted to prevent flexure at
the center. When the Rziha iron strutting is employed, it also serves
for the centering upon which the arch masonry is built.


=Masonry.=--In the Austrian system of tunneling, the lining is built
from the foundations of the side walls upward to the crown of the roof
arch in lengths in consecutive rings equal to the lengths of the
consecutive openings of the full section, or from 12 ft. to 20 ft. long.
Except in infrequent cases in very loose materials the invert is the
last part of the masonry to be built, since to build it first requires
the removal of the strutting which cannot easily or safely be
accomplished until the side walls and roof arch are completed. As the
side wall foundations are built, however, their interior faces are left
inclined, as shown by Figs. 97 and 98, ready for the insertion of the
invert, and are meanwhile kept from sliding inward by the insertion of
blocking between them and the bottom of the strutting. Fig. 98 shows the
nature of this blocking, and also the manner in which the side wall and
roof arch masonry is carried upward. Finally when the roof arch is keyed
and the centers are struck, the strutting is taken down and the invert
is built.


=Advantages and Disadvantages.=--The principal advantages claimed for
the Austrian method of tunneling are: (1) The excavation being conducted
by driving a large number of consecutive small galleries, which are
immediately strutted, there is little disturbance of the surrounding
material; (2) the polygonal type of strutting adopted is easily erected
and of great strength against symmetrical pressures; (3) the masonry,
being built from the foundations up, is a single homogeneous structure,
and is thus better able to withstand dangerous pressures; (4) the
excavation is so conducted that the masons and excavators do not
interfere, and both can work at the same time. The disadvantages which
the method possesses are: (1) The strutting while very strong under
symmetrical pressures, either vertical or lateral, is distorted easily
by unsymmetrical vertical or lateral pressures, and by pressure in the
direction of the axis of the tunnel; (2) the construction of the invert
last exposes the side walls to the danger of being squeezed together,
causing a rotation of the arch of the nature discussed in describing the
Belgian method of tunneling.




CHAPTER XV.

SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD; QUICKSAND TUNNELING;
PILOT METHOD.


ITALIAN METHOD.

The Italian method of tunneling was first employed in constructing the
Cristina tunnel on the Foggia & Benevento R.R. in Italy. This tunnel
penetrated a laminated clay of the most treacherous character, and after
various other soft-ground methods of tunneling had been tried and had
failed, Mr. Procke, the engineer, devised and used successfully the
method which is now known as the Italian or Cristina method. The Italian
method is essentially a treacherous soil method. It consists in
excavating the bottom half of the section by means of several successive
drifts, and building the invert and side walls; the space is then
refilled and the upper half of the section is excavated, and the
remainder of the side walls and the roof arch are built; finally, the
earth filling in the lower half of the section is re-excavated and the
tunnel completed. The method is an expensive one, but it has proved
remarkably successful in treacherous soils such as those of the Apennine
Mountains, in which some of the most notable Italian tunnels are
located. It is, moreover, a single-track tunnel method, since any soil
which is so treacherous as to warrant its use is too treacherous to
permit an opening to be excavated of sufficient size for a double-track
railway, except by the use of shields.


=Excavation.=--The plan of excavation in the Italian method is shown by
the diagram Fig. 99. Work is begun by driving the center bottom heading
No. 1, and this is widened by taking out parts No. 2. Finally part No. 3
is removed, and the lower half of the section is open. As soon as the
invert and side wall masonry has been built in this excavation, parts
No. 2 are filled in again with earth. The excavation of the center top
heading No. 4 is then begun, and is enlarged by removing the earth of
part No. 5. The faces of this last part are inclined so as to reduce
their tendency to slide, and to permit of a greater number of radial
struts to be placed. Next, parts No. 6 are excavated, and when this is
done the entire section, except for the thin strip No. 7, has been
opened. At the ends of part No. 7 narrow trenches are sunk to reach the
tops of the side walls already constructed in the lower half of the
section. The masonry is then completed for the upper half of the
section, and part No. 7 and the filling in parts No. 2 are removed. The
various drifts and headings and the parts excavated to enlarge them are
seldom excavated more than from 6 ft. to 10 ft. ahead of the lining.

[Illustration: FIG. 99.--Diagram Showing Sequence of Excavation in
Italian Method of Tunneling.]

[Illustration: FIG. 100.--Sketch Showing Strutting for Lower Part of
Section.]


=Strutting.=--The bottom center drift, which is first driven, is
strutted by means of frames consisting of side posts resting on floor
blocks and carrying a cap-piece. Poling-boards are placed around the
walls, stretching from one frame to the next. As soon as the invert is
sufficiently completed to permit it, the side posts of the strutting
frames are replaced by short struts resting on the invert masonry as
shown by Fig. 100. To permit the old side posts to be removed and the
new shorter ones to be inserted, the cap-piece of the frame is
temporarily supported by inclined props arranged as shown by Fig. 103.
When parts No. 2 are excavated the roof is strutted by inserting the
transverse caps _a_, Fig. 100, the outer ends of which are carried by
the system of struts _b_, _c_, _d_, and _e_. The longitudinal
poling-boards supporting the ceiling and walls are held in place by the
cap _a_ and the side timber _e_. To stiffen the frames longitudinally of
the tunnel, horizontal longitudinal struts are inserted between them.

The excavation of the upper half of the tunnel section is strutted as in
the Belgian method, with radial struts carrying longitudinal roof bars
and transverse poling-boards. On account of the enormous pressures
developed by the treacherous soils in which only is the Italian method
employed, the radial strutting frames and crown bars must be of great
strength, while the successive frames must be placed at frequent
intervals, usually not more than 3 ft. After the masonry side walls have
been built in the lower part of the excavation, longitudinal planks are
laid against the side posts of the center bottom drift frames, to form
an enclosure for the filling-in of parts No. 2. The object of this
filling is principally to prevent the squeezing-in of the side walls.

[Illustration: FIGS. 101 and 101A.--Sketches Showing Construction of
Centers, Italian Method.]


=Centers.=--Owing to the great pressures to be resisted in the
treacherous soils in which the Italian method is used, the construction
of the centers has to be very strong and rigid. Figs. 101 and 101A show
two common types of center construction used with this method. The
construction shown in Fig. 101 is a strong one where only pressures
normal to the axis of the tunnel have to be withstood, but it is likely
to twist under pressures parallel to the axis of the tunnel. In the
construction shown by Fig. 101A, special provision is made to resist
pressures normal to the plane of the center or twisting pressures, by
the strength of the transverse bracing extending horizontally across the
center.

[Illustration: FIG. 102.--Sketch Showing Invert and Foundation Masonry,
Italian Method.]


=Masonry.=--The construction of the masonry lining begins with the
invert, as indicated by Fig. 100, and is carried up to the roof of parts
No. 2, as already indicated, and is then discontinued until the upper
parts Nos. 4, 5, and 6 are excavated. The next step is to sink side
trenches at the ends of part No. 7, which reach to the top of the
completed side walls. This operation leaves the way clear to finish the
side walls and to construct the roof arch in the ordinary manner of such
work in tunneling. Since this method of tunneling is used only in very
soft ground which yields under load, the usual practice is to construct
the invert and side walls on a continuous foundation course of concrete
as indicated by Fig. 102. The lining is usually built in successive
rings, and the usual precautions are taken with respect to filling in
the voids behind the lining. The thickness of the lining is based upon
the figures for laminated clay of the third variety given in Table II.


=Hauling.=--The system of hauling adopted with this method of tunneling
is very simple, since the excavation of the various parts is driven only
from 6 ft. to 10 ft. ahead, and the work progresses slowly to allow for
the construction of the heavy strutting required. To take away the
material from the center bottom drift, narrow-gauge tracks carried by
cross-beams between the side posts above the floor line are employed.
This same narrow-gauge line is employed to take away a portion of parts
No. 2, the remaining portion being left and used for the refilling after
the bottom portion of the lining has been built, as previously
described. The upper half of the section being excavated, as in the
Belgian method, the system of hauling with inclined planes to the tunnel
floor below, which is a characteristic of that method, may be employed.
It is the more usual practice, however, since the excavation is carried
so little a distance ahead and progresses so slowly, to handle the spoil
from the upper part of the section by wheelbarrows which dump it into
the cars running on the tunnel floor below. Hand labor is also used to
raise the construction materials used in building the upper section. The
tracks on the tunnel floor, besides extending to the front of the
advanced bottom center drift, have right and left switches to be
employed in removing the refilling in parts No. 2, the spoil from the
upper part of the section, and the material of part No. 7. Fig. 103 is a
longitudinal section showing the plan of excavation and strutting
adopted with the Italian method.

[Illustration: FIG. 103.--Sketch Showing Longitudinal Section of a
Tunnel under Construction, Italian Method.]


=Modifications.=--It often happens that the filling placed between the
side walls and the planking, which is practically the space comprised by
parts No. 2, is not sufficient to resist the inward pressure of the
walls, and they tip inward. In these cases a common expedient is to
substitute for the earth filling a temporary masonry arch sprung
between the side walls with its feet near the bottom of the walls, and
its crown just below the level of their tops, as shown by Fig. 107. This
construction was employed in the Stazza tunnel in Italy. In this tunnel
the excavation was begun by driving the center drift, No. 1, Fig. 104,
and immediately strutting it as shown by Fig. 105. The other parts, Nos.
2 and 3, completing the lower portion of the section, were then taken
out and strutted. While part No. 2 was being excavated at the bottom,
and the center part of the invert built, the longitudinal crown bars
carrying the roof of the excavation were carried temporarily by the
inclined props shown by Fig. 106. After completing the invert and the
side walls to a height of 2 or 3 ft., a thick masonry arch was sprung
between the side walls, as shown in transverse section by Fig. 107, and
in longitudinal section by Fig. 106. This arch braced the side walls
against tipping inward, and carried short struts to support the crown
bars. The haunches of the arch were also filled in with rammed earth.
The upper half of the section was excavated, strutted, and lined as in
the standard Italian method previously described. When the lining was
completed, the arch inserted between the side walls was broken down and
removed.

[Illustration: FIG. 104.--Sketch Showing Sequence of Excavation, Stazza
Tunnel.]

[Illustration: FIG. 105.--Sketch Showing Method of Strutting First
Drift, Stazza Tunnel.]

[Illustration: FIGS. 106 and 107.--Sketches Showing Temporary Strutting
Arch Construction, Stazza Tunnel.]


=Advantages and Disadvantages.=--The great advantage claimed for the
Italian method of tunneling is that it is built in two separate parts,
each of which is separately excavated, strutted, and lined, and thus can
be employed successfully in very treacherous soils. Its chief
disadvantage is its excessive cost, which limits its use to tunnels
through treacherous soils where other methods of timbering cannot be
used.


QUICKSAND TUNNELING.

When an underground stream of water passes with force through a bed of
sand it produces the phenomenon known as quicksand. This phenomenon is
due to the fineness of the particles of sand and to the force of the
water, and its activity is directly proportional to them. When sand is
confined it furnishes a good foundation bed, since it is practically
incompressible. To work successfully in quicksand, therefore, it is
necessary to drain it and to confine the particles of sand so that they
cannot flow away with the water. This observation suggests the mode of
procedure adopted in excavating tunnels through quicksand, which is to
drain the tunnel section by opening a gallery at its bottom to collect
and carry away the water, and to prevent the movement or flowing of the
sand by strutting the sides of the excavation with a tight planking.

The sand having to be drained and confined as described, the ordinary
methods of soft-ground tunneling must be employed, with the following
modifications:

(1) The first work to be performed is to open a bottom gallery to drain
the tunnel. This gallery should be lined with boards laid close and
braced sufficiently by interior frames to prevent distortion of the
lining. The interstices or seams between the lining boards should be
packed with straw so as to permit the percolation of water and yet
prevent the movement of the sand.

(2) As fast as the excavation progresses its walls should be strutted
by planks laid close, and held in position by interior framework; the
seams between the plank should be packed with straw.

(3) The masonry lining should be built in successive rings, and the work
so arranged that the water seeping in at the sides and roof is collected
and removed from the tunnel immediately.


=Excavation.=--The best and most commonly employed method of driving
tunnels through quicksand is a modification of the Belgian method. At
first sight it may appear a hazardous work to support the roof arch, as
is the characteristic of this method, on the unexcavated soil below,
when this soil is quicksand, but if the sand is well confined and
drained the risk is really not very great. Next to the Belgian method
the German method is perhaps the best for tunneling quicksand. In these
comparisons the shield system of tunneling is for the time being left
out of consideration. This method will be described in succeeding
chapters. Whenever any of the systems of tunneling previously described
are employed, the first task is always to open a drainage gallery at the
bottom of the section.

Assuming the Belgian method is to be the one adopted, the first work is
to drive a center bottom drift, the floor of which is at the level of
the extrados of the invert. This drift is immediately strutted by
successive transverse frames made up of a sill, side posts, and a cap
which support a close plank strutting or lining, with its joints packed
with straw. Between the side posts of each cross-frame, at about the
height of the intrados of the invert, a cross-beam is placed; and on
these cross-beams a plank flooring is laid, which divides the drift
horizontally into two sections, as shown by Fig. 108; the lower section
forming a covered drain for the seepage water, and the upper providing a
passageway for workmen and cars. The bottom drift is driven as far ahead
as practicable, in order to drain the sand for as great a distance in
advance of the work as possible. After the construction of the bottom
drainage drift the excavation proper is begun, as it ordinarily is in
the Belgian method by driving a top center heading, as shown by Fig.
108. This heading is deepened and widened after the manner usual to the
Belgian method, until the top of the section is open down to the
springing lines of the roof arch. To collect the seepage water from the
center top heading it is provided with a center bottom drain constructed
like the drain in the bottom drift, as shown by Fig. 108. When the top
heading is deepened to the level of the springing lines of the roof
arch, its bottom drain is reconstructed at the new level, and serves to
drain the full top section opened for the construction of the roof arch.
This top drain is usually constructed to empty into the drain in the
bottom drift.

[Illustration: FIG. 108.--Sketch Showing Preliminary Drainage Galleries,
Quicksand Method.]

[Illustration: FIG. 109.--Sketch Showing Construction of Roof Strutting,
Quicksand Method.]


=Strutting.=--The method of strutting the bottom drift has already been
described. For the remainder of the excavation the regular Belgian
method of radial roof strutting-frames is employed, as shown by Fig.
109. Contrary to what might be expected, the number of radial struts
required is not usually greater than would be used in many other soils
besides quicksand. Single-track railway tunnels have been constructed
through quicksand in several instances where the number of radial props
required on each side of the center did not exceed four or five. It is
necessary, however, to place the poling-boards very close together, and
to pack the joints between them to prevent the inflow of the fine sand.
In strutting the lower part of the section it is also necessary to
support the sides with tight planking. This is usually held in place by
longitudinal bars braced by short struts against the inclined props
employed to carry the roof arch when the material on which they
originally rested is removed. This side strutting is shown at the right
hand of Fig. 110.

[Illustration: FIG. 110.--Sketch Showing Construction of Masonry Lining,
Quicksand Method.]


=Masonry.=--As soon as the upper part of the section has been opened the
roof arch is built with its feet resting on planks laid on the
unexcavated material below. This arch is built exactly as in the regular
Belgian method previously described, using the same forms of centers and
the same methods throughout, except that the poling-boards of the
strutting are usually left remaining above the arch masonry. To prevent
the possibility of water percolating through the arch masonry, many
engineers also advise the plastering of the extrados of the arch with a
layer of cement mortar. This plastering is designed to lead the water
along the haunches of the arch and down behind the side walls. In
constructing the masonry below the roof arch the invert is built first,
contrary to the regular Belgian method, and the side walls are carried
up on each side from the invert masonry. Seepage holes are left in the
invert masonry, and also in the side walls just above the intrados of
the invert. At the center of the invert a culvert or drain is
constructed, as shown by Fig. 110, inside the invert masonry. This
culvert is commonly made with an elliptical section with its major axis
horizontal, and having openings at frequent intervals at its top. The
thickness of the lining masonry required in quicksand is shown by Table
II.


=Removing the Seepage Water.=--After the tunnel is completed the water
which seeps in through the weep-holes left in the masonry passes out of
the tunnel, following the direction of the descending grades. During
construction, however, special means will have to be provided for
removing the water from the excavation, their character depending upon
the method of excavation and upon the grades of the tunnel bottom. When
the excavation is carried on from the entrances only, unless the tunnel
has a descending grade from the center toward each end, the tunnel floor
in one heading will be below the level of the entrance, or, in other
words, the descending grade will be toward the point where work is going
on, while at the opposite entrance the grade will be descending from the
work. In the latter case the removal of the seepage water is easily
accomplished by means of a drainage channel along the bottom of the
excavation. In the former case the water which drains toward the front
is collected in a sump, and if there is not too great a difference in
level between this sump and the entrance, a siphon may be used to remove
it. Where the siphon cannot be used, pumps are installed to remove the
water. When the tunnel is excavated by shafts the condition of one high
and one low front, as compared with the level at the shaft, is had at
each shaft. Generally, therefore, a sump is constructed at the bottom of
the shaft; the culvert from the high front drains directly to the shaft
sump, while the water from the low-front sump is either siphoned or
pumped to the shaft sump. From the shaft sump the water is forced up the
shaft to the surface by pumps.


THE PILOT METHOD.

The pilot system of tunneling has been successfully employed in
constructing soft-ground sewer tunnels in America by the firm of
Anderson & Barr, which controls the patents. The most important work on
which the system has been employed is the main relief sewer tunnel built
in Brooklyn, N.Y., in 1892. This work comprised 800 ft. of circular
tunnel 15 ft. in diameter, 4400 ft. 14 ft. in diameter, 3200 ft. 12 ft.
in diameter, and 1000 ft. 10 ft. in diameter, or 9400 ft. of tunnel
altogether. The method of construction by the pilot system is as
follows:

Shafts large enough for the proper conveyance of materials from and into
the tunnel are sunk at such places on the line of work as are most
convenient for the purpose. From these shafts a small tunnel,
technically a pilot, about 6 ft. in diameter, composed of rolled boiler
iron plates riveted to light angle irons on four sides, perforated for
bolts, and bent to the required radius of the pilot, is built into the
central part of the excavation on the axis of the tunnel. This pilot is
generally kept about 30 ft. in advance of the completed excavation, as
shown by Fig. 111. The material around the exterior of the pilot is then
excavated, using the pilot as a support for braces which radiate from it
and secure in position the plates of the outside shell which holds the
sand, gravel, or other material in place until the concentric rings of
brick masonry are built. Ribs of T-iron bent to the radius of the
interior of the brick work, and supported by the braces radiating from
the pilot, are used as centering supports for the masonry. On these ribs
narrow lagging-boards are laid as the construction of the arch proceeds,
the braces holding the shell plates and the superincumbent mass being
removed as the masonry progresses. The key bricks of the arches are
placed in position on ingeniously contrived key-boards, about 12 ins. in
width, which are fitted into rabbeted lagging-boards one after another
as the key bricks are laid in place. After the masonry has been in place
at least twenty-four hours, allowing the cement mortar time to set, the
braces, ribs, and lagging which support it are removed. In the meantime
the excavation, bracing, pilot, and exterior shell have been carried
forward, preparing the way for more masonry. The top plates of the shell
are first placed in position, the material being excavated in advance
and supported by light poling-boards; then the side-plates are butted to
the top and the adjoining side-plates. In the pilot the plates are
united continuously around the perimeter of the circle, while in the
exterior shell the plates are used for about one-third of the perimeter
on top, unless treacherous material is encountered, when the plates are
continued down to the springing lines of the arch. This iron lining is
left in place. The bottom is excavated so as to conform to the exterior
lines of the masonry. The excavation follows so closely to the outer
lines of the normal section of the tunnel that very little loss occurs,
even in bad material; and there is no loss where sufficient bond exists
in the material to hold it in place until the poling-boards are in
position.

[Illustration: ~Bracing.~

~Arch Construction.~

~Longitudinal Section.~

FIG. 111.--Sketch Showing Pilot Method of Tunneling.]

In the Brooklyn sewer tunnel work, previously mentioned, the pilot was
built of steel plates ³⁄₈ in. thick, 12 ins. wide, and 37¹⁄₂ ins. long,
rolled to a radius of 3 ft. Steel angles 4 × 4¹⁄₂ ins. were riveted
along all four sides of each plate, and the plates were bolted together
by ³⁄₄-in. machine-bolts. The plates weighed 136 lbs. each, and six of
them were required to make one complete ring 6 ft. in diameter. In
bolting them together, iron shims were placed between the horizontal
joints to form a footing for the wooden braces for the shell, which
radiate from the pilot. The shell plates of the 15-ft. section of the
tunnel were of No. 10 steel 12 ins. wide and 37 ins. long, with steel
angles 2¹⁄₂ × 2¹⁄₂ × ³⁄₈ ins., riveted around the edges the same as for
the pilot, and put together with ⁵⁄₈-in. bolts. These plates weighed 61
lbs. each, and eighteen of them were required to make one complete ring
15 ft. in diameter. The plates for the 12-ft. section were No. 12 steel
12 ins. wide with 2 × 2 × ¹⁄₄-in. angles. Seventeen plates were required
to make a complete ring.




CHAPTER XVI.

OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS; BOSTON SUBWAY
AND NEW YORK RAPID TRANSIT.


OPEN-CUT TUNNELING.

When a tunnel or rapid-transit subway has to be constructed at a small
depth below the surface, the excavation is generally performed more
economically by making an open cut than by subterranean tunneling
proper. The necessary condition of small depth which makes open-cut
tunneling desirable is most generally found in constructing
rapid-transit subways or tunnels under city streets. This fact
introduces the chief difficulties encountered in such work, since the
surface traffic makes it necessary to obstruct the streets as little as
possible, and has led to the development of the several special methods
commonly employed in performing it.

Subways are usually constructed under and along important streets where
electric cars are running. The engineers have taken advantage of the
presence of these lines to facilitate the construction of subways. In
New York, for instance, the tracks of the electric lines were supported
by cast-iron yokes 4 or 5 ft. apart and were surrounded by concrete,
leaving only a large hollow space in the middle for the wires and
trolleys. The rails from 40 to 60 ft. long formed almost a solid
concrete structure for their entire length. The tracks and the street
surface were supported by horizontal beams inserted underneath the
tracks. These were the caps of bents constructed underground whose
rafters were finally resting on the subgrade of the proposed subway.

The various methods for constructing the subways may be classified as
follows: (1) The single wide trench method; (2) the single narrow
longitudinal trench method; (3) the parallel longitudinal trench method;
(4) the slice method.


=Single Longitudinal Trench.=--The simplest manner by which to construct
open-cut tunnels is to open a single cut or trench the full width of the
tunnel masonry. This trench is strutted by means of side sheetings of
vertical planks, held in place by transverse braces extending across the
trench and abutting against longitudinal timbers laid against the
sheeting plank. The lining is built in this trench, and is then filled
around and above with well-rammed earth, after which the surface of the
ground is restored. An especial merit of the single longitudinal trench
method of open-cut tunneling is that it permits the construction of the
lining in a single piece from the bottom up, thus enabling better
workmanship and stronger construction than when the separate parts are
built at different times. The great objection to the method when it is
used for building subways under city streets is, that it occupies so
much room that the street usually has to be closed to regular traffic.
For this reason the single longitudinal trench method is seldom
employed, except in those portions of city subways which pass under
public squares or parks where room is plenty.

This method was followed in the construction of the New York subway,
Section 2, along Elm St., a new street to be opened to traffic after the
subway had been completed, and at other points where local conditions
allowed it.

[Illustration: FIG. 112.--Diagram Showing Sequence of Construction in
Open-Cut Tunnels.]

A modification of this method was used in Contract Section 6, on upper
Broadway. The street at this point is very wide, so by opening a trench
as wide as the proposed four-track line of the subway there still
remained room enough for ordinary traffic. The electric car tracks were
supported by means of trusses 60 or 70 ft. long, which were laid in
couples parallel to the tracks and which rested on firm soil. The soil
under the car tracks was removed, beginning with transversal cuts to
receive the needles which were tied to the lower chord of the trusses by
means of iron stirrups. After the excavation had reached the subgrade,
posts were erected to support the needles thus forming bents upon which
the tracks rested. The trusses were removed and advanced to another
section of the tunnel, and, in the clear space left, the subway was
built from foundation up.


=The Single Narrow Longitudinal Trench.=--This method was used on
Contract Section 5, of the New York subway in order to comply with the
peculiar conditions of the traffic along 42nd St. On this street, on
account of the New York Central Station, there is a constant heavy
traffic, while pedestrians use the northern sidewalks almost
exclusively. A single longitudinal trench was then opened along the
south side, and from this trench all the work of excavation and
construction was carried on. At first the steel structure of the subway
was erected in the trench and then a small heading was driven and
strutted under and across the surface-car tracks. Afterward heavy
I-beams were inserted, which rested with one end on top of the steel
bents and the other end blocked to the floor of the excavation. These
I-beams were located 5 ft. apart and they supported the surface of the
street by means of longitudinal planks. The soil was removed from the
wide space underneath the I-beams and the subway was constructed from
the foundation up. When the structure had been completed, the packing
was placed between the roof of the structure and the surface of the
street, the I-beams withdrawn and the voids filled in.


=Parallel Longitudinal Trenches.=--The parallel longitudinal trench
method of open-cut tunneling consists in excavating two narrow parallel
trenches for the side walls, leaving the center core to be removed after
the side walls have been built. The diagram, Fig. 112, shows the
sequence of operations in this method. The two trenches No. 1 are first
excavated a little wider than the side wall masonry, and strutted as
shown by Fig. 113. At the bottoms of these trenches a foundation course
of concrete is laid, as shown by Fig. 114, if the ground is soft; or the
masonry is started directly on the natural material, if it is rock. From
the foundations the walls are carried up to the level of the springing
lines of the roof arch, if an arch is used; or to the level of its
ceiling, if a flat roof is used. After the completion of the side walls,
the portion of the excavation shown at No. 2, Fig. 112, is removed a
sufficient depth to enable the roof arch to be built. When the arch is
completed, it is filled above with well-rammed earth, and the surface is
restored. The excavation of part No. 3 inclosed by the side walls and
roof arch is carried on from the entrances and from shafts left at
intervals along the line.

[Illustration: FIG. 113.--Sketch Showing Method of Timbering Open-Cut
Tunnels, Double Parallel Trench Method.]

[Illustration: FIG. 114.--Side-Wall Foundation Construction Open-Cut
Tunnels.]

A modification of the method just described was employed in constructing
the Paris underground railways. It consists in excavating a single
longitudinal trench along one side of the street, and building the side
wall in it as previously described. When this side wall is completed to
the roof, the right half of part No. 2, Fig. 112, is excavated to the
line _AB_, and the right-hand half of the roof arch is built. The space
above the arch is then refilled and the surface of the street restored,
after which the left-hand trench is dug and the side wall and roof-arch
masonry is built just as in the opposite half. Generally the work is
prosecuted by opening up lengths of trench at considerable intervals
along the street and alternately on the left-and right-hand sides. By
this method one-half of the street width is everywhere open to traffic,
the travel simply passing from one side of the street to the other to
avoid the excavation. When the lining has been completed, the center
core of earth inclosed by it is removed from the entrances and shafts,
leaving the tunnel finished except for the invert and track
construction, etc.

Another modification of the parallel longitudinal trenches method was
used in the construction of the New York subway. A narrow longitudinal
trench was excavated on one side of the street near the sidewalk.
Meanwhile the pavement of half of the street was removed and a wooden
platform of heavy planks, supported by longitudinal beams which were
buried in the ground, was substituted. Then small cuts underneath the
car tracks were directed from the side trench and heavy beams or needles
were placed in these cuts, which also reached the longitudinal beams of
the wooden platform. The needles were wedged and blocked to the car
track structure and the beams. They were temporarily supported by cribs
built from underneath as the excavation progressed. When the subgrade
was reached, vertical and batter posts were inserted to support the
needles, thus forming regular timber bents underground. In the space
thus left open the subway was constructed to the middle of the street.
While the work was going on as described, another longitudinal narrow
trench was excavated at some distance on the other side of the street.
From this trench, the work of constructing the other half of the subway
was carried on in the manner just described. After the work had been
completed, the timbers removed, the voids filled in and the pavement of
the street restored, another equal section was attacked on both sides of
the street.


=Transverse Trenches.=--The transverse trench or “slice” method of
open-cut tunneling has been employed in one work, the Boston Subway.
This method is described in the specifications for the work prepared by
the chief engineer, Mr. H. A. Carson, M. Am. Soc. C. E., as follows:--

“Trenches about 12 ft. wide shall be excavated across the street to as
great a distance and depth as is necessary for the construction of the
subway. The top of this excavation shall be bridged during the night by
strong beams and timbering, whose upper surface is flush with the
surface of the street. These beams shall be used to support the railway
tracks as well as the ordinary traffic. In each trench a small portion
or slice of the subway shall be constructed. Each slice of the subway
thus built is to be properly joined in due time to the contiguous
slices. The contractor shall at all times have as many slice-trenches in
process of excavation, in process of being filled with masonry, and in
process of being back-filled with earth above the completed masonry, as
is necessary for the even and steady progress of the work towards
completion at the time named in the contract.”

In regard to the success of this method Mr. Carson, in his fourth annual
report on the Boston Subway work, says:

“The method was such that the street railway tracks were not disturbed
at all, and the whole surface of the street, if desired, was left in
daytime wholly free for the normal traffic.”


=Tunnels on the Surface.=--It occasionally happens when filling-in is to
take place in the future, or where landslides are liable to bury the
tracks, that a railway tunnel has to be built on the surface of the
ground. In such cases the construction of the tunnel consists simply in
building the lining of the section on the ground surface with just
enough excavation to secure the proper grade and foundation. Generally
the lining is finished on the outside with a waterproof coating, and is
sometimes banked and partly covered with earth to protect the masonry
from falling stones and similar shocks from other causes. A recent
example of tunnel construction of this character was described in
“Engineering News” of Sept. 8, 1898. In constructing the Golden Circle
Railroad, in the Cripple Creek mining district of Colorado, the line had
to be carried across a valley used as a dumping-ground for the refuse of
the surrounding mines. To protect the line from this refuse, the
engineer constructed a tunnel lining consisting of successive steel
ribs, filled between with masonry.


=Concluding Remarks.=--From the fact that the open-cut method of
tunneling consists first in excavating a cut, and second in covering
this cut to form an underground passageway, it has been named the
“cut-and-cover” method of tunneling. The cut-and-cover method of
tunneling is almost never employed elsewhere than in cities, or where
the surface of the ground has to be restored for the accommodation of
traffic and business. When it is not necessary to restore the original
surface, as is usually the case with tunnels built in the ordinary
course of railway work, it would obviously be absurd to do so except in
extraordinary cases. In a general way, therefore, it may be said that
the cut-and-cover method of construction is confined to the building of
tunnels under city streets; and the discussion of this kind of tunnels
follows logically the general description of the open-cut method of
tunneling which has been given.


TUNNELS UNDER CITY STREETS.

The three most common purposes of tunnels under city streets are: to
provide for the removal of railway tracks from the street surface, and
separate the street railway traffic from the vehicular and pedestrian
traffic; to provide for rapid transit railways from the business section
to the outlying residence districts of the city; and to provide conduits
for sewage or subways for water and gas mains, sewers, wires, etc.
Within recent years the greatest works of tunneling under city streets
have been designed and carried out to furnish improved transit
facilities.


=Conditions of Work.=--The construction of tunnels under city streets
may be divided into two classes, which may be briefly defined as shallow
tunnels and deep tunnels. Shallow tunnels, or those constructed at a
small depth beneath the surface, are usually built by one of the
cut-and-cover methods; deep tunnels, or those built at a great depth,
beneath the surface are constructed by any of the various methods of
tunneling described in this book, the choice of the method depending
upon the character of the material penetrated, and the local conditions.

In building tunnels under city streets the first duty of the engineer is
to disturb as little as possible the various existing structures and the
activities for which these structures and the street are designed. The
character of the difficulties encountered in performing this duty will
depend upon the depth at which the tunnel is driven. In constructing
shallow tunnels by the cut-and-cover method care has to be taken first
of all not to disturb the street traffic any more than is absolutely
necessary. This condition precludes the single trench method of open cut
tunneling in all places where the street traffic is at all dense, and
compels the engineer to use the methods employed in Paris and New York,
as previously described, or else the transverse trench or slice method
employed in the Boston Subway.

These methods have to be modified when the work is done on streets
having underground trolley and cable roads, and in which are located gas
and water pipes, conduits for wires, etc. Where underground trolley or
cable railways are encountered, a common mode of procedure is to
excavate parallel side trenches for the side walls, and turn the roof
arch until it reaches the conduit carrying the cables or wires. The
earth is then removed from beneath the conduit structure in small
sections, and the arch completed as each section is opened. As fast as
the arch is completed the conduit structure is supported on it. Where
pipes are encountered they may be supported by means of chains,
suspending them from heavy cross-beams, or by means of strutting, or
they may be removed and rebuilt at a new level. Generally the conditions
require a different solution of this problem at different points.

Another serious difficulty of tunneling under city streets arises from
the danger of disturbing the foundations of the adjacent buildings. This
danger exists only where the depth of the tunnel excavation extends
below the depth of the building foundations, and where the material
penetrated is soft ground. Where the tunnel penetrates rock there is no
danger of disturbing the building foundations. To prevent trouble of
this character requires simply that the excavation of the tunnel be so
conducted that there is no inflow of the surrounding material, which
may, by causing a settlement of the neighboring material, allow the
foundations resting on it to sink.

The Baltimore Belt tunnel, described in a preceding chapter, is an
example of the method of work adopted in constructing a tunnel under
city streets through very soft ground. This may be classed as a deep
tunnel. Another method of deep tunneling under city streets is the
shield method, examples of which are given in a succeeding chapter. Two
notable examples of cut-and-cover methods of tunneling are the Boston
Subway and the New York Rapid Transit Ry., a description of which
follows.


=Boston Subway.=--The Boston Subway may be defined as the underground
terminal system of the surface street railway system of the city, and as
such it comprises various branches, loops, and stations. The subway
begins at the Public Garden on Boylston St., near Charles St., and
passes with double tracks under Boylston St. to its intersection with
Tremont St., where it meets the other double-track branch, passing under
Tremont St. and beginning at its intersection with Shawmut Ave. From
their intersection at Tremont and Boylston streets the two double-track
branches proceed under Tremont St. with four tracks to Scollay Square.
At Scollay Square the subway divides again into two double-track
branches, one passing under Hanover St., and the other under Washington
St. At the intersection of Hanover and Washington streets the two
double-track branches combine again into a four-track line, which runs
under Washington St. to its terminus at Haymarket Square, where it comes
to the surface by means of an incline. The subway, therefore, has three
portals or entrances, located respectively at Boylston St., Shawmut
Ave., and Haymarket Square. It also has five stations and two loops, the
former being located at Boylston St., Park St., Scollay Square, Adams
Square, and Haymarket Square, and the latter at Park St. and Adams
Square. The total length of the subway is 10,810 ft.


_Material Penetrated._--The material met with in constructing the subway
was alluvial in character, the lower strata being generally composed of
blue clay and sand, and the upper strata of more loose soil, such as
loam, oyster shells, gravel, and peat. At many points the material was
so stable that the walls of the excavation would stand vertical for some
time after excavation. Surface water was encountered, but generally in
small quantities, except near the Boylston St. portal, where it was so
plentiful as to cause some trouble.

[Illustration: FIG. 115.--Wide Arch Section, Boston Subway.]


_Cross-Section._--The subway being built for two tracks in some places
and for four tracks in other places, it was necessary to vary the form
and dimensions of the cross-section. The cross-sections actually adopted
are of three types. Fig. 115 shows the section known as the wide-arch
type, in which the lining is solid masonry. The second type was known as
the double-barrel section, and is shown by Fig. 116. The third type of
section is shown by Fig. 117. The lining consists of steel columns
carrying transverse roof girders, the roof girders being filled between
with arches, and the wall columns having concrete walls between them.
The wide-arch type and the double-barrel type of sections were employed
in some portions of the Tremont St. line, where the traffic was very
dense, since it was possible to construct them without opening the
street. Much of the wide-arch line was constructed by the use of the
roof shield, which is described in the succeeding chapter on the shield
system of tunneling.

[Illustration: FIG. 116.--Double-Barrel Section, Boston Subway.]


_Methods of Construction._--Several different methods were employed in
constructing the subway. Where ample space was available, the single
wide trench method of cut-and-cover construction was employed, the earth
being removed as fast as excavated. In the streets, except where regular
tunneling was resorted to, the parallel trench or transverse trench
cut-and-cover methods were employed.

In the transverse trench method, trenches about 12 ft. wide were
excavated across the street, their length being equal to the extreme
transverse width of the tunnel lining, and their depth being equal to
the depth of the tunnel floor. These trenches were begun during the
night, and immediately roofed over with a timber platform flush with the
street surface. Under these platforms the excavation was completed and
the lining built. As each trench or “slice” was completed, the street
above it was restored and the platform reconstructed over the succeeding
trench or slice. During the construction of each slice the street
traffic, including the street cars, was carried by the timber platform.

[Illustration: FIG. 117.--Four-Track Rectangular Section, Boston
Subway.]

[Illustration: FIG. 118.--Section Showing Slice Method of Construction,
Boston Subway.]

In the parallel trench method, short parallel trenches were dug for the
opposite side walls, and also for the intermediate columns, and
completely roofed over during the night. Under this roofing the masonry
of the side walls and column foundations and the columns themselves were
erected. When the side walls and columns had been erected, the surface
of the street between them was removed, the roof beams laid, and a
platform covering erected, as shown by Fig. 118. This roofing work was
also done at night. The subsequent work of building the roof arches,
removing the remainder of the earth, and constructing the invert, was
carried on underneath the platform covering which carried the street
traffic in the meantime. The successive repetition of the processes
described constructed the subway.

Where the traffic was very dense on the street above, tunneling was
resorted to. For small portions of this work the excavation was done in
the ordinary way, using timber strutting, but much the greater portion
of the tunnel work was performed by means of a roof shield. In the
latter case, the side walls were first built in small bottom side drifts
and were fitted with tracks on top to carry the roof shield. The
construction and operation of this shield are described fully in the
succeeding chapter on the shield system of tunneling.


_Masonry._--The masonry of the inclined approaches to the subway
consists simply of two parallel stone masonry retaining walls. In the
wide-arch and double-barrel tunnel sections, the side walls are of
concrete and the roof arches are of brick masonry. In the other parts of
the subway the masonry consists of brick jack arches sprung between the
roof beams and covered with concrete, of concrete walls embedding the
side columns, and of the concrete invert and foundations for the
columns. Figs. 115 to 118 inclusive show the general details of the
masonry work for each of the three sections. The inside of the lining
masonry is painted throughout with white paint.


_Stations._--The design and construction of the stations for the Boston
Subway were made the subjects of considerable thought. All the stations
consist of two island platforms of artificial stone having stairways
leading to the street above. The platforms are made 1 ft. higher than
the rails. The station structure itself is built of steel columns and
roof beams with brick roof arches and concrete side walls. Its interior
is lined with white enameled tiles. The intermediate columns are cased
with wood, and have circular wooden seats at their bottoms. Each
stairway is covered by a light housing, consisting of a steel framework
with a copper covering and an interior wood and tile finish.


_Ventilation._--The subway is ventilated by means of exhaust fans
located in seven fan chambers, some of which contain two fans, and
others only one fan. Each of the fans has a capacity of from 30,000 to
37,000 cu. ft. of air per minute, and is driven by electric motor,
taking current from the trolley wires. This system of ventilation has
worked satisfactorily.


_Disposal of Rain Water._--The rain water which enters the subway from
the inclined entrances, together with that from leakage, is lifted from
12 ft. to 18 ft. by automatic electric pumps to the city sewers. The
subway has pump-wells at the Public Garden, at Eliot St., Adams Square,
and Haymarket Square. In each of these wells are two vertical submerged
centrifugal pumps made entirely of composition metal. In each chamber
above, are two electric motors operating the pumps. Each motor is
started and stopped according to the height of water by means of a float
and an automatic release starting box. The floats are so placed that
only one pump is usually brought into use. The other, however, comes
into service in case the first pump is out of order or the water enters
more rapidly than one pump can dispose of it. In the latter case, both
motors continue to run until the same low level has been reached.

Very little dampness except from atmospheric condensation is to be found
on the interior walls or roof of the subway, although numerous
discolored patches, caused by dampness and dust, may be seen on some
parts of the walls. Substantially all of the leakage comes through the
small drains in the invert leading from hollows left in the side walls.
Careful measurement was taken at the end of an unusually wet season to
determine the actual amount of leakage, and the total amount for the
entire subway was found to be about 81 gallons per minute.


_Estimated Quantities._--The estimated quantities of material used in
constructing the subway were as follows:

  Excavation                        369,450 cu. yds.
  Concrete                           75,660  „   „
  Brick                              11,105  „   „
  Steel                               8,105 tons
  Granite                             2,285 cu. yds.
  Piles                             117,925 lin. ft.
  Ribbed tiles                       12,440 sq. yds.
  Plaster                            88,190  „   „
  Waterproofing (asphalt coating)   117,980  „   „
  Artificial stone                    6,790  „   „
  Enameled brick                      2,210  „   „
  Enameled tiles                      2,855  „   „


_Cost of the Subway._--The estimated cost of the subway made before the
work was begun was approximately $4,000,000, and the cost of
construction did not exceed $3,700,000. This includes ventilating and
pump chambers, changes of water and gas pipes, sewers and other
structures, administration, engineering, interest on bonds, and all cost
whatsoever. Dividing this number by the total length we obtain a cost
per linear foot of $342.30.


=New York Rapid Transit Railway.=--The project of an underground rapid
transit railway to run the entire length of Manhattan Island was
originated some years previous to 1890. In 1894, however, a Rapid
Transit Commission was appointed to prepare plans for such a road, and
after a large amount of trouble and delay this commission awarded the
contract for construction to Mr. John B. McDonald of New York City, on
Jan. 15, 1900.


_Route._--The road starts from a loop which encircles the City Hall
Park. Within this loop the tunnel construction is two-track; but where
the main line leaves the loop, all four tracks come to the same level,
and continue side by side thereafter except at the points which will be
noted as the description proceeds. Proceeding from the loop, the
four-track line passes under Center and Elm Streets. It continues under
Lafayette Place, across Astor Place and private property between Astor
Place and Ninth St. to Fourth Ave. The road then passes under Fourth and
Park Avenues until 42d St. is reached. At this point the line turns west
along 42d St., which it follows to Broadway. It turns northward again
under Broadway to the boulevard, crossing the Circle at 59th St. The
road then follows the boulevard until 97th St. is reached, where the
four-track line is separated into two double-track lines.

At a suitable point north of 96th St. the outside tracks rise so as to
permit the inside tracks, on reaching a point near 103d St., to curve to
the right, passing under the north-bound track, and to continue thence
across and under private property to 104th St. From there the two-track
tunnel goes under 104th St. and Central Park to 110th St., near Lenox
Ave.; thence under Lenox Ave to a point near 142d St.; thence across and
under private property and the intervening streets to the Harlem River.
The road passes under the Harlem River and across and under private
property to 149th St., which street it follows to Third Ave., and then
passes under Westchester Ave., where, at a convenient point, the tracks
emerge from the tunnel and are carried on a viaduct along and over
Westchester Ave., Southern Boulevard, and Boston Road to Bronx Park.
This portion of the line, from 96th St. to Bronx Park, is known as the
East Side Line.

From the northern side of 96th St. the outside tracks rise and after
crossing over the inside tracks they are brought together on a location
under the center line of the street and proceed along under the
boulevard to a point between 122d and 123d Streets. At this point the
tracks commence to emerge from the tunnel, and are carried on a viaduct
along and over the boulevard at a point between 134th and 135th Streets,
where they again pass into the tunnel under and along the boulevard and
Eleventh Ave. to a point about 1350 ft. north of the center line of
190th St. There the tracks again emerge from the tunnel, and are carried
on a viaduct across and over private property to Elwood St., and over
and along Elwood St. to Kingsbridge St. to Kingsbridge Ave., private
property, the Harlem Ship Canal and Spuyten Duyvil Creek, private
property, Riverdale Ave., and Broadway to a terminus near Van Cortland
Park. That portion of the line from 96th St. to the above-mentioned
terminus at Van Cortland Park is known as the West Side Line.

The total length of the rapid transit road, including the parts above
and below the surface ground of the streets, as well as both the East
and West Side Lines, is about 22¹⁄₂ miles.


_Material Penetrated._--The soil through which the road was excavated
was a varied one. The lower portion of the road, or the part including
the loop up to nearly Fourth St., was excavated through loose soil, but
from Fourth St. to the ends it was excavated in rock. The loose soil
forming the southern part of Manhattan Island is chiefly composed of
clay, sand, and old rubbish--a soil very easy to excavate. Water was met
at some points, but not in such quantities as to be a serious
inconvenience. From Fourth St. to the ends of both the east and west
side lines, the soil was chiefly composed of rock of gneissoid and
mica-schistose character, these rocks prevailing nearly throughout the
whole of Manhattan Island. The rock, as a rule, was not compact, but
full of seams and fissures, and at many points it was found
disintegrated and alternated with strata of loose soils, and even
pockets of quicksand were met with along the line of the road.


_Cross-Sections._--The section of the underground road is of three
different types,--the rectangular, the barrel-vault, and the circular.
The rectangular section. Fig. 119, is used for the greater part of the
road, of which a portion is for four tracks and a portion for two
tracks. The dimensions adopted for the four tracks are 50 × 13 ft., and
for the double tracks 25 × 13 ft. The barrel-vault section, composed of
a polycentric arch, having the flattest curve at the crown, has been
adopted for the tunnels under Park Avenue--while the semicircular arch
is used for all the other portions of the road to be tunneled. The
circular section of 15-ft. diameter is used under the Harlem River, and
being for single track, two parallel tunnels were built side by side.

[Illustration: FIG. 119.--Double-Track Section, New York Rapid Transit
Railway.]

The main line from the City Hall loop to about 102d St. consists of four
tracks built side by side in one conduit, except for that portion under
the present Fourth Ave. tunnel where two parallel double-track tunnels
are employed. The West Side Line will consist of double tracks laid in
one conduit, except across Manhattan St. and beyond 190th St., where it
is carried on an elevated structure. The East Side Line consists of a
double-track tunnel driven from 102d St., and the boulevard under
Central Park to 110th St. and Lenox Ave., and two parallel circular
tunnels excavated under the Harlem River,--the other portions of the
road being double-track, subway and elevated structure.


_Methods of Excavation._--Both the double-and four-track subway were
built by using the different varieties of the cut-and-cover method. The
single wide-trench method was used for the construction of the
double-track line and also for the construction of the four-track line
where the local conditions allowed it. The single narrow-trench method
was used for the construction of the four-track subway at 42d St., to
meet with the peculiar conditions of the traffic. Almost the total
length of the four-track line of the subway was built by means of the
two parallel side trenches. The slice method, so successfully employed
in the Boston Subway, was used only on 42d St. west of 6th Avenue.


_Lining._--The lining of the subway is of concrete, carried by a
framework of steel. The floor consists of a foundation layer of concrete
at least eight inches thick on good foundation, but thicker, according
to conditions, where the foundation is bad. On top of this is placed
another layer of concrete, with a layer of waterproofing between the
two. In this top layer are set the stone pedestals for the steel
columns, and the members making up the tracks.

In the four-track subway, the steel framework consists of transverse
bents of columns, and I-beams spaced about five feet apart along the
tunnel. The three interior columns of each bent are built-up bulb-angle
and plate columns of H-section. The wall columns are I-beams, as are
also the roof beams; between the I-beams, wall columns, and roof beams
there is a concrete filling, so that the roof of the subway will be made
up of concrete arches resting on the flanges of the I-beams of the roof.
The concrete used is of one part Portland cement, two parts sand, and
four parts broken stones. The double-track subway is built in the same
way, except that only one column is placed between the tracks for the
support of the roof.

All the concrete masonry of the roof, foundations, and side walls
contains a layer of waterproofing, so as to keep perfectly dry the
underground road, and prevent the percolation of water. This
waterproofing is made up as follows: On the lowest stratum of concrete,
whose surface is made as smooth as possible, a layer of hot asphalt is
spread. On this asphalt are immediately laid sheets or rolls of felt;
another layer of hot asphalt is then spread over the felt, and then
another layer of felt laid, and so on, until no less than two, and no
more than six, layers of felt are laid, with the felt between layers of
asphalt. On top of the upper surface of asphalt the remainder of the
concrete is put in place so as to reach the required thickness of the
concrete wall.

[Illustration: FIG. 120.--Park Avenue Deep Tunnel Construction, New York
Rapid Transit Railway.]


_Tunnels._--When the distance between the roof of the proposed structure
and the street was 20 ft. or over, the Standard Subway construction was
replaced by tunnels. Three important tunnels have been constructed along
the line of the New York Rapid Transit and these are located between 33d
and 42d Streets on Park Ave., under Central Park northeast of 104th St.
and under Broadway north of 152d St. The Park Ave. construction (Fig.
120) consists of two parallel double-track tunnels, located on each side
of the street, and about 10 ft. below the present tunnel. The soil being
composed of good rock, the tunnels were driven by a wide heading, and
one bench, since no strutting was required, and the masonry lining, even
of the roof, was left far behind the front of the excavation. The
masonry lining consists of concrete walls and brick arches. The tunnels
under Central Park and under Broadway being driven through a similar
rock, the same method of excavation and the same manner of lining was
used.

The tunnel under the Harlem River was driven through soft ground; and it
was constructed as a submarine tunnel, according to the caisson process.
The tunnels were lined with iron made up of segments, with radial and
circumferential flanges. Concrete was placed inside and flush with the
flanges.

[Illustration: FIG. 121.--Harlem River Tunnel, New York Rapid Transit
Railway.]

The tracks, both in the subway and tunnels, are an intimate part of the
concrete flooring. The rail rests on a continuous bearing of wooden
blocks, laid with the grain running transversely with respect to the
line of the rail, and held in place by two channel iron guard rails. The
guard rails are bolted to metal cross-ties embedded in the concrete.


_Viaduct._--A considerable portion of the double-track branch lines
north of 103d St. is viaduct, or elevated structure. The viaduct
construction on the West Side Line extends, including approaches, from
122d St. to very near 135th St. Of this distance, 2018 ft. 8 ins. are
viaduct proper, consisting of plate girder spans carried by trestle
bents at the ends, and by trestle towers for the central portion. The
columns of the bents and towers are built-up bulb-angle and plate
columns of H-section of the same form as those used in the bents inside
the subway. The approaches proper are built of masonry. The elevated
line proper consists of plate girder spans, supported on plate cross
girders carried by columns.


_Stations._--Many stations are built along the line. These are located
on each side of the street. The entrances at the stations consist of
iron framework, with glass roofs covering the descending stairways. The
passageways leading down are walled with white enameled bricks and
wainscoted with slabs of marble. The stations for the local trains are
located on each side of the road close to the walls, since the outside
tracks are reserved for the local trains, while the middle ones are
reserved for the expresses. The few stations for the express trains are
located between the middle and outside tracks. Stations are provided
with all the conveniences required, having toilet rooms, news stands,
benches, etc., and are lighted day and night by numerous arc lamps.


_General._--The contractor completed the work in four years. No
difficulty was encountered in doing this, since the great extension of
the road and the great width of the avenues under which it runs allowed
work all along the line at the same time. The work, briefly summarized,
comprises the following items:--

  Length of all sections, ft.              109,570
  Total excavation of earth, cu. yds.    1,700,228
  Earth to be filled back, cu. yds.        773,093
  Rock excavated, cu. yds.                 921,128
  Rock tunneled, cu. yds.                  368,606
  Steel used in structure, tons             65,044
  Cast iron used, tons                       7,901
  Concrete, cu. yds.                       489,122
  Brick, cu. yds.                           18,519
  Waterproofing, sq. yds.                  775,795
  Vault lights, sq. yds.                     6,640
  Local stations, number                        43
  Express stations, number                       5
  Station elevators, number                     10
  Track total, lin. ft.                    305,380
    „   underground, lin. ft.              245,514
    „   elevated, lin. ft.                  59,766

In addition to the construction of the railway itself, it was necessary
to construct or reconstruct certain sewers, and to adjust, readjust,
and maintain street railway lines, water pipes, subways, and other
surface and subsurface structures, and to relay street pavements.

The total cost of the work, according to the contract signed by Mr.
McDonald, was $35,000,000. Dividing this amount by the total length of
the road, which is 109,570 lineal feet, we have the unit cost a lineal
foot $315, or a little less than unit of cost of the Boston Subway,
which was $342 per lineal foot.

The road belongs to the city. The contractor acts as an agent for the
city in carrying out the work, and he is the leaser of the road for
fifty years. The work was paid for as soon as the various parts of the
road were completed, and the money was obtained from an issue of city
bonds. During the fifty years’ lease the contractor will pay the
interest plus 1% of the face value of the bonds. This 1% goes to the
sinking-fund, which within the fifty years at compound interest forms
the total sum required for the redemption of bonds.

This first New York Subway has been extended to Brooklyn, and more lines
will be built so as to form a complete underground railway system to
accommodate the ever-increasing traveling crowd of the American
metropolis. No new method of construction has been devised yet. The only
variation from the illustrated methods has been where the subway is
built underneath the Elevated Road which had to be strongly supported
during the construction of the subway. This has been done in two
different ways, either by supporting the columns of the Elevated Road by
means of two wooden A-frames abutting at the top and leaving a large
space close to the foot of the column where a pit was excavated to the
required depth of the subway, or by attaching the columns to long iron
girders placed longitudinally and resting with both ends in firm soil.




CHAPTER XVII.

SUBMARINE TUNNELING: GENERAL DISCUSSION.--THE SEVERN TUNNEL.


GENERAL DISCUSSION.

Submarine tunnels, or tunnels excavated under the beds of rivers, lakes,
etc., have been constructed in large numbers during the last quarter of
a century, and the projects for such tunnels, which have not yet been
carried to completion, are still more numerous. Among the more notable
completed works of this character may be noted the tunnel under the
River Severn and those under the River Thames in England, the one under
the River Seine in France, those under the St. Clair, Detroit, Hudson,
Harlem and East Rivers, and the one under the Boston Harbor for
railways, that under the East River for gas mains, that under Dorchester
Bay, Boston, for sewage, and those under Lakes Michigan and Erie for the
water supply of Milwaukee, Chicago, Buffalo, and Cleveland in America.
For the details of the various projected submarine tunnels of note,
which include tunnels under the English and Irish Channels, under the
Straits of Gibraltar, under the sound between Copenhagen in Denmark and
Malmö in Sweden, under the Messina Straits between Italy and Sicily, and
under the Straits of Northumberland between New Brunswick and Prince
Edward Island, and those connecting the various islands of the Straits
of Behring, the reader is referred to the periodical literature of the
last few years.

Previous to attempting the driving of a submarine tunnel it is necessary
to ascertain the character of the material it will penetrate. This fact
is generally determined by making diamond-drill borings along the line,
and the object of ascertaining it is to determine the method of
excavation to be adopted. If the material is permeable and the tunnel
must pass at a small depth below the river bed, a method will have to be
adopted which provides for handling the difficulty of inflowing water.
If, on the other hand, the tunnel passes through impermeable material at
a great depth, there will be no unusual trouble from water, and almost
any of the ordinary methods of tunneling such materials may be employed.
Shallow submarine tunnels through permeable material are usually driven
by the shield method or by the compressed air method, or by a method
which is a combination of the first and second.

It is not an uncommon experience for a submarine tunnel to start out in
firm soil and unexpectedly to find that this material becomes soft and
treacherous as the work proceeds, or that it is intersected by strata of
soft material. The method of dealing with this condition will vary with
the circumstances, but generally if any considerable amount of soft
material has to be penetrated, or if the inflow of water is very large,
the firm-ground system of work is changed to one of the methods employed
for excavating soft-ground submarine tunnels. The Milwaukee water supply
tunnel, described elsewhere, is a notable example of submarine tunnels,
began in firm material which unexpectedly developed a treacherous
character after the work had proceeded some distance. Occasionally the
task of building a submarine tunnel in the river bed arises. In such
cases the tunnel is usually built by means of cofferdams in shallow
water, and by means of caissons in deep water.

Submarine tunnels under rivers are usually built with a descending grade
from each end which terminates in a level middle position, the
longitudinal profile of the tunnel corresponding to the transverse
profile of the river bottom. Where, however, such tunnels pass under the
water with one end submerged, and the other end rising to land like the
water supply tunnels of Chicago, Milwaukee, and Cleveland, the
longitudinal profile is commonly level, or else descends from the shore
to a level position reaching out under the water.

The drainage of submarine tunnels during construction is one of the most
serious problems with which the engineer has to deal in such works. This
arises from the fact that, since the entrances of the tunnel are higher
than the other parts, all of the seepage water remains in the tunnel
unless pumped out, and from the possibility of encountering faults or
permeable strata, which reach to the stream bed and give access to water
in greater or less quantities. Generally, therefore, the excavation is
conducted in such a manner that the inflowing water is led directly to
sumps. To drain these sumps pumping stations are necessary at the shore
shafts, and they should have ample capacity to handle the ordinary
amount of seepage, and enough surplus capacity to meet probable
increases in the inflow. For extraordinary emergencies this plant may
have to be greatly enlarged, but it is not usual to provide for these at
the outset unless their likelihood is obvious from the start. The
character and size of the pumping plants used in constructing a number
of well-known tunnels are described in Chapter XII.

In this book submarine tunnels will be classified as follows: (1)
Tunnels in rock or very compact soils, which are driven by any of the
ordinary methods of tunneling similar materials on land; (2) tunnels in
loose soils, which may be driven, (_a_) by compressed air, (_b_) by
shields, or (_c_) by shields and compressed air combined; (3) tunnels on
the river bed, which are constructed, (_a_) by cofferdams, or (_b_) by
caissons. To illustrate tunnels of the first class, the River Severn
tunnel in England has been selected; to illustrate those of the second
class, the several tunnels discussed in the chapter on the shield system
of tunneling and the Milwaukee tunnel is sufficient; to illustrate those
of the third class, the Van Buren Street tunnel in Chicago, the Harlem,
the Seine and the Detroit River tunnels are selected.


THE SEVERN TUNNEL.

The Severn tunnel, which carries the Great Western Railway of England,
beneath the estuary of a large river, is 4 miles 642 yards long. It
penetrates strata of conglomerate, limestone, carboniferous beds, marl,
gravel, and sand at a minimum depth of 44³⁄₄ ft. below the deepest
portion of the estuary bed. The following description of the work is
abstracted from a paper by Mr. L. F. Vernon-Harcourt.[12]

  [12] Proceedings Inst. C. E., vol. cxxi.

The first work was the sinking of a shaft, 15 ft. in diameter, lined
with brickwork, on the Monmouthshire bank of the Severn, 200 ft. deep.
After the Monmouthshire shaft had been sunk, a heading 7 ft. square was
driven under the river, rising with a gradient of 1 in 500 from the
shaft on the Monmouthshire shore, so as to drain the lowest part of the
tunnel. Near to the first, a second shaft was sunk and tubbed with iron,
in which the pumps were placed for removing the water from the tunnel
works, connection being made by a cross-heading with the heading from
the first shaft. There was also a shaft on the Gloucestershire shore;
and two shafts inland from the first on the Monmouthshire side, to
expedite the construction of the tunnel. Headings were then driven in
both directions along the line of the tunnel, from the four shafts; and
the drainage heading from the old shaft was continued, in the line of
the tunnel, under the deep channel of the estuary, and up the ascending
gradient towards the Gloucestershire shore, till, in October, 1879, it
had reached to within about 130 yards of the end of the descending
heading from the Gloucestershire shaft. During this period, though the
work had progressed slowly, no large quantity of water had been met with
in any of the headings, in spite of their already extending under almost
the whole width of the estuary. On October 18, 1889, however, a great
spring was tapped by the heading which was being driven landwards from
the old shaft, about 40 ft. above the level of the drainage heading;
and the water poured out from this land spring in such quantity that,
flowing along the heading, falling down the old shaft, and thus finding
its way into the drainage heading and the long heading of the tunnel
under the estuary in connection with it, it flooded the whole of the
workings in communication with the old shaft, which it also filled
within twenty-four hours from the piercing of the spring.

To obtain increased security against any influx of water from the deep
channel of the estuary into the tunnel, the proposed level portion of
the tunnel, rather more than a furlong long under this part, was lowered
15 ft. by increasing the descending gradient on the Monmouthshire side
from 1 in 100 to 1 in 90, and lowering the proposed rail level on the
Gloucestershire side 15 ft. throughout the ascent, so as not to increase
the gradient of 1 in 100 against the load. A new shaft, 18 ft. in
diameter, was sunk slightly nearer the estuary on the Monmouthshire
shore than the old one; two shafts also were sunk on the land side of
the great spring for pumping purposes; and additional pumping machinery
was erected. The flow from the spring into the old shaft was arrested by
a shield of oak fixed across the heading; and at last, after numerous
failures and breakdowns of the pumps, the headings were cleared of
water, after a diver, supplied with a knapsack of compressed oxygen, had
closed a door in the long heading under the estuary; and the works were
resumed nearly fourteen months after the flooding occurred. The great
spring was then shut off from the workings by a wall across the heading
leading to the old shaft; and, owing to the lowering of the level of the
tunnel, a new drainage heading had to be driven from the bottom of the
new shaft at a lower level, which was made 5 ft. in diameter, and lined
with brickwork, whilst the old drainage heading was enlarged to 9 ft. in
diameter, and lined with brickwork, so as to aid in the permanent
ventilation of the tunnel. The lowering of the level, moreover,
converted the bottom tunnel headings into top headings, so that along
more than a mile of the tunnel the semicircular arch, 26 ft. in
diameter, was built first, and then, after lowering the headings, the
invert was laid and the side walls were built up. Bottom headings were
driven along the remainder of the tunnel, and the work was expedited by
means of break-ups. Ventilation was effected in the works by a fan 18
inches in diameter and 7 ft. wide, fixed at the top of the new deep
shaft; the rock was bored by drills worked by compressed air; the
blasting was eventually effected exclusively by tonite, owing to its
being freer from deleterious fumes than any other explosive; and the
workings were lighted by Swan and Brush electric lamps. The tunnel is
lined throughout with vitrified brickwork, between 2¹⁄₄ ft. to 3 ft.
thick, set in cement, and has an invert 1¹⁄₂ ft. to 3 ft. in thickness;
the lining was commenced towards the end of 1880, the headings under the
river were joined in September, 1881, and the last length of the tunnel,
across the line of the great spring, was completed in April, 1885.

Water came in from the river through a hole in a pool of the estuary,
close to the Gloucestershire shore, in April, 1881, during the lining of
a portion of the tunnel, but fortunately before the headings were
joined. This influx was stopped by allowing the water to rise in the
tunnel to tide-level, to prevent the enlargement of the hole, which was
then filled up at low water with clay, weighted on the top with clay in
bags. The great spring broke out again in October, 1883, and flooded the
works a second time; but within four weeks the water had been pumped out
and the spring again imprisoned. During this period an exceptionally
high tide, raised still higher by a southwesterly gale, inundated the
low-lying land on the Monmouthshire side of the estuary, and, flowing
down one of the inland shafts, flooded a section of the tunnel, but the
pumps removed this water within a week.

In order to construct the portion of tunnel traversing the line of the
great spring, the water was diverted into a side heading below the level
of the tunnel, leading to the old shaft, whence it was pumped, and the
fissure below the tunnel was filled with concrete, over which the
invert was built. An attempt to imprison the spring, on the completion
of this length of tunnel, having resulted in imposing an excessive
pressure on the brickwork, leading to fractures and leakage, a shaft, 29
ft. in diameter, was sunk at the side of the tunnel at this point in
1886, and pumps were erected powerful enough to deal with the entire
flow of the spring.

The tunnel was opened for traffic in December, 1886, and gives access to
a double line of railway, connecting the lines converging to Bristol
with the South Wales railway and the western lines. The pumping power
provided at the shaft connected with the great spring, and at four other
shafts, is capable of raising 66,000,000 gallons of water per day, the
maximum amount pumped from the tunnel being 30,000,000 gallons a day.
The ventilation of the tunnel is effected by fans placed in the two main
shafts on each bank of the estuary, and the fan in the Monmouthshire
shaft is 40 ft. in diameter, and 12 ft. wide. The tunnel gives passage
to a large traffic, numerous through-trains between the north and
southwest of England making use of it.




CHAPTER XVIII.

SUBMARINE TUNNELING (Continued); THE COMPRESSED AIR METHOD.--THE
MILWAUKEE WATER-WORKS TUNNEL.


Tunnels excavated at shallow depth from the bed of the river are liable
to cave in under the great weight of the water and material above the
roof. Besides, the progress of the work will be greatly interfered with
by the water which may reach the tunnel passing through the loose soil
in large quantities. To contend with these two sources of trouble,
different methods of constructing subaqueous tunnels have been devised;
they are: by compressed air, by shield, and finally by a combination of
these two methods, viz., by shield and compressed air.

The compressed air method was suggested by Mr. Haskin, the promoter and
the first builder of the Hudson River tunnel. In 1874, when he began to
sink the shaft for the construction of his tunnel, several subaqueous
tunnels had already been driven by means of shields. Mr. Haskin had
ideas of his own, and thought he could dispense with the shield and
could trust to compressed air, since he was firmly convinced that
compressed air alone could expel the water and temporarily support the
roof of the excavation prior to the building of the lining masonry. In
other words, he expected to substitute a core of compressed air for the
core of earth removed. In the patent granted him for this method of
tunneling, he expresses himself as follows: “The distinguishing feature
of my system is that, instead of using temporary facings of timber or
other rigid material, I rely upon the air pressure to resist the caving
in of the wall and infiltration of water until the masonry wall is
completed. The pressure is, of course, to be regulated by the
exigencies of the occasion. The effect of such a pressure has been found
to drive water in from the surface of the excavation, so that the sand
becomes dry.”

The compressed air method was soon found to be inefficient, even in the
construction of the Hudson tunnel where the roof of the excavation was
supported by timbering in the manner indicated in the pilot system. Thus
large subaqueous railway tunnels cannot be driven exclusively by the
compressed air method; still it has been successfully employed in the
construction of small tunnels driven for aqueduct purposes. But the use
of compressed air marked a great progress in the art of submarine
tunneling.


THE MILWAUKEE WATER-WORKS TUNNEL.

The following description of the Milwaukee Water-Works Tunnel is an
example of subaqueous tunnels driven through good soil in the usual
manner employed in land tunnels; but afterward when treacherous material
was encountered, the work was continued by means of compressed air.

The new water supply intake tunnel for the city of Milwaukee, Wis., is
one of the most difficult examples of tunnel construction which American
engineering practice has afforded. The difficulties were in a large
measure unexpected when the work was decided upon and put under way. The
tunnel began and ended in a hard, impervious clay, practically a rock,
and all the preliminary investigations led to the conclusion that the
same favorable material would be encountered for its entire length. With
such material a brick-lined tunnel 7¹⁄₂ ft. in diameter presented no
unusual problems; but after about 1640 ft. had been excavated from the
shore end the tunnel ran out of the hard clay, and for the next 600 ft.
or more a variety of water-bearing material was encountered, which tried
the skill and patience of the engineer to their utmost. Other
difficulties were indeed met with, but these were of minor importance in
comparison with that of safely and successfully penetrating the
water-bearing drift.

The work of sinking the shore shafts and excavating the first 1600 ft.
of tunnel did not prove especially difficult. A hard, compact, and
rock-like clay, bearing very little moisture, was encountered all along,
and was blasted and removed in the ordinary manner. The only mishap
which occurred with this portion of the work was the destruction of the
contractor’s boiler plant by fire on Jan. 12, 1891, which allowed the
tunnel to fill with water, and delayed work about a month. By Oct. 21,
1891, 1640 ft. had been driven, averaging about 6²⁄₃ ft. per day, all in
the hard clay. No timbering had been necessary, and except for the first
100 ft. of the tunnel there was very little seepage. On the afternoon of
Oct. 21 water was observed coming out from one of the drill holes in the
heading, but no attention was paid to it. Shortly after a blast was
fired, and was immediately followed by a rush of water from the heading.
An unsuccessful attempt was made to check the flow, and the pumps were
started; but they were unable to keep the water down, and after seven
hours’ hard work the tunnel was abandoned. By the next morning the
tunnel and shaft were full of water.

Several attempts were made to empty the tunnel; but the limited pumping
capacity was not equal to the task, and it was finally decided to
install larger pumps. The pumping had, however, shown that about 1000
gallons of water a minute was coming through the leak. With the
increased pumping plant the tunnel was finally laid dry Feb. 13, 1892.
Upon examination the head of the drift was found to be in the same
undisturbed condition in which it was left when the water broke in three
months before.

A brick bulkhead was built into the end of the brickwork of the tunnel,
and provided with a timber door for passage, and two 10-in. pipes for
the outlet of the water. With these openings closed, the flow was
checked sufficiently to allow the placing of pumps at the bottom of the
shore shaft. Meanwhile the pressure of the water against the bulkhead
caused dangerous leakage, and so after the pumps were in position the
10-in. pipes were opened, relieving the pressure and allowing the water
its normal rate of flow. Trouble with the pumps now arose, and after
various stoppages and breaks the discharge pipe finally fell, disabling
the whole plant. It became necessary to close the 10-in. pipes in the
bulkhead and draw up the pumps. This allowed the tunnel to again fill
with water.

After thoroughly overhauling the pumping machinery, the contractor again
laid the tunnel dry on March 19; and after the pumps had been
permanently placed so as to take care of the water, an examination of
the work was made. It was found that the water was coming from the
north, and with the hope of avoiding the difficulties of the old
heading, it was decided to make a détour of the south. On April 16 work
was begun at a point about 90 ft. back from the face, and deflecting the
line about 38° toward the south. About 38 ft. from the angle of junction
a brick bulkhead with two 8-in. openings was built into the new bore.
The work progressed successfully for about 75 ft., when water was again
encountered; and upon pushing forward the heading, gravel and sand came
in such quantities that it was found impracticable to continue the work
further. On June 1 the bulkhead was permanently closed, and the work in
this direction was abandoned.

A further and closer examination was now made of the heading first
abandoned. Upon breaking through the rock-like clay it was found that
the water came from an underground stream flowing from the north through
a well defined channel in red clay. This channel was about 13 ft. above
the grade of the tunnel; and above it in every direction visible was a
bed of hard, dry, red clay, while immediately in front of the face of
the work was a bank of coarse gravel. Fig. 122 is a sketch of the
channel and stream where they entered the work. In this last drawing the
photograph has been followed exactly, no particular being exaggerated in
the slightest. The water from this stream was clear and pure; and a
chemical analysis showed that it was not lake water, but must come from
some separate source.

[Illustration: FIG. 122.--Sketch Showing Underground Stream, Milwaukee
Water-Works Tunnel.]

While the engineer did not consider the difficulty of proceeding along
the old line insurmountable, it was decided to be less difficult on the
whole to go back from 150 ft. to 175 ft. and deflect the line to the
north and upward, so as to pass over the underground entrance. Instead
of allowing the water to flow at its normal rate and take care of it by
pumping, the contractors desired to reduce the pumping, and to this end
they constructed a bulkhead just west of the deflection toward the south
with a view of shutting off the water. The water, however, accumulated
with a pressure of some 50 lbs. per sq. in. and penetrated the filling
around the brick lining of the tunnel, preventing the cutting through of
the lining for the new line. A second bulkhead was then built about 20
ft. west of the first, but with not much better results, for upon
closing it the water was found to leak through the brickwork for a long
distance west. Finally on Aug. 2, 1892, the contractors lifted their
pumps and allowed the tunnel to fill again with water.

No further work was done on the tunnel by the contractors, although they
continued work on the lake shaft for some months. Difficulties had,
however, arisen here, which will be described further on; and finally a
disagreement arose between the contractors and the city over the delay
in prosecuting the tunnel work and over one or two other questions,
which resulted in the City Council suspending their contract and
ordering the Board of Public Works to go ahead with the work.

The first step to be taken by the engineer was to purchase adequate
pumping machinery and empty the tunnel. This was effected Jan. 17, 1894;
and as soon as practicable thereafter the two bulkheads were removed and
the tunnel cleaned, tram-car tracks laid, and everything prepared for
work. It was now determined to go ahead on the original line of the
tunnel if possible, and the bulkhead here was removed and work begun.
Meanwhile, a safety bulkhead had been built to replace the first one
torn away. This was provided with a door and drainage pipes. Work was
begun on the original heading, but had proceeded only a little way when
the water broke in, driving out the workmen. This was removed three or
four times, when the flow suddenly increased to 3000 gallons per minute.
An examination of the lake bottom above the break showed that it had
settled down, indicating that the new break connected with the lake
bottom, and making further work along the original line out of the
question.

The question now arose what it was best to do. It was impracticable to
use a shield, as the material ahead of the break required blasting, and
the pressure from above was enormous. On account of its expense and
difficulty of application the freezing process did not seem advisable,
and the plenum process was likewise out of the question on account of
the great pressure which would be required at this depth. The détour to
the south which had been made by the contractor had been unsuccessful,
and had left the ground in a treacherous condition. To depress the
tunnel was not advisable, for it was not by any means certain that the
bed of gravel could be avoided in that way; and, moreover, it would be
necessary to ascend again further on, and thus leave a trap which would
effectually cut off escape to those at work on the face if water again
broke into the tunnel.

It was finally decided that the old plan of deflecting the line toward
the north and upward so as to pass over the underground stream should be
tried. A hole was therefore cut through the tunnel lining 1433 ft. from
the shore, and work was begun on a détour of 20° toward the north and an
upward grade of 10%. Fair progress was made on this new line, gradually
ascending into solid rock, until May 10, when the test borings, which
were constantly made in every direction from the face, showed that sand
was being approached. A brick bulkhead was therefore built into the
masonry as a safeguard, should it happen that water was encountered in
large quantities. As the borings seemed to indicate that the top surface
of the rock underlying the sand was nearly level, the lower half of the
tunnel was first excavated, leaving about 18 ins. of the rock to serve
as a roof (Sketch _a_, Fig. 123), and the brick invert was built for a
distance of 52 ft. The rock roof was then carefully broken through for
short distances at a time, and short sheeting driven ahead into the
sand, which proved to be a very fine quicksand flowing through the
smallest openings. Extreme care had to be taken in this work, but little
by little the brickwork was pushed ahead until at a distance of 90 ft.
from the point where the sand was first met, and 208 ft. from the old
tunnel, the sand stopped and the heading entered a hard clay.

All this work had been done on an ascending grade, and the ascent was
continued about 40 ft. farther in the clay. By this time a sufficient
elevation was gained to pass over the underground stream, and the tunnel
line was changed to head toward the lake shaft, and the grade reduced to
a level. The underground stream was passed without trouble and the
tunnel continued for a distance of 54 ft. without difficulty. On July 10
the clay in the heading suddenly softened, and before the miners could
secure it by bracing, the water rushed in, followed by gravel, filling
up solidly some 34 ft. of the tunnel before it was stopped by a timber
bulkhead hastily built.

[Illustration: ~Longitudinal Section Showing Method of Construction in
Rock Covered with Quicksand.~

~Sketch “a”.~

~Section A-B-C-D.~

~Sketch “c”.~

~Longitudinal Section of Tunnel.~

~Sketch “b”.~

~Cross Section Showing Manner of Constructing Lining around Boulder.~

~Sketch “d”.~

FIG. 123.--Sketch Showing Methods of Lining, Milwaukee Water-Works
Tunnel.]

Upon examining the lake bottom a cavity over 60 ft. deep and 10 ft. in
diameter was found directly over the end of the tunnel, which had been
caused by the gravel breaking into the tunnel. Having now reached an
elevation where it was possible to use compressed air, it was determined
to put in double air-locks and use the plenum process. The locks were
built, and some 670 cu. yds. of clay were dumped into the hole in the
lake bottom. On Aug. 4 the air-locks were tried with 26 lbs. air
pressure; but, upon a temporary release of the pressure, the water
passed around the locks and back of the tunnel lining for some distance,
and even forced through the lining, carrying considerable clay and fine
sand with it. Upon sounding the lake bottom it was found that the cavity
had again increased to a depth of 65 ft., whereupon an additional 600
cu. yds. of clay were dumped into it.

On account of the water leaking through the brickwork, the only dry
place to cut through the brickwork and build in an air-lock was just
ahead of the brick bulkhead. This lock was completed Aug. 27, and to
avoid encountering the danger of the direct connection with the lake at
the end of the drift, it was decided to make another détour to the
north. On Aug. 28, therefore, the brick on the north side of the tunnel
12 ft. back from the end of the brickwork was cut through under 25 lbs.
air pressure, and work proceeded in good, hard clay. The original
air-lock was cut out and a new lock built into this clay about 34 ft.
from the last détour, to be used in case of further difficulties. After
building the tunnel for about 80 ft. from the détour, the soundings
again indicated the approach to gravel and water, and on Oct. 14 the
water broke through from the bottom in such volume and with such force
that the men ran out, closing every air-lock and the valves of every
drain in their haste to escape, until the brick bulkhead was reached. It
was with great difficulty that the portion of the tunnel up to the last
air-lock was recovered and cleaned out.

It was now recognized that a pressure of from 38 to 40 lbs. of air would
be needed to hold this water, and accordingly another compressor was
added to the plant. With a pressure of 36 lbs. the water was driven out
and the work again started. At this time also an additional 350 cu. yds.
of clay were dumped into the hole in the lake bottom. Altogether, 1620
cu. yds. of clay had been put into this hole.

Loose gravel and boulders, some of immense size, were now encountered,
and the work became exceedingly difficult on account of the great escape
of air. The interstices between the gravel and boulders were not filled
with silt or sand, but contained water. Moreover, this material extended
upward to the lake bottom, as was shown by the escape of air at the
surface of the lake. For an area of several hundred square feet the
surface of the water resembled a pot of boiling water. At times the air
would escape very rapidly; and again only a few bubbles would show.

It need hardly be said that the work in this gravel was very slow. It
was impossible to blast or to tear out the large boulders whole, as so
much surface would be exposed that an inrush of water would take place
despite the air pressure. The method of procedure was to excavate a
heading and build the brick roof arch first, and then to take out the
bench and build the invert. Fig. 123 gives a number of sketches showing
how the work was done. A short piece of heading was taken out, the top
and face of the bench being meanwhile plastered with clay (Sketches _b_
and _c_, Fig. 123) to reduce the escape of air, and then the roof arch
was built and supported on side sills resting on the bench. Bit by bit
the roof arch was pushed forward until some little distance had been
completed, then the heading was plastered with clay and the bench taken
out little by little and the invert built. All the gravel except the
small area upon which work was actually in progress was kept thoroughly
plastered with clay; and as the air escaped through the completed
brickwork very rapidly, water was allowed to cover a portion of the
invert (see Sketch _c_, Fig. 123), so as to reduce the area of escape.

When a large boulder was reached, which lay partly within and partly
without the tunnel section, the lining was built out and around it, as
shown in Sketch _d_, Fig. 123. The boulder was then broken and taken
out. All through this gravel bed the cross-section of the lining is made
irregular by the construction of these pockets in the lining to get
around boulders. Sometimes they were on one side and sometimes on the
other, or on both, or at the top or bottom. In fact, there was no
regularity. Despite the hazard and danger of this work, continual
progress was made, though sometimes it was only 4 ft. of completed
tunnel per week, working night and day; and, if some cases of caisson
disease be excepted, the only mishap occurring was a fire which got into
the timber packing behind the lining and caused some trouble. From the
gravel the tunnel ran into clay and quicksand, and then into hard, dry
clay similar to that encountered near the shore. Some difficulty was had
with the quicksand, but it was successfully overcome; and when the hard
clay was struck, the trouble, as far as the work from the shore shaft
was concerned, was virtually over.

Meanwhile, a different set of afflictions had come upon the engineer and
contractors in sinking the lake shaft and driving the heading toward
shore. This shaft was intended to be built by sinking a cast-iron
cylinder 10 ft. in diameter, made up of sections bolted together. Work
was begun July 5, 1892, and the sinking was accomplished first by
weighting the cylinder, and afterwards by pumping out the sand and water
within it until the pressure from the outside broke through under the
cutting edge and forced the sand into the cylinder, allowing it to sink
a little. From 10 to 30 cu. yds. of sand were carried into the cylinder
each time, and finally it was feared that if the process continued, the
crib, which had been previously erected, would be undermined. On Sept.
6, therefore, the contractors were ordered to discontinue this method of
work. No change was made, however, until Oct. 1, when the cylinder had
reached a depth of 68 ft., and by this time there was quite a large
cavity underneath the crib. This was refilled, and the cylinder pumped
out, and excavation begun inside of it. On Oct. 11 a 2¹⁄₂-ft. deep ring
of brickwork was laid underneath the cutting edge; but in trying to put
in another ring beneath the first, two days later, the sand and water
broke through the bottom, driving the men out, and filling the cylinder
to a depth of 16 ft. with sand. The pumps were started, but the water
could not be lowered to a greater depth than 60 ft.

At the request of the contractors, the city engineer had a boring made
at the center of the shaft to determine the character of the material to
be further penetrated. This boring showed that sand mixed with loam and
gravel would be found for a depth of 26 ft., then would come 15 ft. of
red clay, and finally a layer of hard clay like that penetrated by the
shore end of the tunnel. About the middle of December the contractors
made another attempt to pump the shaft, but finding that the water came
in at the rate of 25 gallons a minute, abandoned the attempt. In the
latter part of February preparations were made to put an air-lock in the
shaft and use compressed air. Hardly had the work been begun by this
system when, on April 20, 1893, a terrific easterly storm swept the top
of the crib bare of the buildings and machinery, and drowned all but one
of the 15 men at work there.

This disaster delayed the work for some time, but in June the
contractors erected a new building and new machinery, and resumed work.
Very little progress was made; and the air escaped so rapidly that it
loosened the sand surrounding the shaft and reduced the friction to such
an extent that on July 28 the entire cylinder lifted bodily about 6 ft.,
and sand rushed in, filling the lower part of the cylinder to within 45
ft. of the lake surface. No further work was done by the contractors
although they submitted a proposition to sink a steel cylinder inside
the cast-iron cylinder and extending from 5 ft. above datum to 100 ft.
below datum for $300 per ft. This proposition was refused by the city;
and since work on the tunnel proper had been abandoned by the
contractors some time before, as had already been described, the city
suspended their contract on Oct. 19.

On Oct. 30 a contract was made with Mr. Thos. Murphy of Milwaukee, Wis.,
to sink a steel cylinder inside the old iron cylinder. The water was
first pumped out of the old cylinder, and a timber bulkhead built at
the bottom. On this the steel cylinder was built, and then the bulkhead
was removed. Air pressure was put on, and the excavation proceeded
successfully until the bottom layer of clay was met with, when all
chances for trouble ceased.

The cylinder, as it was completed, penetrated 9 ft. into the hard clay,
and was underpinned with brickwork for a depth of 29 ft. or more, to a
point 4 ft. below the grade line of the tunnel. At the lower end, the
section of the shaft was changed from a circle to a square. Later the
steel cylinder was lined with brick.

On March 28, 1894, an agreement was made with Mr. Thos. Murphy to
construct the tunnel from the lake shaft toward the shore. Except that
considerable water was encountered, which, owing to inadequate pumping
machinery, filled the tunnel and shaft at two different times, and had
to be removed, no very great difficulty was had with this part of the
work.

On July 28, 1895, the headings from the lake and shore shafts met.
Meanwhile the cast-iron pipe intake, the intake crib, etc., had been
completed, and practically all that remained to be done was to clean the
tunnel and lift the pumping machinery at the shore shaft. During the
cleaning, the air pressure had been kept up on account of the leakage
through the brick lining, and, indeed, the pressure was kept up until
the last possible moment, and everything made ready for removing the
air-locks, bulkheads, pumps, etc., in the least possible time. The pumps
were the last to come out.




CHAPTER XIX.

SUBMARINE TUNNELING (Continued).


THE SHIELD SYSTEM.


=Historical Introduction.=--The invention of the shield system of
tunneling through soft ground is generally accredited to Sir Isambard
Brunel, a Frenchman born in 1769, who emigrated to the United States in
1793, where he remained six years, and then went to England, in which
country his epoch-making invention in tunneling was developed and
successfully employed in building the first Thames tunnel, and where he
died in 1849, a few years after the completion of this great work. Sir
Isambard is said to have obtained the idea of employing a shield to
tunnel soft ground from observing the work of ship-worms. He noticed
that this little animal had a head provided with a boring apparatus with
which it dug its way into the wood, and that its body threw off a
secretion which lined the hole behind it and rendered it impervious to
water. To duplicate this operation by mechanical means on a large enough
scale to make it applicable to the construction of tunnels was the plan
which occurred to the engineer; and how closely he followed his animate
model may be seen by examining the drawings of his first shield, for
which he secured a patent in 1818. Briefly described, this device
consisted of an iron cylinder having at its front end an auger-like
cutter, whose revolution was intended to shove away the material ahead
and thus advance the cylinder. As the cylinder advanced the perimeter of
the hole behind was to be lined with a spiral sheet-iron plating, which
was to be strengthened with an interior lining of masonry. It will be
seen that the mechanical resemblance of this device to the ship-worm, on
which it is alleged to have been modeled, was remarkably close.

In the same patent in which Sir Isambard secured protection for his
mechanical ship-worm he claimed equal rights of invention for another
shield, which is of far greater importance in being the prototype of the
shield actually employed by him in constructing the first Thames tunnel.
This alternative invention, if it may be so termed, consisted of a group
of separate cells which could be advanced one or more at a time or all
together. The sides of these cells were to be provided with friction
rollers to enable them to slide easily upon each other; and it was also
specified that the preferable motive power for advancing the cells was
hydraulic jacks. To summarize briefly, therefore, the two inventions of
Brunel comprehended the protecting cylinder or shield, the closure of
the face of the excavation, the cellular division, the hydraulic-jack
propelling power, and cylindrical iron lining, which are the essential
characteristics of the modern shield system of tunneling. The next step
required was the actual proof of the practicability of Brunel’s
inventions, and this soon came.

Those who have read the history of the first Thames tunnel will recall
the early unsuccessful attempts at construction which had discouraged
English engineers. Five years after Brunel’s patent was secured a
company was formed to undertake the task again, the plan being to use
the shield system, under the personal direction of its inventor as chief
engineer. For this work Brunel selected the cellular shield mentioned as
an alternative construction in his original patent. He also chose to
make this shield rectangular in form. This choice is commonly accounted
for by the fact that the strata to be penetrated by the tunnel were
practically horizontal, and that it was assumed by the engineer that a
rectangular shield would for some reason best resist the pressures which
would be developed. Whatever the reason may have been for the choice,
the fact remains that a rectangular shield was adopted. The tunnel as
designed consisted of two parallel horseshoe tunnels, 13 ft. 9 ins. wide
and 16 ft. 4 ins. high and 1200 ft. long, separated from each other by a
wall 4 ft. thick, pierced by 64 arched openings of 4 ft. span, the whole
being surrounded with massive brickwork built to a rectangular section
measuring over all 38 ft. wide and 22 ft. high.

The first shield designed by Brunel for the work proved inadequate to
resist the pressures, and it was replaced by another somewhat larger
shield of substantially the same design, but of improved construction.
This last shield was 22 ft. 3 ins. high and 37 ft. 6 ins. wide. It was
divided vertically into twelve separate cast-iron frames placed close
side by side, and each frame was divided horizontally into three cells
capable of separate movement, but connected by a peculiar articulated
construction, which is indicated in a general way by Fig. 124. To close
or cover the face of the excavation, poling-boards held in place by
numerous small screw-jacks were employed. Each cell or each frame could
be advanced independently of the others, the power for this operation
being obtained by means of screw-jacks abutting against the completed
masonry lining. Briefly described, the mode of procedure was to remove
the poling-boards in front of the top cell of one frame, and excavate
the material ahead for about 6 ins. This being done, the top cell was
advanced 6 ins. by means of the screw-jacks, and the poling-boards were
replaced. The middle cell of the frame was then advanced 6 ins. by
repeating the same process, and finally the operation was duplicated for
the bottom cell. With the advance of the bottom cell one frame had been
pushed ahead 6 ins., and by a succession of such operations the other
eleven frames were advanced a distance of 6 ins., one after the other,
until the whole shield occupied a position 6 ins. in advance of that at
which work was begun. The next step was to fill the 6-in. space behind
the shield with a ring of brickwork.

[Illustration: FIG. 124.--Longitudinal Section of Brunel’s Shield, First
Thames Tunnel.]

The illustration, Fig. 124, is the section parallel to the vertical
plane of the tunnel through the center of one of the frames, and it
shows quite clearly the complicated details of the shield construction.
Two features which are to be particularly noted are the suspended
staging and centering for constructing the roof arch, and the top plate
of the shield extending back and overlapping the roof masonry so as to
close completely the roof of the excavation and prevent its falling.
Notwithstanding its complicated construction and unwieldy weight of 120
tons, this shield worked successfully, and during several months the
construction proceeded at the rate of 2 ft. every 24 hours. There were
two irruptions of water and mud from the river during the work, but the
apertures were effectually stopped by heaving bags of clay into the
holes in the river bed, and covering them over with tarpaulin, with a
layer of gravel over all. The tunnel was completed in 1843, at a cost of
about $5600 per lineal yard, and 20 years from the time work was first
commenced, including all delays.

[Illustration: FIG. 125.--First Shield Invented by Barlow.]

The next tunnel to be built by the shield system was the tunnel under
London Tower constructed by Barlow and Greathead and begun in 1869. In
1863 Mr. Peter W. Barlow secured a patent in England for a system of
tunnel construction comprising the use of a circular shield and a
cylindrical cast-iron lining. The shield, as shown by Fig. 125, was
simply an iron or steel plate cylinder. The cylinder plates were thinned
down in front to form a cutting edge, and they extended far enough back
at the rear to enable the advance ring of the cast-iron lining to be set
up within the cylinder. In simplicity of form this shield was much
superior to Brunel’s; but it seems very doubtful, since it had no
diametrical bracing of any sort, whether it would ever have withstood
the combined pressure of the screw-jacks and of the surrounding earth in
actual operation without serious distortion, and, probably, total
collapse. It should also be noted that Barlow’s shield made no provision
for protecting the face of the excavation, although the inventor did
state that if the soil made it necessary such a protection could be
used. The patent provided for the injection of liquid cement behind the
cast-iron lining to fill the annular space left by the advancing
tail-plates of the shield. Although Barlow made vigorous efforts to get
his shield used, it was not until 1868 that an opportunity presented
itself. In the meantime the inventor had been studying how to improve
his original device, and in 1868 he secured additional patents covering
these improvements. Briefly described, they consisted in partly closing
the shield with a diaphragm as shown by Fig. 126. The uninclosed portion
of the shield is here shown at the center, but the patent specified that
it might also be located below the center in the bottom part of the
shield. The idea of the construction was that in case of an irruption of
water the upper portion of the shield could be kept open by air
pressure, and work prosecuted in this open space until the shield had
been driven ahead sufficiently to close the aperture, when the normal
condition of affairs would be resumed. This was obviously an improvement
of real merit. The partial diaphragm also served to stiffen the shield
somewhat against collapse, but the thin plate cutting-edges and most of
the other structural weaknesses were left unaltered. To summarize
briefly the improvements due to Barlow’s work, we have: the construction
of the shield in a single piece; the use of compressed air and a partial
diaphragm for keeping the upper part of the shield open in case of
irruptions of water; and the injection of liquid cement to fill the
voids behind the lining.

[Illustration: ~Longitudinal Section.~

~Cross Section.~

FIG. 126.--Second Shield Invented by Barlow.]

Turning now to the London Tower tunnel work, it may first be noted that
Barlow found some difficulty in finding a contractor who was willing to
undertake the job, so little confidence had engineers generally in his
shield system. One man, however, Mr. J. H. Greathead, perceived that
Barlow’s device presented merit, although its design and construction
were defective, and he finally undertook the work and carried it to a
brilliant success. The tunnel was 1350 ft. long and 7 ft. in diameter,
and penetrated compact clay. Work was begun on the first shore shaft on
Feb. 12, 1869, and the tunnel was completed the following Christmas, or
in something short of eleven months, at a cost of £14,500.

The shield used was Barlow’s idea put into practical shape by Greathead.
It consisted of an iron cylinder, or, more properly, a frustum of a cone
whose circumferential sides were very slightly inclined to the axis, the
idea being that the friction would be less if the front end of the
shield were slightly larger than the rear end. The shell of the cone was
made of ¹⁄₂-in. plates. The thinned plate cutting-edge of Barlow’s
shield was replaced by Greathead with a circular ring of cast iron.
Greathead also altered the construction of the diaphragm by arranging
the angle stiffeners so that they ran horizontally and vertically, and
by fastening the diaphragm plates to an interior cast-iron ring
connected to the shell plates. This was a decided structural
improvement, but it was accompanied with another modification which was
quite as decided a retrogression from Barlow’s design. Greathead made
the diaphragm opening rectangular and to extend very nearly from the top
to the bottom of the shield, thus abandoning the element of safety
provided by Barlow in case of an irruption of water. Fortunately the
material penetrated by the shield for the Tower tunnel was so compact
that no trouble was had from water; but the dangerous character of the
construction was some years afterwards disastrously proven in driving
the Yarra River tunnel at Melbourne, Australia. To drive his shield
Greathead employed six 2¹⁄₂-in. screw-jacks capable of developing a
total force of 60 tons. The tails of the jack bore against the completed
lining, which consisted of cast-iron rings 18 ins. wide and ⁷⁄₈ in.
thick, each ring being made up of a crown piece and three segments. The
different segments and rings were provided with double (exterior and
interior) flanges, by means of which they were bolted together. The
soil behind the lining was filled with liquid cement injected through
small holes by means of a hand pump.

[Illustration: FIG. 127.--Shield Suggested by Greathead for the Proposed
North and South Woolwich Subway.]

[Illustration: FIG. 128.--Beach’s Shield Used on Broadway Pneumatic
Railway Tunnel.]

The remarkable success of the London Tower tunnel encouraged Barlow to
form in 1871 a company to tunnel the Thames between Southwark and the
City, and Greathead, in 1876, to project a tunnel under the same
waterway known as the North and South Woolwich Subway. Barlow’s
concession was abrogated by Parliament in 1873, without any work having
been done. Greathead progressed far enough with his enterprise to
construct a shield and a large amount of the iron lining when the
contractors abandoned the work. From the brief description of his shield
given by Greathead to the London Society of Civil Engineers, it
contained several important differences from the shield built by him for
the London Tower tunnel, as is shown by Fig. 127. The changes which
deserve particular notice are the great extension of the shield behind
the diaphragm, the curved form of the diaphragm, and the use of
hydraulic jacks. Greathead had also designed for this work a special
crane to be used in erecting the cast-iron segments of the lining.

[Illustration: FIG. 129.--Shield for City and South London Railway.]

While these works had been progressing in England, Mr. Beach, an
American, received a patent in the United States for a tunnel shield of
the construction shown by Fig. 128, which was first tried practically in
constructing a short length of tunnel under Broadway for the nearly
forgotten Broadway Pneumatic Underground Railway. This shield, as is
indicated by the illustration, consisted of a cylinder of wood with an
iron-cutting-edge and an iron tail-ring. Extending transversely across
the shield at the front end were a number of horizontal iron plates or
shelves with cutting-edges, as shown clearly by the drawing. The shield
was moved ahead by means of a number of hydraulic jacks supplied with
power by a hand pump attached to the shield. By means of suitable valves
all or any lesser number of these jacks could be operated, and by thus
regulating the action of the motive power the direction of the shield
could be altered at will. Work was abandoned on the Broadway tunnel in
1870. In 1871-2 Beach’s shield was used in building a short circular
tunnel 8 ft. in diameter in Cincinnati, and a little later it was
introduced into the Cleveland water-works tunnel 8 ft. in diameter. In
this latter work, which was through a very treacherous soil, the shield
gave a great deal of trouble, and was finally so flattened by the
pressures that it was abandoned. The obviously defective features of
this shield were its want of vertical bracing and the lack of any means
of closing the front in soft soil.

[Illustration: FIG. 130.--Shield for St. Clair River Tunnel.]

[Illustration: ~Longitudinal Section.~

~Cross Section.~

FIG. 131.--Shield for Blackwall Tunnel.]

With the foregoing brief review of the early development of the shield
system of tunneling, we have arrived at a point where the methods of
modern practice can be studied intelligently. In the pages which follow
we shall first illustrate fully the construction of a number of shields
of typical and special construction, and follow these illustrations with
a general discussion of present practice in the various details of
shield construction.

[Illustration: ~Transverse Section.~

~Longitudinal Section.~

FIG. 132.--Elliptical Shield for Clichy Sewer Tunnel, Paris.]

[Illustration: ~Longitudinal Section.~

~Cross Section.~

FIG. 133.--Semi-elliptical Shield for Clichy Sewer Tunnel.]

Mr. Raynald Légouez, in his excellent book upon the shield system of
tunneling, considers that tunnel shields may be divided into three
classes structurally, according to the character of the material which
they are designed to penetrate. In the first class he places shields
designed to work in a stiff and comparatively stable soil, like the
well-known London clay; in the second class are placed those constructed
to work in soft clays and silts; and in the third class those intended
for soils of an unstable granular nature. This classification will, in a
general way, be kept by the writer. As a representative shield of the
first class, the one designed for the City and South London Railway is
illustrated in Fig. 129. The shields for the London Tower tunnel, the
Waterloo and City Railway, the Glasgow District Subway, the Siphons of
Clichy and Concorde in Paris, and the Glasgow Port tunnel, are of the
same general design and construction. To represent shields of the second
class, the St. Clair River and Blackwall shields are shown in Figs. 130
and 131. The shields for the Mersey River, the Hudson River, and the
East River tunnels also belong to this class. To represent shields of
the third class, the elliptical and semi-elliptical shields of the
Clichy tunnel work in Paris are shown by Figs. 132 and 133. The
semi-circular shield of the Boston Subway is illustrated by Fig. 134.

[Illustration: ~Half Transverse Section A-B.~

~Half Rear-End Elevation.~

~Details of Casting Supporting Ends of Jacks.~

~Details of Castings under Ends of Girders.~

~Longitudinal Section C-D.~

FIG. 134.--Roof Shield for Boston Subway.]


=Prelini’s Shield.=--In closing this short review mention will be made
of a new shield designed and patented by the Author and shown in Fig.
135. It is an articulated shield composed of two separated shields whose
outer shells overlap each other. The shields are connected together by
means of hydraulic jacks attached all around the two diaphragms. Between
these diaphragms is a large inclosed space called a safety chamber,
where the men can withdraw in case of accidents and where the air can be
immediately raised to the required pressure. This is an advantage in
case of blow-outs, because the flooding of the tunnel is prevented,
while the accident is limited to only a few feet from the front. On
account of the shield being advanced half at a time it is always under
control and is thus better directed through grade and alignment.
Besides, this shield will not rotate around its axis and consequently it
can be built of any shape, thus permitting the construction of
subaqueous tunnels of any cross-section and even with a wider
foundation, which is impossible to-day with the ordinary rotating
shields of circular cross-section.

[Illustration: FIG. 135.--Transversal and Longitudinal Section of
Prelini’s Shield.]


SHIELD CONSTRUCTION.


=General Form.=--Tunnel shields are usually cylindrical or
semi-cylindrical in cross-section. The cylinder may be circular,
elliptical, or oval in section. Far the greater number of shields used
in the past have been circular cylinders; but in one part of the sewer
tunnel of Clichy, in Paris, an elliptical shield with its major axis
horizontal, was used, and the German engineer, Herr Mackensen, has
designed an oval shield, with its major axis vertical. A semi-elliptical
shield was employed on the Clichy tunnel, and semi-circular shields were
used on the Baltimore Belt Line tunnel and the Boston Subway in America.
Generally, also, tunnel shields are right cylinders; that is, the front
and rear edges are in vertical planes perpendicular to the axis of the
cylinder. Occasionally, however, they are oblique cylinders; that is,
the front or rear edges, or both, are in planes oblique to the axis of
the cylinder. One of these visor-shaped shields was employed on the
Clichy tunnel.


=The Shell.=--It is absolutely necessary that the exterior surface of
the shell should be smooth, and for this reason the exterior rivet heads
must be countersunk. It is generally admitted, also, that the shell
should be perfectly cylindrical, and not conical. The conical form has
some advantage in reducing the frictional resistance to the advance of
the shield; but this is generally considered to be more than
counterbalanced by the danger of subsidence of the earth, caused by the
excessive void which it leaves behind the iron tunnel lining. For the
same reason the shell plate, which overlaps the forward ring of the
lining, should be as thin as practicable, but its thickness should not
be reduced so that it will deflect under the earth pressure from above.
Generally the shell is made of at least two thicknesses of plating, the
plates being arranged so as to break joints, and, thus, to avoid the use
of cover joints, to interrupt the smooth surface which is so essential,
particularly on the exterior. The thickness of the shell required will
vary with the diameter of the shield, and the character and strength of
the diametrical bracing. Mr. Raynald Légouez suggests as a rule for
determining the thickness of the shell, that, to a minimum thickness of
2 mm., should be added 1 mm. for every meter of diameter over 4 meters.
Referring to the illustrations, Figs. 128 to 132 inclusive, it will be
noted that the St. Clair tunnel shield, 21¹⁄₂ ft. in diameter, had a
shell of 1-in. steel plates with cover-plate joints and interior angle
stiffeners; the shell of the East River tunnel shield, 11 ft. in
diameter, was made up of one ¹⁄₂-in. and one ³⁄₈-in. plate; the
Blackwall tunnel shield, 27 ft. 9 ins. in diameter, had a shell
consisting of four thicknesses of ⁵⁄₈-in. plates; and the Clichy tunnel
shield, with a diameter of 2.06 meters, had a shell 2 millimeters
thick.


=Front-End Construction.=--By the front end is meant that portion of the
shield between the cutting-edge and the vertical diaphragm. The length
of this portion of the shield was formerly made quite small, and where
the material penetrated is very soft, a short front-end construction yet
has many advocates; but the general tendency now is to extend the
cutting-edge far enough ahead of the diaphragm to form a fair-sized
working chamber. Excavation is far more easy and rapid when the face can
be attacked directly from in front of the diaphragm than where the work
has to be done from behind through the apertures in the diaphragm. So
long as the roof of the excavation is supported from falling, experience
has shown that it is easily possible to extend the excavation safely
some distance ahead of the diaphragm. In reasonably stable material,
like compact-clay, the front face will usually stand alone for the short
time necessary to excavate the section and advance the shield one stage.
In softer material the face can usually be sustained for the same short
period by means of compressed air; or the face of the excavation,
instead of being made vertical, can be allowed to assume its natural
slope. In the latter case a visor-shaped front-end construction, such as
was used on some portions of the Clichy tunnel, is particularly
advantageous. The following figures show the lengths of the front ends
of a number of representative tunnel shields.

  City and South London    1     ft.
  St. Clair River         11.25   „
  Hudson River             5²⁄₃   „
  Mersey River             3      „
  East River               3²⁄₃   „
  Blackwall                6.5    „

Two general types of construction are employed for the cutting-edge. The
first type consists of a cast-iron or cast-steel ring, beveled to form a
chisel-like cutting-edge and bolted to the ends of the forward shell
plates. This construction was first employed in the shield for the
London Tower tunnel, and has since been used on the City and South
London, Waterloo and City, and the Clichy tunnels. The second
construction consists in bracing the forward shell plates by means of
right triangular brackets, whose perpendicular sides are riveted
respectively to the shell plates and the diaphragm, and whose inclined
sides slant backward and downward from the front edge, and carry a
conical ring of plating. The shields for the St. Clair River, East
River, and Blackwall tunnels show forms of this type of cutting-edge
construction. A modification of the second type of construction, which
consists in omitting the conical plating, was employed on some of the
shields for the Clichy tunnel. This modification is generally considered
to be allowable only in materials which have little stability, and which
crumble down before the advance of the cutting-edge. Where the material
is of a sticky or compact nature, into which the shield in advancing
must actually cut, the beveled plating is necessary to insure a clean
cutting action without wedging or jamming of the material.


=Cellular Division.=--It is necessary in shields of large diameter to
brace the shell horizontally and vertically against distortion. This
bracing also serves to form stagings for the workmen, and to divide the
shield into cells. The following table shows the arrangement of the
vertical and transverse bracing in several representative tunnel
shields.

  +------------------+----------+-------+-------+-------+
  |  NAME OF TUNNEL. | DIAMETER.| HORI- |PLATES,| VERT. |
  |                  |          |ZONTAL.| DIST. |BRACES.|
  |                  |          |       | APART.|       |
  +------------------+----+-----+-------+-------+-------+
  |                  |Ft. | In. |  No.  |  Ft.  |  No.  |
  |Hudson River      |19  |11   |   2   |  6.54 |   2   |
  |Clichy            |19.4| 0   |   2   |  6.54 |  None |
  |St. Clair River   |21  | 6   |   2   |  6.98 |   3   |
  |Waterloo (Station)|24  |10¹⁄₂|   2   |  7.12 |  None |
  |Blackwall         |27  | 8   |   2   |  6.0  |   3   |
  |East River        |11  |  ³⁄₄| None  |  ...  |   1   |
  +------------------+----+-----+-------+-------+-------+

Referring first to the horizontal divisions, it may be noted that they
serve different purposes in different instances. In the Clichy tunnel
shield the horizontal divisions formed simply working platforms; in the
Waterloo tunnel shield they were designed to abut closely against the
working face by means of special jacks, and so to divide it into three
separate divisions; in the St. Clair tunnel they served as working
platforms, and also had cutting-edges for penetrating the material
ahead; and in the Blackwall tunnel shield they served as working
platforms, and had cutting-edges as in the St. Clair tunnel shield, and
in addition the middle division was so devised that the two lower
chambers of the shield could be kept under a higher pressure of air than
the two upper chambers. Passing now to the vertical divisions, they
serve to brace the shell of the shield against vertical pressures, and
also to divide the horizontal chambers into cells; but unlike the
horizontal plates they are not provided with cutting-edges. The St.
Clair, Hudson River, and Blackwall tunnel shields illustrate the use of
the vertical bracing for the double purpose of vertical bracing and of
dividing the horizontal chambers into cells. The Waterloo tunnel shield
is an example, of vertical bracing employed solely as bracing. The
vertical division of the East River tunnel shield was employed in order
to allow the shield to be dissembled in quadrants.


=The Diaphragm.=--The purpose of the shield diaphragm is to close the
rear end of the shield and the tunnel behind from an inrush of water and
earth from the face of the excavation. It also serves the secondary
purpose of stiffening the shell diametrically. Structurally the
diaphragm separates the front-end construction previously described from
the rear-end construction, which will be described farther on; and it is
usually composed of iron or steel plating reinforced by beams or
girders, and pierced with one or several openings by which access is had
to the working face. In stable material, where caving or an inrush of
water and earth is not likely, the diaphragm is omitted. The shield of
the Waterloo tunnel is an example of this construction. In more
treacherous materials, however, not only is a diaphragm necessary, but
it is also necessary to diminish the size of the openings through it,
and to provide means for closing them entirely. Sometimes only one or
two openings are left near the bottom of the diaphragm, as in the St.
Clair and Mersey tunnel shields; and sometimes a number of smaller
openings are provided, as in the East River and Hudson River tunnel
shields.

In highly treacherous materials subject to sudden and violent irruptions
of earth from the excavation face, it sometimes is the case that
openings, however small, closed in the ordinary manner, are
impracticable, and special construction has to be adopted to deal with
the difficulty. The shields for the Mersey and for the Blackwall tunnels
are examples of such special devices. In the Mersey tunnel a second
diaphragm was built behind the first, extending from the bottom of the
shield upward to about half its total height. The aperture in the first
diaphragm being near the bottom, the space between the second and first
diaphragms formed a trap to hold the inflowing material. The Blackwall
tunnel shield, as previously indicated, had its front end divided into
cells. Ordinarily the face of the excavation in front of each cell was
left open, but where material was encountered which irrupted into these
cells a special means of closing the face was necessary. This consisted
of three poling-boards or shutters of iron held one above the other
against the face of the excavation. These shutters were supported by
means of strong threaded rods passing through nuts fastened to the
vertical frames, which permitted each shutter to be advanced against or
withdrawn from the face of the excavation independently of the others.
Various other constructions have been devised to retain the face of the
excavation in highly treacherous soils, but few of them have been
subjected to conclusive tests, and they do not therefore justify
consideration.


=Rear-end Construction.=--By the rear end of the shield is meant that
portion at the rear of the diaphragm. It may be divided into two parts,
called respectively the body and the tail of the shield. The chief
purpose of the body of the shield is to furnish a place for the location
of the jacks, pumps, motors, etc., employed in manipulating the shield.
It also serves a purpose in distributing the weight of the shield over a
large area. To facilitate the passage of the shield around curves, or
in changing from one grade to another, it is desirable to make the body
of the shield as short as possible. In the Mersey, Clichy, and Waterloo
tunnel shields, and, in fact, in most others which have been employed,
the shell plates of the body have been reinforced by a heavy cast-iron
ring, within and to which are attached the jacks and other apparatus.
The latest opinion, however, seems to point to the use of brackets and
beams for strengthening the shell for the purpose named, rather than to
this heavy cast-iron construction. In the Hudson River, St. Clair River,
and East River tunnel shields, with their long and strongly braced
front-end construction to carry the jacks, the body of the shield, so to
speak, is omitted and the rear-end construction consists simply of the
tail plating. In the Blackwall shield, the body of the shield shell
provides the space necessary for the double diaphragms and the cells
which they inclose. In a general way, it may be said that the present
tendency of engineers is to favor as short and as light a body
construction as can be secured.

The tail of the shield serves to support the earth while the lining is
being erected; and for this reason it overlaps the forward ring of the
lining, as shown clearly by most of the shields illustrated. To fulfill
this purpose, the tail-plates should be perfectly smooth inside and
outside, so as to slide easily between the outside of the lining plates
and the earth, and should also be as thin as practicable, in order not
to leave a large void behind the lining to be filled in. In soils which
are fairly stable, the tail construction is often visor-shaped; that is,
the tail-plates overlap the lining only for, say, the roof from the
springing lines up, as in one of the shields for the Clichy tunnel. In
unstable materials the tail-plating extends entirely around the shield
and excavation. The length of the tail-plating is usually sufficient to
overlap two rings of the lining, but in one of the Clichy tunnel shields
it will be noticed that it extended over three rings of lining. This
seemingly considerable space for thin steel plates is made possible by
the fact that the extreme rear end of the tail always rests upon the
last completed ring of lining.

In closing these remarks concerning the rear-end construction, the
accompanying table, prepared by Mr. Raynald Légouez, will be of
interest, as a general summary of principal dimensions of most of the
important tunnel shields which have been built. The figures in this
table have been converted from metric to English measure, and some
slight variation from the exact dimensions necessarily exists. The
different columns of the table show the diameter, total length, and the
length of each of the three principal parts into which tunnel shields
are ordinarily divided in construction as previously described:

  +---------------------+-----------------------------------+
  |                     |         LENGTH IN FEET.           |
  |   NAME OF SHIELD.   +---------+-----+-----+------+------+
  |                     |DIAMETER.|TAIL.|BODY.|FRONT.|TOTAL.|
  +---------------------+---------+-----+-----+------+------+
  |Concorde Siphon      |   6.75  | 2.51| 2.55|  1.16|  6.67|
  |Clichy Siphon        |   8.39  | 2.51| 2.55|  1.16|  6.16|
  |Mersey               |   9.97  | 5.61| 2.98|  2.98| 11.58|
  |East River           |  10.99  | 3.51| 0.32|  3.67|  7.51|
  |City and South London|  10.99  | 2.65| 2.82|  1.01|  6.49|
  |Glasgow District     |  12.07  | 2.65| 2.82|  1.01|  6.49|
  |Waterloo and City    |  12.99  | 2.75| 2.98|  1.24|  6.98|
  |Glasgow Harbor       |  17.25  | 2.75| 2.98|  1.08|  8.49|
  |Hudson River         |  19.91  | 4.82| 2.98|  5.67| 10.49|
  |St. Clair River      |  21.52  | 4.00| 2.98| 11.25| 15.25|
  |Clichy Tunnel        |23.7-19.8| 4.00| 2.98|  6.88| 17.22|
  |Clichy Tunnel        |23.8-19.4| 7.44|11.90|  4.46| 23.65|
  |Blackwall            |  27.00  | 6.98| 5.90|  6.59| 19.48|
  |Waterloo Station     |  24.86  | 3.34| 5.51|  1.14| 10.00|
  +---------------------+---------+-----+-----+------+------+

A shield of 60 or 100 tons weight can hardly be directed along the line
of the proposed tunnel and also through curves and grades, especially
when driven through loose or muddy soils. The tunnels of the New York
and Hudson River Railroad under the Hudson, and the tunnel of the New
York Rapid Transit Railway under the East River, show marked evidence of
how troublesome this work is. To avoid these and other inconveniences
encountered in every shield, the Author has designed a new shield which
was briefly described at page 251.

[Illustration: FIG. 136.--Elevation and Section of Hydraulic Jack, East
River Gas Tunnel.]


=Jacks.=--The motive power usually employed in driving modern tunnel
shields is hydraulic jacks. In some of the earlier shields screw-jacks
were used, but these soon gave way to the more powerful hydraulic
device. The manner of attaching the hydraulic jacks to the shield is
always to fasten the cylinder castings at regular intervals around the
inside of the shell, with the piston rods extending backward to a
bearing against the forward edge of the lining. In the older forms of
shield, having an interior cast-iron reinforcing ring construction, the
jack cylinder castings were always attached to this cast-iron ring; but
in many of the later shields constructed without this cast-iron
reinforcing ring, the cylinder castings are attached to the shell by
means of bracket and gusset connections. The number and size of the
jacks employed, and the distance apart at which they are spaced, depend
upon the size of the shield and the character of the material in which
it is designed to work. In stiff and comparatively stable clays, the
skin friction of the shield is comparatively small, and an aggregate
jack-power of from 4 to 5 tons per square yard of the exterior friction
surface of the shield has usually been found ample. The cylinders are
spaced about 5³⁄₄ ft. apart, and have a working diameter of from 5 to 6
ins., with a water pressure of about 1000 lbs. per sq. in. In soft,
sticky material, giving a high skin friction, the aggregate jack-power
required per square yard of exterior shell surface rises to from 18 to
24 tons; the jacks are spaced about 3 ft. apart; and the working
cylinder diameter and water pressure are, respectively, about 6 or 7
ins., and from 4000 lbs. to 6000 lbs. per sq. in. With these high
pressures, power pumps are necessary to give the required water
pressure; but where the pressure required does not exceed 1000 lbs. per
sq. in., hand pumps may be, and usually are, employed. Fig. 136 shows
the hydraulic jacks used in the East River Gas Tunnel at New York. The
number of jacks required depends upon the diameter of the shield, and,
of course, upon the distance apart which they are placed. In the City
and South London tunnel shield six jacks were used, and in the Blackwall
shield 24 were used. The mechanical construction of the jacks for tunnel
shields presents no features out of the usual lines of such devices used
elsewhere. The jacks used on the East River tunnel shield are shown by
Fig. 136.

Two general methods are employed for transmitting the thrust of the
piston rods against the tunnel lining. The object sought in each is to
distribute the thrust in such a manner that there is no danger of
bending the thin front flange of the forward lining ring. In English
practice the plan usually adopted is to attach a shoe or bearing casting
to the end of the piston rod, which will distribute the pressure over a
considerable area. An example of this construction is the shield for the
City and South London tunnel. In the East River and St. Clair River
tunnels built in America, the tail of the piston rod is so constructed
that the thrust is carried directly to the shell of the lining.


LINING.

Either iron or masonry may be used for lining shield-driven tunnels but
present practice is almost universally in favor of iron lining. As
usually built, iron lining consists of a series of successive cast-iron
rings, the abutting edges of which are provided with flanges. These
flanges are connected by means of butts, the joints being packed with
thin strips of wood, oakum, cement, or some other material to make them
water-tight. Each lining ring is made up of four or more segments, which
are provided with flanges for bolted connections similar to those
fastening the successive rings. Generally the crown segment is made
considerably shorter than those forming the sides and bottom of the
ring. The erection of the iron segments forming the successive rings of
the lining may be done by hand in tunnels of small diameter where the
weights to be handled are comparatively light, but in tunnels of large
size special cranes attached to the shield or carried by the finished
lining are employed. The construction of the iron lining for the Hudson
River tunnel is illustrated in Chapter XX., and that for the St. Clair
River tunnel is shown by Fig. 137.

[Illustration: ~Part Transverse Section.~

~Longitudinal Section.~

FIG. 137.--Cast-Iron Lining, St. Clair River Tunnel.]




CHAPTER XX.

SUBMARINE TUNNELING (Continued).

THE SHIELD AND COMPRESSED AIR METHOD. THE HUDSON RIVER TUNNEL OF THE
PENNSYLVANIA RAILROAD.


The shield and compressed air method of excavating subaqueous tunnels is
used when the distance is small between the roof of the tunnel and the
bed of the river. These tunnels are usually driven from the shafts sunk
from each shore. It is very seldom they can be driven also by an
intermediate shaft. This, however, was done in the case of the Belmont
tunnel under the East River. Here the tunnels passed under the
man-of-war reef where a working shaft was sunk.

The plant is located at some convenient point near the head shaft. It
consists of a set of boilers to provide the power for the different
machines. They are low and high pressure compressors, the former supply
the air through the tunnel; the latter, the air for working the drills,
in case rock is encountered, and power for hauling and hoisting
purposes. The various pumps force the water for the hydraulic rams that
drive the shield and work the erector. They also remove the water from
the tunnel which always collects in variable quantities at the bottom of
the excavation. Besides the machines for light and ventilation purposes,
the head shaft is provided with an overhead construction where are
housed the hoisting machines, the telephone and other means of
communication with the work at the front. Usually a long trestle is
built in connection with the head shaft, leading to the dumping place
and yard. On this inclined elevated structure are located, also, the
tracks upon which will run the small cars used inside the tunnel for
hauling purposes.

The shafts are excavated on a square, rectangular or circular plan and
are usually lined with masonry. It is only recently that shafts
excavated through loose soils have been lined with the same cast-iron
lining used in the tunnels, the only difference being that the rings
were laid flat on the ground and attached to those already sunk.

After the shaft has been sunk to the required level, the tunnel is
driven toward the river by any one of the methods used for land work. At
some convenient distance from the shaft, the dimensions of the tunnel
are enlarged for a length of 20 or 30 ft. In this larger space, called
the shield chamber, the shield is assembled, mounted, and, when
completed, it is slowly pushed toward the river. The tunnel is excavated
from the shield chamber on, with dimensions equal to the exterior shell
of the shield.

The construction of the shield and the hydraulic jacks used for its
advance are explained in a preceding chapter.

In very loose soils, a solid bulkhead of masonry is built across the
tunnel, after the shield has advanced to a certain distance and some
rings of the cast-iron lining have been erected. The bulkhead is
provided with three air locks--two near the floor of the tunnel, for
working purposes, and one near the roof, called the emergency lock,
which, as the name suggests, is used only in case of danger. The air
locks are steel cylinders from 10 to 15 ft. long and 6 ft. in diameter,
made up of boiler plates. They are provided with doors at each end,
besides the pipes for the admission and exit of compressed air. The
working locks also have narrow-gauge tracks for hauling purposes. In
rock or more consistent soil the bulkhead is constructed after the
shield is far ahead, since there is no immediate necessity, under these
conditions, to use the compressed air. In both the loose and good soils,
when the shield has been advanced over 500 ft. from the bulkhead, a
second bulkhead, with air locks, is erected in the tunnel. The first is
left in place but used only in case of emergency.

To direct the shield along the center line and through curves and
grades, accurate measurements are taken, and the distance between the
shield and the last ring inserted in the iron lining is regulated
accordingly. The alignment inside the tunnel is maintained in a very
simple way. For this purpose, points corresponding to the center line
are marked on the roof at distances of 100 ft. Nearly 100 ft. from the
shield, a transit is set up on a strong scaffold spanning the tunnel,
and it is supported by the flanges of the iron lining. A plumb-line is
hung from one of the points of the roof already determined, as
indicating the center line; and the transit man aligns his instrument
with this plumb-line; after this he “plunges” his telescope. A rodman
next places a horizontal rod of special construction between the flanges
of the last ring of the lining. This rod has in the center an open slot
which carries a glass with a black vertical line. The slot is graduated,
the zero of graduation remains in the center while the vertical line is
moved right and left. The rodman places a lamp behind the slot and the
transit-man tells him how to move the dark line until it coincides with
the axis of the tunnel. If the ring, just erected, be a little out of
alignment, it is readjusted by pushing the shield a little more on the
side that has swerved from the axis of the tunnel. As the shield is
pushed forward, it is kept in place by four men with graduated rods, one
man on each side of the shield, one on top and the other on the floor.
As the shield progresses, they repeat aloud in succession, the distance
indicated on the rods, which is the distance from the shield to the
outer circumferential flange of the last ring of the lining. When an
advance of one foot has been made, readings are taken at every inch; and
when very near the required distance, they are taken at every quarter of
an inch. In this way it is not difficult to bring the shield back into
line, in case it may have shifted a little to the right or left. When
curves are met, the rings are no longer cylindrical segments but tores,
so that the segments at one side are longer than those on the other. In
this case, the shield is advanced more on one side by a quantity equal
to the difference of the two sides of the ring to be erected. At each
advance the shield is moved 2 ft. or 2¹⁄₂ ft. ahead, the distance
corresponding to the length of the cast-iron rings of the lining. Within
the space now open between the shield and the lining another ring is
inserted. The ring is composed of different segments provided with
flanges and holes bored so they can be bolted together. The segments of
the lining are very heavy and difficult to handle but they are easily
set by means of the erector.

When the erector is not mounted on the shield, it is located in the
middle of a girder placed across the iron rings of the lining and just
at the rear end of the shield. The girder, at both extremities, has
flanged wheels resting on rails which are placed on brackets. These
brackets are attached temporarily to the flanges of the iron lining. The
erector is provided with an arm capable to swing in a full circle. Its
movements are regulated by two hydraulic jacks, located horizontally on
the spanning girder. On the extreme end of the revolving arm are
projections with holes for the bolts. Each segmental plate of the lining
has a kind of plug in the center which is cast together with the plate
and is provided with holes for the bolt. In placing the segmental plates
of the lining, the arm of the erector is swung over the plate to be
lifted, then two bolts are passed through the holes in the projection of
the erector, and through those in the plug. The arm of the erector is
then moved upwards until the plate, free from all obstacles, is swung
very near its intended position. There it is adjusted and held until
bolts are inserted to fix it to the plates of the preceding ring.

In connection with the method of excavating submarine tunnels by means
of shield and compressed air, the excavation varies with the quality of
soil encountered. In compact rock the usual heading and bench method, so
common in land tunnels, is also employed in this case. The shield is
left behind in presence of good rock.

The men at the front attack the rock with air drilling machines and
charges of dynamite. The holes are driven at a smaller depth than in
land work; very light charges of dynamite are used and only a few holes
fired at each round. Every precaution is taken in order not to disturb
the shield and the bed of the river any more than is possible, because
at a shallow depth the blast would tend to widen the existing crevices
in the rock and thus permit an inflow of water. When the rock is
fissured or disintegrated and the roof of the excavation at the front
requires timbering, the shield should be kept closer to the front. In
this way the quantity of timber for strutting is greatly reduced, so
lessening the probabilities of fires. It is very difficult, in
compressed air, to extinguish fires and in almost every instance the
only way is to flood the tunnel. This was done at the Manhattan end of
the tunnel under the East River for the extension to Brooklyn of the New
York Subway.

The excavation is made by hand in loose but compact soils such as clay.
The men work on platforms located at the front of the shield and they
are protected from the caving-in of the roof by a hood added for working
through loose soils. The men excavate the material which is shoveled
inside the tunnel and is carried away in small cars. The shield is very
close to the front of the excavation in loose soil. The East Boston
tunnel, under Boston Harbor, connecting with the Boston Subway, was
excavated through blue clay. The minimum distance between the bottom of
the water and the roof of the excavation was 18 ft. The tunnel was
excavated by means of compressed air and the shield which was only used
for the roof. It slid on top of concrete side walls built in two drifts
which were excavated nearly 100 ft. ahead of the shield. The tunnel was
lined with concrete, the arch being reinforced by longitudinal steel
rods which received the thrust of jacks used for advancing the shield.
The material in the drifts under the shield and the bench was removed by
hand and carried away in small cars.

Subaqueous tunnels driven through very loose soils can be excavated by
simply leaving the doors open while the shield is pushed ahead. The
material, dislodged by the cutting edge of the shield, is forced through
the doors and falls on the floor whence it is removed in small cars. In
very loose soils the excavation has been made in a still more economic
way; the shield with closed doors is simply squeezed through the soil.
This method is financially convenient, because all the excavating and
hauling operations are eliminated and the tunnel progresses from 40 to
50 ft. per day, but clearly indicates a lack of stability. In this
manner, the Hudson River tunnel of the New York and New Jersey Railroad
was constructed.

The pressure of the air in the tunnel depends upon the depth and as a
rule it varies between 20 and 40 or even more pounds per square inch
above atmospheric pressure. Working in compressed air causes a peculiar
disease commonly known as “bends” or “caisson disease” often proving
fatal. To prevent and remedy the disease, the engineers should order a
set of rules to be strictly observed. The preventative measures should
be, first, to employ only sober, strong and healthy men, never one who
has not successfully passed the examination of the attending physician;
second, to order the lock tenders never to allow any man in or out of
the tunnel unless he has spent at least ten minutes within the locks.
Both compression and decompression should be thorough and it cannot be
in less than this time. A stop of only a few minutes in the locks is not
sufficient and this incomplete compression or decompression is the real
cause of the bends. The men become careless after they have been in the
compressed air for some time, and they try to reduce this tiresome
operation to a minimum, hence the duty of the engineer to strictly
enforce this rule. The remedial measures should consist of constant
medical attendance near the shafts and the erection of a compressed air
hospital where the men affected by bends for lack of decompression may
be attended and cured.


THE HUDSON RIVER TUNNELS OF THE PENNSYLVANIA RAILROAD.[13]

The tunnels constructed under the Hudson River for the Pennsylvania
Railroad, consist of two parallel tubes driven side by side 14 ft.
apart. The tubes are of circular cross-section, 23 ft. exterior
diameter, and are lined with cast-iron rings. The tunnels were driven
from two shafts, one on the eastern shore of the Hudson River near 32nd
St. and 11th Ave., New York; the other at Weehawken, New Jersey, near
the piers of the Erie Railroad. The horizontal distance between the
shafts was 6550 ft. The permanent one at Weehawken was built on a square
plan, 130 ft. to a side. It was lined with concrete masonry and the
walls were battered in such a way as to become the shape of an inverted
frustum of a pyramid. It was provided with five openings at the bottom,
four of these are used by trains that run in the open, the fifth one
leads to a power house near by. During the construction of the tunnels
one-third of this shaft was used for the land portion of the tunnel
under Bergen Hill, while the remaining two-thirds were devoted to the
construction of the tunnel under the river. The working shaft on
Manhattan Island was a side shaft of rectangular plan 30 ft. by 22 ft.,
the tunnel proper being connected by two drifts 10 ft. by 10 ft. each.
The shield rooms 23 ft. long, were situated on both sides of the river
just in front of the shafts. On the New York side, the shields, one for
each tube, were built inside the iron lining of the shield chamber, and
the hoisting tackle was slung from the iron lining. The erection on the
Weehawken side was done in the bare rock excavation where timber
falsework was used. After the shields were finished and in position, the
first two rings of the lining were erected in the tail of the shield.
These rings were firmly braced to the rock and the chamber lining; then
the shields were shoved ahead by their own jacks, another ring was built
and so on.

  [13] Condensed from paper by James Forgie, “Eng. News,” Vol. LVI, and
  by H. B. Hewett and W. L. Brown, “Proc. Am. Soc. C. E.”, Vol. XXXVI.

[Illustration: ~Rear Elevation of Shield.~

~Vertical Section.~

~Half Section A-B.~

~Half Section C-D.~

~Horizontal Section.~

FIG. 138.--General Elevations and Sections of Shield.]


=Shield.=--The shields used in these tunnels were designed by Mr. James
Forgie, M. Inst. C. E. and M. Am. Soc. C. E., and were provided with
three innovations: the segmental doors, the sliding platforms and the
removable hood. The shields, Fig. 138, were circular, 23 ft. 6¹⁄₄ ins.
in external diameter, and were 16 ft. long, exclusive of the hood. The
tail of the shield overlapped the lining, the maximum being 6 ft. 4¹⁄₂
ins. during ordinary working; the minimum, 2 ft. during the operation of
taking any ram out for repairing. The shields had only one transverse
bulkhead made up of two continuous horizontal platforms and three
vertical partitions stiffening angular web plates fore and aft the ram
chambers. They were connected by angles and skin plates which formed a
ring-shaped frame 25 ins. thick radially and nearly 5 ft. long. Between
the vertical and horizontal partitions were left openings which either
were partially or entirely closed by segmental doors pivoted on an axis
parallel to the face of the shield bulkhead. There were nine of such
openings on each shield, the clear width being 2 ft. 7 ins., the height
varying from 2 ft. 2 ins. to 3 ft. 4 ins., according to the location.
The hood at the front of the shield was designed so as to be detached
underground and was made of complete segments to permit easy erection or
detachment. The hood was extended as far as the upper platform, thus
protecting only the roof of the excavation. It was attached to the
shield by means of bolts, and, when removed, it was replaced by the
cast-steel cutting-edge, built in 24 sections and placed all around the
shield. The eight sliding platforms, another characteristic of this
shield, could be extended 2 ft. 9 ins. in front of the shield by means
of hydraulic rams, and, when so extended, were able to stand a pressure
of 7900 lbs. per sq. ft. These sliding platforms were used as hoods for
the protection of the men working through loose soils, while in rock
they enabled the drilling and blasting to be carried on at three levels.
A water trap or bird fountain was constructed, at the rear of the
bulkhead of the shield, by means of angle irons to which steel plates
were bolted. The opening to the face was so spacious that in an
emergency the men could readily escape by getting over this trap into
safety. Besides, with the assistance of compressed air, it was
sufficient to perfectly trap the water-bearing ground, in case the face
collapsed. Including rams and erector, the total weight of the shield
was 193 tons.


=Hydraulic Rams.=--The shield was operated by hydraulic pressure. The
machines were designed for a maximum pressure of 5000 lbs., to a
minimum of 2000 lbs., while the average working pressure was 3500 lbs.
per sq. in. The forward movement of the shield was obtained by means of
24 single-acting rams 8¹⁄₂ in. in diameter and with 38 in. stroke. Each
ram exerted a pressure of nearly 100 tons, so that the combined action
of the 24 rams was equal to 2400 tons. Each sliding platform was
operated by two single-acting rams 3¹⁄₂ ins. in diameter and with 2 ft.
9 in. stroke. The rams were attached to the rear face of the shield and
the front ends of the cylinders to the front ends of the sliding
platforms, and since the cylinders were movable and free-sliding so also
were the platforms.


=Erector.=--The erector, a box-shaped frame mounted on a central shaft,
revolved in bearings attached to the shield. Inside this frame there was
a differential hydraulic plunger of 4 in. and 3 in. diameters and 48 in.
stroke. To the plunger head were attached two channels which slide
inside the box frame and to the projecting ends of which the grip was
attached. At the opposite end of the box frame was attached a
counter-weight which balances about 700 lbs. of the tunnel segment at 11
ft. radius. The erector was revolved by two single-acting rams fixed
horizontally to the back of the shield, above the erector pivot, through
double chains and chain wheels which were keyed to the erector shaft.


=Air Locks.=--Two bulkhead walls, forming the rear closure of the
pneumatic sections, were built in each end of each tunnel, one just
ahead of the shield chamber, the other about 1200 ft. ahead of the
first. The walls were built of Portland concrete 10 ft. thick, and they
were grouted with Portland cement, under a pressure of nearly 100 lbs.
per sq. in., to make them thoroughly air-tight. Each wall had in it
three locks; for man, material and emergency. Each was equipped with
hand valves arranged to be operated from either outer end or from
within. The floors of the man and material locks were on a level with
the working platform of the tunnel, about 3 ft. 6 ins. above the invert;
the floor of the emergency lock was about 5 ft. above the horizontal
axis of the tunnel. The locks were made of steel plates and shapes, with
iron fittings riveted and bolted together. The man lock was 11 ft. long
of elliptical cross-section, 6 ft. vertical diameter and 5 ft.
horizontal; the material lock was 25 ft. long, with circular
cross-section, 7 ft. diameter, and the emergency lock was 20 ft. long,
of elliptical cross-section, 4 ft. vertical and 3 ft. horizontal
diameters. Fig. 139 shows the elevation of the air lock used in the
Pennsylvania tunnel.

[Illustration: ~Sectional Elevation~

~Horizontal Section~

FIG. 139.--Plan and Elevation of First Bulkhead Wall in South Tube
Manhattan.]


=Excavation.=--In driving these tunnels almost any kind of material was
encountered, viz., rock, partly rock, and partly loose soil, sand and
gravel, and finally silt.


=Rock.=--Much of the rock excavation was made before the shields were
erected in order to avoid the handling of rock through the narrow
openings of the shield doors. Throughout the cross-section the shield
traveled on a cradle of concrete in which 2 or 3 steel rails were
imbedded. At the points where the excavation had been made for the full
section of the tunnel, it was only necessary to trim off the projecting
corners of rock. Where only the bottom heading had been driven the
excavation was completed just in front of the shield; the drilling below
the axis level being done from the heading itself, and above that from
the front sliding platforms of the shield. The holes were placed near
together and were drilled short; very light charges of powder were used
in order to lessen the chance of knocking the shield about too much.


=Mixed Face.=--When the rock dipped to such an extent that the front of
the tunnel was excavated partly in rock and partly in loose soil, the
compressed air was turned on, starting with a pressure varying from 12
to 18 lbs. When the surface of the rock was penetrated, the soft face
was held up at first by horizontal boards braced from the shield until
the shield was shoved. The braces were then taken out and, after the
shield had been shoved, were replaced by others. As the amount of soft
ground in the surface increased, the system of timbering was gradually
changed to one of 2-in. poling-boards. These rested on top of the shield
and were supported by vertical breast-boards which in turn were held by
6-in. by 6-in. walings, braced through the upper doors to the iron
lining and from the sliding platforms of the shield.


=Sand and Gravel.=--Sand and gravel were only met at Weehawken, where
two different methods were used. The first method was employed when the
roof of the excavation was through sand. It consisted of excavating the
ground 2 ft. 6 ins. ahead of the cutting-edge, the roof being held in
place by longitudinal poling-boards. These boards rested on the outside
of the skin at their back end, and at the forward end on vertical
breast-boards, braced from the sliding platforms and through the shield
doors to cross timbers in the tunnel.

The second method of timbering was used in the presence of gravel at the
upper part of the excavation. In such a case, the excavation was only
carried 1 ft. 3 ins. (half a shove) ahead of the cutting-edge, the roof
being supported by transverse boards held by pipes which rested in holes
left in the shield. After a small section of the ground had been
excavated a board supported by a pipe that was inserted underneath and
wedged to it was placed against the ground. These polings were kept
below the level of the hood, so that when the shield was shoved, they
would come inside of it; in addition they were braced with vertical
posts from the sliding platform. The upper part of the face was held by
longitudinal breast-boards braced from the sliding platform by vertical
pieces. The lower part of the face was supported by vertical sheeted
poling, braced to the tunnel through the lower doors. Straw and clay
were used in front of the boards to prevent the escape of air which was
very large, when the tunnel was excavated through sand and gravel. The
average rate of progress in these materials was 5.1 ft. per day.


=Silt.=--When silt was encountered, the shield was shoved into the
ground without any excavation being done by hand ahead of the diaphragm.
As the shield advanced the silt was forced through the doors into the
tunnel. Forcing the shield through the silt resulted in raising the bed
of the river, the amount that the bed was raised depending on the
quantity of material brought into the shield. When the whole volume of
the excavation was brought in, the surface of the bed was not affected;
when about 50% was taken in, the surface was raised about 3 ft.; if the
shield was driven blind, the bed was raised about 7 ft. When the shield
was driven blind, the tunnel began to rise for about 2 ins., and the
iron lining was distorted, the vertical diameter increasing and the
horizontal one decreasing by about 1¹⁄₄ ins. It was found, however, that
the tunnel was not affected when part of the excavation was taken, but
if all of it was taken in or the shield was shoved with open doors, the
tunnel was lowered. A powerful aid was thus found for the guidance of
the shield; for, if high, the shield could be brought down by increasing
the quantity of muck taken in, if low, by decreasing it.

The junction of the shields under the river was made as follows: When
the two shields of one tunnel, which had been driven from opposite sides
of the river, approached within 10 ft. of each other, they were stopped;
a 10-in. pipe was driven between them, and a final check of lines and
levels was made through the pipe. One shield was then started up with
all doors closed, while the doors of the stationary shield were opened
for the muck driven ahead by the moving shield. This was continued until
the cutting-edges came together. All doors in both shields were then
opened and the shield mucked out. The cutting-edges were taken off and
the shields moved together again, edge of skin to edge of skin. As the
sections of the cutting-edges were taken off, the space between the skin
edges was poled with 3-in. stuff. When everything except the skin had
been removed, iron lining was built up inside the skins; the gap at the
junction was filled with concrete and long bolts were used from ring to
ring on the circumferential joint.


=Lining.=--The tunnels were lined with cast-iron circular rings of the
segmental bolted type. In some special cases, cast steel was used
instead of cast iron. The rings were made 30 ins. long, with an internal
diameter of 21 ft. 2 ins. and an external one of 23 ft. The rings were
composed of nine equal segments of 77¹⁄₂ ins. external circumferential
length each, except the two segments adjoining the key which were equal
to the other segments with the difference, that one end joint was not
radial but formed so as to make an opening 12.25 ins. wide at the
outside and 12.60 ins. at the inside, which was closed by the key
segment. Each segment had six bolts in the circumferential joint, the
key had one, so that there were 67 bolts in one circumferential joint.
Each of the twelve longitudinal or radial joints had five bolts, in all
127 bolts per ring. The circumferential flanges of each plate were
strengthened by two transverse webs or feathers on each flange. Each
segment was provided with a 1¹⁄₂ in. grout hole closed with a screw
plug. In order to pass around curves, whether horizontal or vertical, or
to correct deviation from the line or grade, tapering was used; by this
is meant the placing of rings in the tunnels which were wider than the
standard rings, either at one side (horizontal tapers or liners), or at
the top (depressors), or at the bottom (elevators). Tapers ¹⁄₂, ³⁄₄ or
even 1 in. were used. The taper rings were made by casting a ring with
one circumferential flange much thicker than usual and then machining it
off to the taper.


=Grouting.=--From the exterior of the tunnel already lined with
cast-iron rings, grout was forced through the holes closed by
screw-plugs, at a pressure of 90 lbs. per sq. in. The grout was composed
of 1 Portland cement and 1 sand by volume and was forced in by a
specially constructed machine, so it formed a shell of cement nearly 3
ins. thick around the exterior of the iron lining. The grouting began at
the lower segment; the cement was forced in until it reached the hole
above, then the hole was plugged, and the grouting was carried on from
the consecutive hole and so on until all the tunnel was finally encased
in grout, as it filled every crevice between the outside of the lining
and the ground as excavated. The cast-iron rings of the tunnel were
covered with a concrete lining which was placed in the following order:
First, on the invert; second, on the duct benches; third, on the arch;
fourth, on the ducts; fifth, on the face of the bench. Before any
concrete was placed, the surface of the iron was cleaned by scrapers and
wire brushes and by washing it with water. The invert was built in
sections 30 ft. long and the duct benches were constructed soon after.
These duct benches were built with several steps for the ducts to be
laid later. They were built by means of a traveling stage on wheels
which ran on tracks on the working platform of the tunnel. The arch was
constructed soon after. First the portion from the duct benches to the
haunches, then the arch proper, was built on traveling centers on tracks
laid on the steps of the duct benches. The concrete was received in
³⁄₄-cu.-yd. dumping buckets, from the flat cars on which they were run;
the buckets were hoisted to the level of the lower platform of the arch
by a small Lidgerwood compressed air hoister. At this level the concrete
was dumped on a traveling car or stage and moved in that to the point
on the form where it was to be placed. For the lower part of the arch
the concrete was thrown directly into the form from this traveling part
of the stage. Fig. 140 shows the cross-section of the tunnel with the
iron lining and concrete.

[Illustration: Section in Sand and Gravel or Rock

Section in Hudson River Silt, with foundations

FIG. 140.--Typical Cross-Sections of One Tube of Pennsylvania Railroad
Tunnel Under the Hudson River.]


=Hauling.=--A working platform, made up of 5-ft. sections, was built
inside the tunnel and kept close to the shield. On this platform two
lines of industrial railway tracks with switches and sidings at the
locks, and a heading, were laid for hauling materials and spoils. These
lines converged into a single track in passing through the air locks. At
the shaft elevators, they terminated in a steel plate floor to avoid
switches. Between the locks of the bulkheads was installed an
electrically driven cable system, to haul the loaded muck up grade and
to empty the flat cars. From the first bulkhead to the shaft, the cars
were hauled up grade by a steam hauling engine. At the Manhattan end
there was one 10-H.P. engine for each tunnel, while at Weehawken one
25-H.P. engine served for both tunnels. Each shaft contained two
elevators driven by a double-cable, reversible single-drum
steam-hoisting engine. A grouty frame was built over the shafts, and on
the platforms over this frame were narrow-gauge tracks, extending from
the elevators to the muck-chutes and to points where the lining segments
were loaded on the cars. The elevators were arranged to stop at both the
ground and the grouty platform levels. The rolling-stock at each of the
tunnels consisted of 75 flat cars for moving the tunnel segments, and of
about 50 muck cars, each of 1¹⁄₄ cu. yd. capacity.


=Plant.=--The plants located at each end of the tunnel near the shafts
were almost identical. Each consisted of three 500-H.P. Stirtling
boilers, which supplied steam at 150 lbs. pressure. Feed water was
supplied by three 13¹⁄₂ metropolitan injectors, and two Blake duplex
pumps. Two Worthington surface condensers, each of 2250 sq. ft.
condensing surface, took care of the exhaust from the engines and
compressors. Condensing water was pumped from the river through a 16-in.
pipe. The high-pressure air was supplied by a duplex Ingersoll-Sergeant
compressor, with cross-compound steam end 14 × 26 × 30 ins. and simple
water-jacket air cylinders 13¹⁄₄ × 36 ins. Its capacity at 100 r.p.m.
was 1085 cu. ft. free air per minute. The maximum pressure was 130 lbs.
per sq. in. The air for the pneumatic working was supplied by three 14 ×
26 × 30 in. duplex Ingersoll-Sergeant compressors. The maximum capacity
of the three was 12,000 cu. ft. free air per minute at 125 r.p.m. and a
discharge pressure of 50 lbs. per sq. in. The suction air was taken from
the outside about 10 ft. above the roof of the engine house. Three
aftercoolers, 32¹⁄₂ ins. × 11 ft. 4 ins., each having 809 sq. ft.
cooling surface of tinned brass tubes, cooled the low-pressure discharge
to within 10° F. of the temperature of the cooling-water. From the
aftercoolers, the air passed into three steel receivers each 54 × 12
ft., placed outside the engine room and fitted with weighing safety
valves. The receivers were connected to two 10-in. mains; one serving
the north, the other the south tunnel. A fourth receiver of the same
size was built to receive the discharge of the high-pressure compressor,
through a 4-in. pipe. The high-pressure water required for the shield
was furnished by three Blake direct-acting, duplex pumps with outside
packed plungers. The steam end was 16 × 18 ins., the water end 2¹⁄₁₆ ×
18 ins. At 55 r.p.m. pumping against 5000 lbs. per sq. in., the capacity
of each pump was 57 gals. per minute. Two of them, one on each tunnel,
were sufficient to run the shields and the third was held in reserve.
The high-pressure water was conveyed to the front by means of a 2-in.
double, extra strong pipe which was buried between the engine room and
the shaft, in a trench, to prevent freezing in cold weather. The
electric current for light and power was supplied by two 100-K.W.
250-volt G.E. direct-current generators directly connected to Ball &
Wood high-speed engines running at 250 r.p.m. The switchboard had two
machine panels, two distributing panels and one panel carrying a circuit
breaker for the traction circuit.


=Illumination.=--The tunnel was lighted by electricity, there being two
rows of lamps, one in the crown and one in the south axial fine. The
lamps were 16-c.p., 240-volt, two-wire system, and were spaced 35 ft.
apart in the crown and 12¹⁄₂ ft. apart on the axial line. In addition,
five nests of 5 lamps each were used at the front. Candles were supplied
for miscellaneous and emergency uses. The sockets for electric globes
were fitted to a wooden reflector, coated with white enamel paint on the
inside.




CHAPTER XXI.

SUBMARINE TUNNELING (Continued); TUNNELS AT VERY SHALLOW DEPTH. THE
COFFERDAM METHOD. THE PNEUMATIC CAISSON METHOD. THE JOINING TOGETHER
SECTIONS OF TUNNELS BUILT ON LAND.


The tunnels on the river bed or at such a shallow depth that only a few
feet of material will remain between the bottom of the river and the
roof of the tunnel can be built in three different ways, viz., (1) by a
cofferdam; (2) by pneumatic caissons; (3) by sinking and joining
together whole sections of tunnels that were built on land.


=The Cofferdam Method.=--=The Van Buren Street Tunnel, Chicago
River.=--According to the cofferdam method, the work is attacked at one
of the shores, and the tunnel built in sections of such length as not to
interfere with the flow of water or the navigation of the river. Round
the entire exterior line of the first section a double-walled cofferdam
is built, and strongly braced transversely, so as to withstand the
pressure of the water. When the water is pumped out, a single-walled
cofferdam is built within the first, leaving sufficient distance between
the two to allow of the construction of the masonry. The soil is then
removed within the inner cofferdam, and the tunnel constructed from the
foundation. When the end of the tunnel reaches the channel end of the
cofferdam, a crib-wall is erected over the end of the completed tunnel.
This crib, in turn, forms the end wall of another cofferdam, built in
continuation of the first, so as to allow the second section to be
proceeded with, and at the same time to facilitate the removal of the
cofferdams of the first section. The work goes on continuously in this
way until the distant shore is reached.


VAN BUREN STREET TUNNEL, CHICAGO.

The Van Buren Street tunnel, built to carry a double-track street
railway under the Chicago River, was completed in 1894 by the cofferdam
method. The special features of the tunnel[14] are: (1) the unusually
large dimensions of the cross-section of 30 ft. × 15 ft. 9 ins.; (2) its
construction inside of cofferdams of great length and width; (3) the
construction under some very high buildings calling for great care and
very strong temporary and permanent supports.

  [14] “Eng. News,” April 12, 1892.

The special feature of the work for our present purpose was the
construction of the tunnel across the river. To accomplish this a
cofferdam was built out from the west shore of the river to its middle,
and the tunnel constructed within it like the building of any other
structure within a cofferdam. Transverse and longitudinal sections of
this cofferdam are shown by Fig. 141. As will be seen, it was a simple
double-wall cofferdam, with a clear width between the walls of 58 ft.,
and braced transversely as shown. Inside of this a single-wall cofferdam
of piles was constructed, with a clear width just sufficient to allow
the construction of the masonry within it. When the tunnel end reached
the channel end of the cofferdam, a crib-wall was built over the end of
the completed tunnel, as shown by the drawings. This crib-wall was
intended to form the end wall of another cofferdam, which was built out
from the east shore, and within which the remaining half of the tunnel
was built as the first half had been. The drawings show the character of
the tunnel masonry and of the centering upon which it was built.

[Illustration: ~TRANSVERSE SECTION OF COFFERDAM AND TUNNEL~

~SECTION SHOWING METHOD OF CONSTRUCTING CRIB DAM.~

FIG. 141.--Sections of Cofferdam, Van Buren St. Tunnel, Chicago.]

The Van Buren Street tunnel was the last of the three tunnels under the
Chicago River, constructed according to the cofferdam method. At the
time the tunnels were constructed the bed of the river was 17 ft.
deep. In connection with the harbor and river improvements, the Federal
Government ordered the Chicago River to be lowered so as to give a depth
of 26 ft. of water. This necessitated the lowering of the tunnel roof
and the excavation for a deeper floor which was a very difficult
operation. This work was described in “Eng. News,” Sept., 1906.


THE PNEUMATIC CAISSON METHOD.--THE TUNNEL UNDER THE HARLEM RIVER.

In the early seventies Prof. Winkler proposed to construct a tunnel
under the River Danube to connect the various portions of the Vienna,
Austria, underground railway, and to use caissons in the construction.
Prof. Winkler proposed to build caissons from 30 ft. to 45 ft. long,
with a width depending upon the lateral dimensions adopted for the
tunnel masonry. The caisson was to be made of metal plates and angle
iron with riveted connections on all sides except those running
vertically transverse to the tunnel axis, whose connections were to be
bolted. In the middle of the roof an opening was to be left; this was
for the shaft having the air-locks to allow the passage of men,
materials, and compressed air.

Across the river two parallel rows of piles were to be driven into the
river bed, to fix the place where the caisson was to be sunk. Then the
first caisson near the shore was to be lowered in the ordinary way, and
a second caisson was to be immediately sunk very close to the first one.
When both caissons had reached the plane of the tunnel floor, the sides
which were in contact were to be unbolted and removed, and the small
space between made water-tight. The chambers of the two caissons were to
be opened into a single large one communicating above by means of two
shafts. At the same time that the masonry was being built in the first
two caissons, from the inverted arch up, a third caisson was to be sunk;
and when by excavation it had reached the plane of the projected tunnel
floor, the partitions were to be removed so that the three caissons were
in communication, forming a large single caisson. Then the outer
partition of the first caisson was to be removed, and the masonry of the
submarine tunnel connected with the portion of the tunnel built on land.
In a similar manner all the caissons were to be sunk; and when the last
one was placed, and the masonry lining constructed, and connected with
the portion of the tunnel built on the other shore of the river, the
partition walls were to be battered down, and the submarine tunnel
completely constructed and open to traffic.


=The Harlem River Tunnel.=--The pneumatic caissons method was employed
in the construction of the tunnel under the Harlem River for the New
York Rapid Transit Railway. The tunnel proper consisted of two parallel
tubes riveted to each other and surrounded by a cradle of concrete as
shown in Fig. 121, page 216. The tunnel was built in three
sections:--The first, from the Manhattan shore well towards the middle
of the river; the second, from the shore of the Bronx towards the middle
of the river; and the last, the section uniting the other two and
completing the tunnel.

Each section was built within a specially constructed working-chamber,
consisting of timber side walls forming a wooden caisson, so constructed
that compressed air could be used. This working-chamber of Mr. McBean
presented some novel features, inasmuch as the caisson was not built on
land, but under water.

In building the tunnel, the Harlem River was dredged to a certain depth,
so as to leave only 6 ft. or 8 ft. of excavation to be done before
reaching the line of sub-grade of the proposed structure. Two service
platforms were built on piles 10 ft. apart longitudinally, and cut off
at a point above mean high-water mark, braced in the usual manner, and
covered with heavy planks, to serve as the floor of the platform. On
this platform were placed rails for the trains used in the
transportation of materials. These platforms were also used in
maintaining the perfect alignment of the caissons.

Within the platforms and along the dredged channel four longitudinal
rows of piles were sunk. These piles were accurately brought to line by
beams bolted together, and placed across and above the water-level. A
few beams were also added for the purpose of bracing the piles
transversely, after which they were cut off under water and capped.

[Illustration: FIG. 142.--Showing Working Platforms and Piles Sunk in
the Dredged Channel.]

Fig. 142 shows the manner in which the working platforms were
constructed, and also the rows of piles sunk in the dredged channel.
Between the piles a very strong frame was placed, made up of waling
pieces and two transverse beams 14 ins. by 14 ins. each, placed one
below the other at a distance of 5 ft. 8 ins., and strongly braced
together. Guiding-beams were fixed on each side of the frame for the
sheeting piles. The frames were built in sections of different lengths,
and placed directly above the cap-pieces of the pile-bents sunk in the
dredged channel.

The longitudinal sides of the caisson were constructed by sinking two
rows of sheeting piles, each row being close to a service platform. The
sheeting piles were made up of yellow-pine timbers 12 ins. by 12 ins.;
three piles bolted together formed a section 3 ft. wide. Each section
was grooved and tongued, so as to be firmly connected with the adjacent
sections to be sunk. The lower ends of the piles were cut wedge-shaped,
with a sharp edge to offer a small resistance while penetrating the
soil. The sheeting-piles were then cut off under water, which operation
was successfully carried out by means of a circular saw operated by a
pile-driving machine. The sheeting was also extended between two
platforms to make a bulkhead, and in this way the four sides of the
caisson were built up. Particular attention was always given to the
alignment of the sheeting piles, which was obtained by guiding the piles
with the timbers placed longitudinally, one below the water-line in
connection with the frames located between the pile-bents, and the
second along the inner edge of the service platform, as shown in Fig.
143.

[Illustration: FIG. 143.--Showing Sheeting-Piles for the Sides of the
Caisson and Trussed Beam for the Roof.]

The caisson was completed by placing a roof covering the sides. This
roof was 40 ins. thick, made up of three layers of 12-in. beams placed
transversely to the axis of the caisson, while between the beams planks
2 ins. thick were placed lengthwise and bolted together, so as to make a
firm, solid structure. The roof was built ashore, in sections each
varying from 39 ft. to 130 ft. long. The edges of the roof fitted the
sides of the caisson perfectly; and when each section was towed to the
proper spot, it was sunk and made secure. Under the roof were placed six
longitudinal beams, 12 ins. by 14 ins., called “rangers,” resting on the
cap-pieces of the pile-bents that were laid across the space of the
proposed tunnel; while the extreme rangers were used for the purpose of
fitting above the sheeting-piles of the caisson, in order to make the
latter water-tight. The two extreme rangers were provided with
=T=-irons, the flat side being laid on the sheeting-piles, while the web
penetrated the ranger by reason of the weight of the load resting on the
roof, for the purpose of sinking it to the required point. Earth was
next heaped on the roof, and in this way a large working-chamber was
prepared, as shown in Fig. 144.

[Illustration: FIG. 144.--Showing the Caisson with the Working-Chamber.]

The working-chamber built on the Manhattan side of the Harlem River was
216 ft. long, provided with two rectangular shafts 7 ft. by 17 ft.,
rendered water-tight, and rising above the high-water mark of the river.
Within these shafts the air-locks of the tunnel tubes were placed, so
that the work could be carried on by means of compressed air. The
pressure of the air was used to expel the water, being sufficiently
intense to equilibrate a column of water equal to the depth of the
lowest point of the roof of the caisson.

When the working-chamber was constructed, the tunnel proper was begun by
excavating the soil down to the required level; the concrete was then
laid on. It was just at this point, when a large part of the roof was
constructed and supported only by the sheeting-piles of the sides of the
caisson, that the writer of this article feared that this novel method
of tunneling would prove a failure. The tendency of the timber to float,
aided as it was by the air pressure within the caisson, was counteracted
only by the weight of the earth heaped on the roof, and by the friction
of the soil against the feet of the sheeting-piles. This friction was
only a small quantity, as the soil was loose, so that it was considered
rather risky and dangerous to place reliance on such a feeble quantity.
This fear was, unfortunately, justified on two occasions, when on
cutting off a portion of the pile-bents some of the sheeting-piles got
loose and water flooded the whole chamber, but, happily, without loss of
life. As the chamber was one of large dimensions, the workmen had time
enough to effect their escape. It may be remarked that during these
troubles only a few of the sheeting-piles were displaced, while the
caisson itself offered a stout and successful resistance, due to its
being strongly braced transversely. The accidents were, therefore,
limited to a few piles, instead of affecting the entire caisson. On the
occasion of the first, the repairs were effected by sinking the piles to
a greater depth, continuing down until rock was encountered. After that,
the water was pumped out and the work resumed. In repairing the second
accident, the sheeting-piles were driven down to bed-rock, and the
surrounding soil strengthened by cement forced through the loose soil
around the piles. This remedy proved effective, and no further trouble
occurred to delay the work on the Manhattan side of the Harlem River.

[Illustration: FIG. 145.--Showing the Tunnel Constructed within the
Caisson.]

On the concrete bed of the tunnel the segments of the metal lining were
placed and surrounded by concrete, as required by the plans and
specifications (Fig. 145). The contractors had planned to unbolt the
roof from its holdings, to remove by means of dredgers the earth which
had been heaped on it, and thus set the roof afloat, after which it was
to be towed within the two working platforms already erected on the
Bronx shore. But Mr. McBean devised a simpler and more economic, but at
the same time more dangerous, way of constructing this second section of
the tunnel. He thought that the upper half of the tunnel proper could be
used instead of the timber roof, thereby reducing the capacity of the
working chamber, and limiting the use of compressed air. In this way he
dispensed with the removal of timber, and also of the earth heaped on
the roof.

In building this Bronx section, a channel was dredged along the line of
the tunnel to a depth of 5 ft. from the foundation-bed of the proposed
tunnel. The working platforms were constructed on both sides of this
channel, quite similar to those erected on the other half of the tunnel;
and between them pile-bents were sunk, capped with 12-in. by 12-in.
beams. Over the cap-pieces rangers were placed longitudinally, which
also rested on the sides of the wooden working caisson, Fig. 146. The
sheeting-piles were cut off at level, but much lower down than in the
first half of the tunnel.

The roof was built on floats made of 12-in. by 12-in. timber laid
transversely 4 ft. apart and supporting a floor of 3-in. by 12-in.
planks rendered water-tight. The sides of the floats were made by
verticals, 4 ins. by 6 ins., and planks, 3 ins. by 12 ins., carefully
caulked. A temporary floor was built on the base of the float,
consisting of transverse beams, 16 ins. by 16 ins., placed 8 ft. apart.
A center piece, 10 ins. by 16 ins., was laid so as to correspond with
the axis of the tunnel; and on each side of it, other parallel beams, 16
ins. by 16 ins., corresponding to each center of the circular metal
lining of the tunnel; the beams, longitudinal and transversal, were
strongly bolted together. The temporary floor was completed by boarding
the spaces left between the various beams.

[Illustration: FIG. 146.--Showing Sides of the Caisson and Supports for
the Roof.]

On this float, the upper half of the tunnel was constructed by erecting
the segments of the metal lining, which were strongly supported, so as
to prevent any settling or distortion; the concrete was then built up in
a large flange with vertical suspension rods, four to each bar. The
rings of the tunnel were 6 ft. each, the weight of each lining being
41,000 lbs., the concrete covering 618 cubic feet. The second part of
the tunnel was 300 ft. long, with roof constructed in three
sections--two of 90 ft. in length each and the third of 84 ft. Each of
these sections alternated with a smaller section, 12 ft. long, provided
with air-locks. The shortest of the three sections was the first one set
up, and was constructed close to the Bronx side of the Harlem River. For
this purpose the two extreme ends of the section were closed by means of
steel plates forming diaphragms, built 6 ft. inward, thus leaving one
ring projecting out at each end. Openings were left on the top of these
projecting rings for access by divers. The exterior of the upper half
section of the permanent tunnel was filled with water until it was
lowered into position. It was directed by means of tackles attached to
vertical eye-bars, which were strongly fixed to the flanges of the
springing line of the arch, and bolted to the beams of the temporary
floor. In this way the roof was towed into place, and lowered by means
of stone ballast, until it rested on the cap-pieces and frames of the
pile abutments, the sides of the roof remaining just on top of the
sheeting-piles that formed the sides of the caisson, as shown in Fig.
147. Perfect alignment was obtained by wires strung at each end and
along the side of the roof, corresponding to points fixed on the working
platforms and sighted with transits. Such accuracy was obtained that the
circumferential flanges of the outer 6-ft. ring were brought into
contact with those of the 12-ft. section already constructed. A diver
then entered by the opening left in the projecting ring, and bolted this
section of the roof to the preceding one. By removing the iron
diaphragm, the consecutive sections were united into one. When the diver
completed his work, the opening was closed up, and compressed air used
to keep the water out of the box included between the roof and the
temporary flooring.

[Illustration: FIG. 147.--Showing the Roof of the Caisson Formed by the
Upper Half of the Tunnel.]

The remaining sections of the tunnel roof were built in the same way,
until the last abutted against the part of the work constructed within
the caisson under the high wooden roof on the Manhattan side of the
river. The following method was adopted for the purpose of connecting
the few parts of the tunnel which had been differently constructed. The
diaphragm at the end of the last section of the tunnel roof was
constructed so as to abut against the last circumferential flanges of
the iron lining without leaving a projecting ring. It was continued
above the metal and concrete lining of the roof in a rectangular form,
and of the same height and width as the wooden bulkhead of the
working-chamber on the Manhattan side of the river. The diaphragm was
made of riveted plates and angles, with an opening 20 ins. by 30 ins.,
bolted so as to be removable at will. The diaphragm was of the same
height as the roof and was connected with a roof-plate to the rangers
supporting the thick wooden roof. Other steel plates, placed vertically,
were riveted to the diaphragm and bolted to the caisson. All this work
was carried on by divers. The wooden bulkhead was cut to the
springing-line of the arch; and between the two parts of the tunnel,
built by different methods, a bulkhead was placed, made of steel plates
14 ins. long, which prevented the entrance of water into the
working-chamber.

[Illustration: FIG. 148.--Showing the Tunnel Completed by Building the
Lower Half within the Caisson.]

When the different sections were joined together, and all the openings
closed and made water-tight, cement-grout was poured on the roof, and
earth was heaped up to a height of 5 ft. The 300 ft. of the roof,
resting on sheeting-piles and provided with diaphragms at the extreme
ends, formed a water-tight working-chamber, or caisson, communicating
with the exterior by means of the shafts and air-locks. The lower
portion of the tunnel was built under air-pressure. The pile-bents were
first cut off at the plane of the tunnel sub-grade, after which the
foundation-bed of concrete was laid. The lower segments of the iron
lining were then placed in position, and the structure made continuous
by building up the lateral walls, consisting of concrete (Fig. 148). No
accidents occurred while building the second part of the tunnel.

The Harlem River tunnel was completed in contract time, although the
opening of the subway was delayed by difficulties encountered in
tunneling through rock in the borough of the Bronx. The writer
endeavored to obtain information regarding the expense per linear foot,
but all his efforts were rewarded with a general assurance that it
proved to be the cheapest method.


SINKING AND JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND. THE
SEINE. THE DETROIT RIVER TUNNELS.

In the year 1896, Mr. Erastus Wyman secured a patent for building
subaqueous tunnels close to the river, by sinking and joining together
small sections of tunnels previously built on land. Each section would
have been provided with a long vertical tube for the air-lock when
compressed air was to be admitted to expel the water and permit the
construction of the lining within the sunken shell. Thus each section of
the tunnel would have acted as a pneumatic caisson; being, however, an
improvement on Professor Winkler’s suggestion inasmuch as the caisson
was a portion of the tunnel itself, instead of a simple inclosure for
facilitating the construction of the shield. Mr. Wyman proposed to use
this method in the construction of a tunnel between South Brooklyn and
Stapleton, Staten Island; a charter was granted him but the tunnel was
never built.


=The Tunnel under the Seine River.=--The caisson method of building
tunnels under water was used at Paris, France, in the construction of
the Metropolitan Railroad under the Seine River.

The caissons designed by Mr. L. Chagnaud were for a double track line.
They were sunk, ends to ends, and formed a portion of the tunnel lining
which was enveloped by a framework of metal embedded in concrete.
Built-up frames carried a shell of steel plating on the sides, from toes
to springing lines, and on the sides and roof of the working-chamber. A
temporary plate diaphragm closed the open ends. This construction formed
a vessel capable of floating with a very light draft.

The method of sinking the caissons was as follows: The caisson was
erected on the river bank and when completed it was launched and towed
into position between pile stagings which served the double purpose of
guiding the descent at the beginning of the sinking and of forming a
working platform. The caisson when launched and, consequently, before
the cast-iron lining had been put in place within it, weighed 280 metric
tons; but, beyond some difficulty in taking it under the bridges in the
way, the towing was accomplished without serious trouble.

[Illustration: FIG. 149.--Transversal Section of the Caissons for the
Tunnel under the Seine River.]

Previous to placing the caisson in position between the stagings, the
portion of the river bed it was to rest upon had been leveled by
dredging. Once in position, the first work was the erecting of the
cast-iron lining segments within the framework. Work was then begun by
filling the annular space between the lining and the shell with
concrete; this additional weight gradually sunk the caisson to the river
bottom. The working shafts, made up of steel cylinders, were placed as
the sinking progressed to this point.

[Illustration: ~Section A-B.~

~Section C-D.~

~Plan at Joint.~

FIG. 150.--Showing the Joining of the Caissons at the Pont Mirabeau
Tunnel under the Seine River.]

After the caissons had been sunk to the required place and in
continuation of one another, a space of nearly 5 ft. was left between
them. The construction of the tunnel within the bank of earth
separating the two caissons was as follows: A cofferdam was built around
this space. It was formed by two diaphragms closing the ends of the
tunnel, and by two longitudinal walls sunk as temporary caissons, one on
each side of the tunnel and inclosing their ends. This cofferdam was
covered with a metal working-chamber whose lower edges rested on top of
the four walls of the cofferdam. The joints were made tight by means of
rubber or packed clay. The water in the cofferdam was then pumped out,
the earth excavated, and the masonry built in continuation of the two
ends of the tunnel sections. The submerged sections of the tunnel which
were allowed to remain full of water to render them more stable and to
save effort in pumping them, were now made dry; the diaphragms were
removed from the ends of the caisson tunnels and the work made
continuous. Fig. 149 shows the cross-section of the caissons.

At the Pont Mirabeau crossing of the Seine, a slightly different method
was used, described in “Eng. News,” May 18, 1911. The caissons were sunk
to the required line and grade with an intervening longitudinal space of
15³⁄₄ ins. between two adjoining caissons. At each end of this space,
which was filled with the river marl, was sunk against the edges of the
caissons a hollow cylinder 20 ins. outside diameter. The interior of
these cylinders was excavated and filled with concrete, thus forming a
continuous wall on both sides of the two adjoining caissons. The earth
from the intervening space was then removed and concrete deposited from
bottom opening tremies up to the level of the top of the caisson. After
nearly one month the tunnel was entered from the shaft and an opening
the shape and size of the tunnel section cut through the diaphragms of
the 15³⁄₄-in. wall and the concrete tunnel lining made continuous
between the two sections. Fig. 150 shows the method of joining the
caissons.


=The Detroit River Tunnel.=[15]--With some modifications which permitted
dispensing with compressed air, the tunnel under the Detroit River was
built for the Michigan Central Railroad, connecting Detroit with
Windsor, Canada. The tunnel is 6625 ft. long; of this, however, only
2625 ft. are under the river, while the approach on the American side is
2000 ft. long and that on the Canadian side, 4000 ft. The tunnel
consists of two parallel circular tubes 23 ft. in diameter, built up of
³⁄₈-in. steel plate. They are placed 26 ft. apart, center to center, and
are connected by diaphragms at 12-foot intervals.

  [15] Condensed from a paper by B. H. Ryder.

Each section of the subaqueous tunnel is approximately 262 ft. long.
There are ten of these sections and an eleventh a little over 60 ft.
long. These tubes were built at the shipyards of the Great Lakes
Engineering Works at St. Clair, about 30 miles from Detroit. After the
assembling was completed, the ends of each tube were closed by temporary
wooden bulkheads to make them float, and the outside sheathed
horizontally with heavy timbers bolted to the diaphragms. This sheathing
running lengthwise of the tube made a form or pocket, into which the
inclosing jacket of concrete was placed. The sections were then launched
and towed down to the tunnel site and sunk separately in a trench on the
river bottom that had been previously dredged to receive them. This
trench was dug to a width of 50 ft. and depth varying from 25 to 50 ft.
by clamshell buckets, swung from a scow, working to a depth below the
water level of 60 to 90 ft.

As a foundation for the sections, a grillage was constructed on the
surface and sunk in place in the trench by derricks swung from a scow.
The grillage was placed underneath each joint between the sections and
built up of I-beams imbedded in concrete. This grillage is the width of
the trench and about 30 ft. long, with posts projecting downward from
the four corners, and these were seated into the river bottom, by means
of pile drivers, to the desired grade.

Then the eleven sections of the tunnel were lowered and connected, one
at a time. By the aid of air tanks placed on each section the movement
was controlled until the final sinking upon the grillage in the trench.
This operation called into play the greatest engineering skill and
ingenuity. When it is considered that the current velocity at the river
bed is about 2 ft. per second and much higher along the surface, some
idea can be gained of the problems to be overcome. The movement of the
enormous sections must be absolutely under control. Thirty-five-ton
blocks of concrete were sunk in the river bottom up and down stream to
act as anchors, and through them cables were rigged and connected back
to the hoisting engines on the derrick scows. These were prevented from
moving by spuds at each corner, securely driven into the river bottom at
depths sometimes as great as 90 ft. Controlling cables were also run
from the sections to the tremie scow to pull one structure close to the
adjoining section previously sunk, and the divers made the necessary
connection. Fig. 151 shows cross-sections and plans of the tunnel as
given in “Eng. Record,” March 2, 1907.

[Illustration: ~HALF CROSS SECTION Y-Y~

~HALF CROSS SECTION Z-Z~

~HALF HORIZONTAL SECTION X-X~

~HALF TOP VIEW~

FIG. 151.--Cross-Sections and Plans of the Detroit River Tunnel.]

Steel masts had been previously attached to each end of the sections to
enable the engineers on shore to determine the alignment and locate the
exact position during the sinking.

Concrete was then deposited in the pockets, completely surrounding the
tubes, forming a solid monolithic structure from end to end.

This was done by means of the tremie process.

A 32-ft. by 160-ft. scow was equipped with a concrete mixing plant and
the tremie pipes, three in number, through which the concrete was
deposited. Each pipe is 12 ins. in diameter, of spiral riveted steel, 80
ft. long. These pipes could be raised or lowered, reaching from the
receiving hoppers on the scow to the bottom of the trench. When the
pipes were filled with concrete and lowered into position, a continuous
flow was maintained. As fast as the concrete escaped at the bottom end
of the pipe it was replenished at the top; this process continuing until
the entire space surrounding the section was filled to the desired
level, and under the pressure produced not only by the depth of water
under which it was submerged, but also by the weight of the long column
of concrete contained in the tubes. It is interesting to note that this
is the first time a large amount of concrete has been deposited at a
depth of 70 ft. by this method, and upon the accomplishment of this task
in a measure depended the successful building of the tunnel.

Inside the tubes was placed a lining of reinforced concrete 20 ins.
thick. Side walls were built up from this ring to provide ducts, which
carry the electrical cables for the distribution of power, lighting,
signal and telegraph wires. They also serve to provide a footwalk along
the side of the tunnel.

There are cross passages in the tunnel every 200 ft., and also various
niches for the different equipment needed in connection with the
signaling, telephone and fire alarm system. The tunnel is lighted with
800 16-candle-power incandescent lights.

The track construction is new. There is no ballast used, the ties being
laid in concrete. A ditch in the center of each track carries the
rainfall that will flow down from the summits to sumps which are drained
by centrifugal pumps.

One remarkable feature of its construction is that compressed air was
not used in the building of the subaqueous tunnel, but it was necessary
in building the approach tunnels. This is contrary to the usual program
where compressed air is required in subaqueous work, and not ordinarily
used in approach or land tunnel construction.

The trains are operated by very heavy electric locomotives, operated by
the third-rail system.

The tunnel was constructed under the supervision of W. S. Kinnear, Chief
Engineer of the Detroit River Tunnel Co.; Butler Bros. of New York were
the general contractors.




CHAPTER XXII.

ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER CONSTRUCTION.


In the excavation of tunnels it often happens that the disturbance of
the equilibrium of the surrounding material by the excavation develops
forces of such intensity that the timbering or lining is crushed and the
tunnel destroyed. To provide against accidents of this kind in a
theoretically perfect manner would require the engineer to have an
accurate knowledge of the character, direction and intensity of the
forces developed, and this is practically impossible, since all of these
factors differ with the nature and structure of the material penetrated.
The best that can be done, therefore, is to determine the general
character and structure of the material penetrated, as fully as
practicable, by means of borings and geological surveys, and then to
employ timbering and masonry of such dimensions and character as have
withstood successfully the pressures developed in previous tunnels
excavated through similar material. If, despite these precautions,
accidents occur, the engineer is compelled to devise methods of checking
and repairing them, and it is the purpose of this chapter to point out
briefly the most common kinds of accidents, their causes, and the usual
methods of repairing them.


=Accidents During Construction.=--Accidents may happen both during or
after construction, but it is during construction, when the equilibrium
of the surrounding material is first disturbed, and when the only
support of the pressures developed is the timber strutting that they
most commonly occur.


=Causes of Collapse.=--Collapse in tunnels may be caused: (1) by the
weight of the earth overhead, which is left unsupported by the
excavation; (2) by defective or insufficient strutting; and (3) by
defective or weak masonry.

(1) The danger of collapse of the roof of the excavation is influenced
by several conditions. One of these is the method of excavation adopted.
It is obvious that the larger the volume of the supporting earth is,
which is removed, the greater will be the tendency of the roof to fall,
and the more intense will be the pressures which the strutting will be
called upon to support. Thus the English and Austrian methods of
tunneling, where the full section is excavated before any of the lining
is placed, and where, as the consequence, the strutting has to sustain
all of the pressures, present more likelihood of the roof caving in than
any of the other common methods.

The character and structure of the material penetrated also influence
the danger of a collapse. A loose soil with little cohesion is of course
more likely to cave than one which is more stable. Rock where strata are
horizontal, or which is seamy and fissured, is more likely to break down
under the roof pressures than one with vertical strata and of
homogeneous structure. Soft sod containing boulders whose weight
develops local stresses in the roof timbering is likely to be more
dangerous than one which is more homogeneous. A factor which greatly
increases the danger of collapse, especially in soft soils, is the
presence of water. This element often changes a soil which is
comparatively stable, when dry, into one which is highly unstable and
treacherous. The liability of the material to disintegration by
atmospheric influences and various other conditions, which will occur to
the reader, may influence its stability to a dangerous extent, and
result in collapse.

(2) Collapse is often the result of using defective or insufficient
strutting. Of course, in one sense, any strutting which fails under the
pressures developed, however enormous they may be, can be said to be
insufficient, but as used here the term means a strutting with an
insufficient factor of safety to meet probable increases or variations
in pressure. Insufficient strutting may be due to the use of too light
timbers, to the spacing of the roof timbers too far apart, to the
yielding of the foundations, to insufficient bearing surface at the
joints, etc. Collapse is often caused by the premature removal of the
strutting during the construction of the masonry. The masons, to secure
more free space in which to work, are very likely, unless watched, to
remove too many of the timbers and seriously weaken the strutting.

(3) The third cause of collapse is badly built masonry. Poor masonry may
be due to the use of defective stone or brick, to the thinness of the
lining, to poor mortar, to weak centers which allow the arch to become
distorted during construction, to poor bonding of the stone or bricks,
to the premature removal of the centers, to driving some of the roof
timbers inside it, etc.


=Prevention of Collapse.=--Tunnels very seldom collapse without giving
some previous warning of the possible failure, and also of the manner in
which the failure is likely to occur. From these indications the
engineer is often able to foresee the nature of the danger and take
steps to check it. The danger may occur either during excavation or
after the lining is built. During excavation the danger of collapse is
indicated beforehand by the partial crushing or deflection of the
strutting timbers. If the timbers are too light or the bearing surfaces
are too small, crushing takes place where the pressures are the
greatest, and the timbers bend, burst, or crack in places, and the
joints open in other places. The remedy in such cases is to insert
additional timbers to strengthen the weak points, or it may be necessary
to construct a double strutting throughout. When the distance spanned by
the roof timbers is too great, failure is generally indicated by the
excessive deflection of these timbers, and this may often be remedied by
inserting intermediate struts or props. In some respects the best
remedy under any of these conditions is to construct the masonry as
soon as possible.

When collapse is likely to occur after the masonry is completed, its
probability is generally indicated by the cracking and distortion of the
lining. A study of the cause is quite likely to show that it is the
percolation of water through the material surrounding the lining which
causes cavities behind the lining in some places, and an increase of the
pressures in other places. When it is certain that this water comes from
the surface streams above, these streams may often be diverted or have
their beds lined with concrete to prevent further percolation. When
percolating water is not the cause of the trouble, a usually efficient
remedy is to sink a shaft over the weak point, and refill it with
material of more stable character. These, and the remedies previously
suggested, are designed to prevent failure without resorting to
reconstruction. When they or similar means prove insufficient,
reconstruction or repairs have to be resorted to.


=Repairing Failures.=--Tunnels may collapse in several ways: (1) The
front and sides of the excavation may cave in; (2) the floor or bottom
may bulge or sink; (3) the roof may fall in; (4) the material above the
entrances may slide and fill them up.

(1) One of the most common accidents is the caving of the front and
sides of the excavation. This may often be prevented by taking care that
the face of the excavation follows the natural slope of the material
instead of being more or less nearly vertical. When, however, caving
does occur it may usually be repaired by removing the fallen material,
strongly shoring the cavity, and filling in behind with stone, timber,
or fascines.

(2) The bulging or rising of the bottom of the tunnel may usually be
considered as a consequence of the squeezing together of the side walls.
It usually occurs in very loose soils, and is chiefly important from the
fact that the reconstruction of the side walls is made necessary. The
sinking of the tunnel bottom is a more serious occurrence. It seldom
happens unless there is a cavity beneath the floor, due either to
natural causes or to the fact that mining operations have gone on in the
hill or mountain penetrated by the tunnel. When the bottom of the tunnel
sinks, three cases may be considered: (_a_) when the sinking is limited
to the middle of the tunnel floor; (_b_) when only a portion of the
foundation masonry is affected; and, (_c_) when the entire lining is
disturbed. In the first case repairs are easily made by filling in the
cavity with new material. In the second case the unimpaired portion of
the masonry is temporarily supported by shoring while the injured
portion is removed and rebuilt on a firm foundation. The remaining
cavity is then filled. In the case of the complete failure of the
lining, the method of repairing employed when the roof falls, and
described below, is usually adopted.

(3) The most dangerous of all failures is the falling of the tunnel
roof. In such casualties two cases may be considered: (_a_) When the
falling mass completely fills the tunnel section, and (_b_) when it
fills only a portion of the section.

[Illustration: FIG. 152.--Tunneling through Caved Material by Heading.]

When the whole section is filled by the fallen material, the problem may
be considered as the excavation of a new tunnel of short length inside
the old tunnel, and under rather more difficult conditions. The first
task, particularly if men have been imprisoned behind the fallen
material, is to open communication through it between the two uninjured
portions of the tunnel. It is advisable to do this even when there is no
danger to life because of imprisoned workmen, since it enables the work
of repairing to be conducted from both directions. The excavation of a
passageway through the fallen material is rendered difficult, both
because the fallen material is of an unstable character, and also
because it is usually filled with the lining masonry, timbering, etc.
When, therefore, the accident has happened before the full section of
the original material has been removed, the first heading or drift is
driven through this original material rather than through the fallen
débris. Any of the regular soft-ground methods of tunneling may be
employed, but it is usually better to select one which allows the
masonry to be built with as little excavation as possible at first. For
this reason the German method of tunneling is particularly suited to
repair work of this nature. The Belgian method may also be used to
advantage, particularly when the caving extends to the surface of the
ground above, and the upper portion of the débris is, therefore,
practically the same material as that through which the original tunnel
was driven. The greatest defect of the Belgian method for making repairs
is that the roof arch is supported by a rather unstable mass of mingled
earth, stone, and timber, which constitutes the bottom layer of the
fallen material. The method of strutting the work when the German or
Belgian method is used is shown by Fig. 152. It sometimes happens that
the fallen débris is so unstable that it will not carry safely the arch
masonry in the Belgian method or the strutting in the German method, and
in these cases one of the full-section methods of excavation is usually
adopted. The nature of the strutting employed is shown by Fig. 153. When
the section has been opened and the new masonry built, great care should
be taken to fill the cavity behind the masonry with timber or stone; and
should the disturbance reach to the ground surface it is often a good
plan to sink a shaft through the disturbed material, and fill it with
more stable material.

[Illustration: FIG. 153.--Tunneling through Caved Material by Drifts.]

When the fallen débris fills only a part of the section, the first thing
to provide against is the occurrence of any further caving; and this is
usually done by building a protecting roof above the line of the future
roof masonry. Figs. 154 and 155 show two methods of constructing this
temporary roof, which it will be noticed is filled above with cordwood
packing. As soon as the temporary roof is completed, the lining masonry
is constructed.

[Illustration: FIGS. 154 and 155.--Filling in Roof Cavity Formed by
Falling Material.]

[Illustration: FIG. 156.--Timbering to Prevent Landslides at Portal.]

(4) Landslides which close the tunnel entrance are repaired in a variety
of ways. Fig. 156 shows a common method of preventing the extension of a
landslide which has been started by the excavation for the entrance
masonry. Fig. 157 shows a method often adopted when the slope is quite
flat and the amount of sliding material is small. It consists
essentially of removing the fallen material and building a new portal
farther back; that is, the open cut is extended and the tunnel is
shortened. When the amount of the sliding material is very large, the
contrary practice of lengthening the tunnel and shortening the open cut,
as shown by Fig. 158, may be adopted.

[Illustration: FIG. 157.--Shortening Tunnel Crushed by Landslide at
Portal.]


=Accidents After Construction.=--Accidents after the completion of the
tunnel may be divided into two classes: first, those which entirely
obstruct the passage of trains, of which the collapse of the roof is the
most common; and second, those which allow traffic to be continued while
the repairs are being made, such as the bulging inward of a portion of
the lining without total collapse. In the first case the first duty of
the engineer is to open communication through the fallen débris, so that
passengers at least may be transferred from one part of the tunnel to
the other and proceed on their way. This is done by driving a heading,
and strongly timbering it to serve as a passageway. If the tunnel is
single tracked this heading is afterwards enlarged until the whole
section is opened. In double-track tunnels the method generally adopted
is to open first one side of the section and timber it strongly, so as
to clear one track for traffic. While the trains are running through
this temporary passageway the other half of the section is opened and
repaired; the traffic is then shifted to the new permanent track, and
the temporary structure first employed is replaced with a permanent
lining. When the accident is such that the repairs can be made without
obstructing traffic entirely, various modes of procedure are followed.
In all cases great care has to be exercised to prevent accident to the
trains and to the tunnel workmen. The work should be done in small
sections so as to disturb as little as possible the already troubled
equilibrium of the soil; the strutting should be placed so as to give
ample clearing space to passing trains, and the trains themselves should
be run at slow speeds past the site of the repairs. To illustrate the
two kinds of accidents and the methods of repairing them, which have
been mentioned, the accidents at the Giovi tunnel in Italy and at the
Chattanooga tunnel in America have been selected.

[Illustration: FIG. 158.--Extending Tunnel through Landslide at Portal.]


=Giovi Tunnel Accident.=--In September, 1869, at a point about 220 ft.
from the south portal of the Giovi tunnel, a disturbance of the masonry
lining for a length of about 52 ft. was observed. Accurate measurements
showed that the lining was not symmetrical with respect to the vertical
axis of the sectional profile. It was concluded that owing to some
disturbance of the surrounding soil unsymmetrical vertical and lateral
pressures were acting on the masonry. Close watch was kept of the
distorted masonry, which for some time remained unchanged in position.
In 1872, however, new crevices were observed to have developed, and
shortly afterwards, in January, 1873, the injured portion of the masonry
caved in, obstructing the whole tunnel section. The fallen material
consisted chiefly of clay in a nearly plastic state. The surface of the
ground above was observed to have settled. Investigation showed also
that the cause of the caving was the percolation of water from a nearby
creek. The water had soaked the ground, and decreased its stability to
such an extent that the masonry lining was unable to withstand the
increased vertical and lateral pressures.

The mode of procedure decided upon for repairing the damage was: (1) To
open at least one track for the temporary accommodation of traffic; (2)
To remove permanently the causes which had produced the collapse; (3) To
build a new and much stronger lining. Close to the western side wall,
which was still standing, the débris was removed, and the opening
strongly strutted in order to allow the laying of a single track to
reëstablish communication. At the same time a shaft was sunk from the
surface above the caved portion of the tunnel, for the double purpose of
facilitating the removal of the fallen material and of affording
ventilation. The depth of the surface above the tunnel was 41.6 ft.,
which made the construction of the shaft a comparatively easy matter.
The shaft itself was 6¹⁄₂ ft. wide and 18 ft. long, with its longer
dimensions parallel to the tunnel, and it was lined with a rectangular
horizontal frame and vertical-poling board construction. After
temporary communication had been opened on the western track of the
tunnel, the remainder of the fallen earth was removed and the excavation
strutted. The new masonry lining was then built.

To remove permanently the cause of the cave-in, which was the
percolation of water from a close-by stream, this stream was diverted to
a new channel constructed with a concrete bed and side walls.

The failure of the original lining occurred by cracks developing at the
crown, haunches, and springing lines. The new lining was made
considerably thicker than the original lining, and at the points where
failure had first occurred in the original arch cut-stone _voussoirs_
were inserted in the brickwork of the new arch as described in Chapter
XIII.


=Chattanooga Tunnel.=--The Western & Atlantic Ry. passes through the
Chattanooga mountains by means of a single-track tunnel 1,477 ft. long,
constructed in 1848-49. The lining consisted of a brickwork roof arch
and stone masonry side walls. After the tunnel had been opened to
traffic, this lining bulged inward at places, contracting the tunnel
section to such an extent that it was decided to reconstruct the
distorted portions. After careful surveys and calculations had been
made, it was decided to take down and reconstruct about 170 ft. of the
lining.

Owing to contracted space in the tunnel, it was necessary to remove all
men, tools, and material, whenever trains were to pass through; and in
order to do this a work-train of three cars was fitted up with necessary
scaffolds, and supplied with gasoline torches for lighting purposes.
Mortar was mixed on the cars, and all material remained on them until
used. Débris torn out of the old wall was loaded on the cars, and hauled
to the waste dump. A siding was built near the West end of the tunnel
for the use of this train, and a telephone system was installed between
the entrances and the working-train. On account of the contracted
working-space and the greater ease with which brick could be handled,
it was decided to rebuild the walls out of brick instead of stone.

In tearing out the old wall a hole was first cut through the three
bottom courses of the arch and gradually widened. When the opening
became four or five feet long, a small jack was placed near the center
of it and brought to a bearing against the arch to sustain it. After
cutting the opening to a length of from 7 to 10 ft. depending on the
stability of the earth backing, the jack was removed and a piece of 8×16
in. timber placed under the arch and brought up to a bearing with jacks.
One end of the timber rested on the old wall, the other on a seat built
into the adjoining section of new wall. Wedges were then driven under
the ends of timber and the jacks removed. With this timber in place, the
old wall could be taken down with ease, the only trouble being that
small stones and earth fell in from above and behind the arch. This was
obviated by placing a 2 in. plank across the opening and just back of
the 8×16 in. timber. At several points, however, the earth backing was
saturated with water, and it became necessary to put in lagging as the
old wall was removed. This timbering would be taken out as the new work
was built up.

A suitable foundation for the new wall was secured at a depth from 2 to
4 ft., and a concrete footing was used. The section of the new wall was
then built up as near as possible to the 8×16 in. timber; the timber was
then removed and the new wall built up and keyed under the arch.

The new wall had a minimum width of 2¹⁄₂ ft. at the top, and 4 ft. at
the base of rail, and was provided with weep holes at intervals. To
facilitate matters, work was carried on simultaneously at two or three
different places, the intention being to get one place torn out and
ready for the bricklayers by the time they completed a section of the
new wall at another place.

In rebuilding the arch, sections extending from the springing line up as
far as was necessary to obtain the desired clearance, and from 2¹⁄₂ to
4 ft. in length, were removed. Near the sides, the earth above the arch
was a stiff clay, which was self-sustaining; but near the center there
occurred a stratum of gravel and clay saturated with water. This gave
considerable trouble, falling through almost continuously until
timbering could be placed. One end of this timber rested on the old
arch, the other on the adjoining section of the new work. As the new
work was to be set 6 to 13 ins. back from the old, it was necessary to
block up this distance on top of the old arch, to carry the end of the
lagging timber, in order that the timber should be clear of the new
arch.

Owing to the small clearance between the car roof and the arch, a
special form of centering was required, one that would occupy as small
space as possible. Bar iron 1 in. thick, 4 ins. wide, and 20 ft. long
was curved to a radius of 6¹⁄₂ ft., and on the underside of this was
riveted a 6-in. plate ¹⁄₄ in. thick. This plate projected 1 in. on the
sides of the centering, and carried the ends of the 1 in. boards used
for lagging. The rivets were counter-sunk on the outside of the
centering to present a smooth surface next the arch.

In keying up a section of the new work, a space about 18 ins. square had
to be left open for the use of the workmen. As soon as the next section
had been torn out, this space was built up. In building up the last
section, this space had to be filled from below, which proved to be a
tedious undertaking. The opening was gradually reduced to a size of 10 ×
18 in., and the top ring then completed and keyed up, the adhesion of
mortar holding the bricks in place until the key could be driven home.
The next ring was treated in a similar manner, and so on to the face
ring. Altogether 412 lin. ft. of the walls and 178 lin. ft. of the arch
were taken down and rebuilt, amounting in all to 607 cu. yds. of masonry
at the total cost of $7,440, or about $12.25 per cu. yds.

The regular trains arrived so frequently at the tunnel that slightly
over two hours was the longest working-time between any two trains, and
usually less than one hour at a time was all that it could be worked. In
addition to the regular trains, a large number of extra trains, moving
troops, had to be accommodated. Work was in progress eight months, and
during that time there was no delay to a passenger train. The repairs
were completed in August, 1899. The work was under the direction of Mr.
W. H. Whorley, engineer of the Western & Atlantic R. R., and foreman of
construction, A. H. Richards. A recent examination failed to reveal any
sign of settlement cracks at the junction points of the new and old
work.




CHAPTER XXIII.

RELINING TIMBER-LINED TUNNELS WITH MASONRY.


The original construction of many American railway tunnels with a timber
lining to reduce the cost and hasten the work has made it necessary to
reline them, as time has passed, with some more permanent material. In
most cases the work of removing the old lining and replacing it with the
new masonry has had to be done without interfering with the running of
trains, and a number of ingenious methods have been developed by
engineers for accomplishing this task. Three of these methods which have
been employed, respectively, in relining the Boulder tunnel on the
Montana Central Ry., in Montana, the Mullan tunnel on the Northern
Pacific Ry., in Montana, and the Little Tom tunnel on the Norfolk &
Western R. R., in Virginia, have been selected as fairly representative
of this class of tunnel work.


=Boulder Tunnel.=--This tunnel penetrates a spur of the main range of
the Rocky Mountains, at an elevation at the summit of grade of 5,454
ft., and is 6,112 ft. in length. Its alignment is a tangent, with the
exception of 150 ft. of 30′ curve at the north end. The material
penetrated is blue trap-rock with seams for 4,950 ft. from the north
end, and syenitic boulders with the intervening spaces filled with
disintegrated material for the remaining 1,160 ft. The dimensions and
character of the old timber lining and of the new masonry lining
replacing it are shown in Figs. 159 and 160.

The form of masonry adopted consisted of coarse rubble side walls of
granite, 13 ft. 8 ins. high, and generally 20 ins. thick, with a full
center circular arch of four rings of brick laid in rowlock form. When
greater strength was needed the thickness of the side walls was
increased to 30 ins. and that of the arch to six rings of brick.

[Illustration: ~Cross Section.~

~Longitudinal Section.~

~Cross Section.~

~Cross Section.~

FIGS. 159 and 160.--Relining Timber-Lined Tunnel.]

The first plan adopted in putting in the masonry was to remove all the
timbering; but owing to the large number of falls and slides this was
abandoned, and the plan followed was to leave in the three roof segments
of the timbering with the overlying cord-wood packing and débris. In
carrying on the work the first step was to remove the side timbers. This
was done by supporting the roof timbers, as shown in Fig. 159; that is,
the first and fourth arch rib of an 8-ft. section containing four arch
ribs were supported by temporary posts. The intermediate arch ribs were
supported against the downward pressure by 6 × 6 in. timbers, extending
from the side ribs near the tops of the temporary posts to the opposite
sides of the intermediate roof segments, as shown in the longitudinal
section, Fig. 160. To resist the pressure from the sides, 4 × 6 in.
braces were placed across the tunnel from near the center of the
intermediate segments to the upper ends of the hip segments, as shown in
the cross-section, Fig. 159. The hip segments were then sawed off below
the notch, and the side timbering removed and the masonry built.

The stone was conveyed into the tunnel on flat cars, and laid by means
of small derricks located on the cars. Two derricks were used, one for
each side wall, and the work on both walls was carried on
simultaneously.

The arch was built upon a centering, the ribs of which were 5¹⁄₂ ins.
less in diameter than the distance between the side walls, so as to
permit the use of 2³⁄₄ ins. lagging. Each center had three ribs, made in
1-in. or 2-in. board segments, 10 ins. thick and 14 ins. deep. These
ribs were mounted on frames, which followed the opposite walls, and were
4 ft. apart, making the total length of the center out to out about 9
ft. The frames, upon which the ribs were supported, are shown in Fig.
161. As will be seen, they were mounted on dollys to enable the center
to be moved from one section to another. Jacks were used to raise and
lower the center into its proper position.

[Illustration: ~Cross Section.~

~Longitudinal Section.~

FIG. 161.--Relining Timber-Lined Tunnel, Great Northern Ry.]

The arch was built up from the springing lines on both sides at the same
time, four masons being employed. The rings were built beginning with
the intrados, which was brought up, say, a distance of about 2 ft. from
the springing line. Then the back of the ring was well plastered with
from ³⁄₈ in. to ¹⁄₂ in. of mortar, and the second ring brought up to the
same height and plastered on the back, and so on until the last ring was
laid. After bringing the full width of the arch up some distance, new
laggings were placed on the ribs for an additional height of 2 ft. and
the same process was repeated. All the space between the extrados of the
masonry arch and the old lining was compactly filled with dry rubble.
When high enough so that the hip segments had a foot or more bearing on
the masonry the segments were securely wedged and blocked up against the
brickwork, and the longitudinal 4 × 6 in. timbers removed. The
remaining space was now clear for completion of the arch, and both sides
were brought up until there was not sufficient space for four masons to
work, when the keying was completed by two masons beginning at the
completed and working back toward the toothed end. The brickwork was
built from the top of a staging-car.

[Illustration: ~Cross Section.~

~Longitudinal Section.~

FIG. 162.--Relining Timber-Lined Tunnel, Great Northern Ry.]

In a few instances where slides occurred after the removal of the slide
timbering, the method of re timbering the tunnel shown in Fig. 162 was
adopted. Two side drifts were first run 2¹⁄₂ ft. wide by 4 ft. high, and
the plate timbers placed in position and blocked. Cross drifts were then
run, and the roof segments placed, and the core down to the level of the
bottoms of the side drifts taken out. The lower wall plates were then
placed and the hip segments inserted. The bench was then taken down by
degrees, the side plates being held by jacks, and the posts placed one
at a time. As the masonry at the points where slides occur consists of
30-in. walls and six-ring arch, the timbering was 22 ft. wide in the
clear, with other dimensions as shown in Fig. 162.

Only a single crew of brick and stone masons was employed. In order to
prepare the sections for these masons it was necessary to have timber
and trimming crews at work throughout the whole day of 24 hours, so that
an engine and two train crews were in constant attendance. The single
mason crews were able to complete 8 ft. of side wall and arch in 24
hours. The number of men actually employed at the tunnel was 35. This
included electric-light maintenance, and all other labor pertaining to
the work. The tunnel was lighted by an Edison dynamo of 20 arc light
capacity, one arc light being placed on each side of the tunnel at all
working-places. Each lamp carried a coil of wire 20 or 30 ft. long to
allow it to be shifted from place to place without delay.


=Mullan Tunnel.=--This tunnel is 3,850 ft. long, and crosses the main
range of the Rocky Mountains, about 20 miles west of Helena, Mont. The
tunnel is on a tangent throughout, and has a grade of 20% falling toward
the east. The summit of the grade, west of the tunnel, is 5,548 ft.
above sea level, and the mountain above the line of the tunnel rises to
an elevation of 5,855 ft. Owing to the treacherous nature of the
material through which the tunnel passed, it had been a constant menace
to traffic ever since its construction in 1883, and numerous delays to
trains had been caused by the falls of rock and fires in the timber
lining. For these reasons it was finally decided to build a permanent
masonry lining, and work on this was begun in July, 1892.

[Illustration: ~_With Wall Plates._~

~_Without Wall Plates._~

~Old Timber Sections.~

~_Minimum Section._~

~_Average Section._~

~Permanent Work.~

FIG. 163.--Relining Timber Lined Tunnel, Great Northern Ry.]

The original timbering consisted of sets spaced 4 ft. apart _c._ to
_c._, with 12 × 12 in. posts supporting wall plates, and a five-segment
arch of 12 × 12 in. timbers joined by 1¹⁄₂-in. dowels. The arch was
covered with 4-in. lagging, and the space between this and the roof was
filled with cordwood. Except where the width had been reduced by
timbering placed inside the original timbering to increase the strength,
the clear width was 16 ft., and the clear height 20 ft. above the top of
the rail. Fig. 163 shows the timbering and also the form of masonry
lining adopted. The side walls are of concrete and the arch of brick.
This new masonry, of course, required the removal of all the original
timbering. The manner of doing this work is as follows: A 7-ft. section,
_A B_, Fig. 164, was first prepared by removing one post and supporting
the arch by struts, _S S_. After clearing away any backing, and
excavating for the foundation of the side wall, two temporary posts, _F
F_, were set up, and fastened by hook bolts. Fig. 146, _L_, and a
lagging was built to form a mold for the concrete. Several of these
7-ft. sections were prepared at a time, each two being separated by a
5-ft. section of timbering.

[Illustration: ~Section, with Concrete Car.~

~With Wall Plate.~

~Without Wall Plate.~

~Longitudinal Section.~

FIG. 164.--Construction of Centering Mullan Tunnel.]

The mortar car was then run along, and enough mortar (1 cement to 3
sand) was run by the chute into each section to make an 8-in. layer of
concrete. As the car passed along to each section, broken stone was
shoveled into the last preceding section until all the mortar was taken
up. The walls were thus built up in 8-in. layers, and became hard enough
to support the arches in about 10 to 14 days. The arches were then
allowed to rest on the wall, and the posts of the remaining 5-ft.
sections were removed, and the concrete wall built up in the same way as
before.

The average progress per working-day was 30 ft. of side wall, or about
45 cu. yds.; and the average cost, including all work required in
removing the timber work, train service, lights and tools, engineering
and superintendence, and interest on plant, was $8 per cubic yard.

[Illustration: FIG. 165.--Centering Mullan Tunnel.]

The centering used for putting in the brick arches is shown in Fig. 165.
From 3 ft. to 9 ft. of arch was put in at a time, the length depending
upon the nature of the ground. To remove the old timber arch, one of the
segments was partly sawed through; and then a small charge of giant
powder was exploded in it, the resulting débris, cordwood, rock, etc.,
being caught by a platform car extending underneath. From this car the
débris was removed to another car, which conveyed it out of the tunnel.
The center was then placed and the brickwork begun, the cement car shown
in Fig. 164 being used for mixing the mortar. The size of the bricks
used was 2¹⁄₂ + 2¹⁄₂ + 9 ins., four rings making a 20-in. arch and
giving 1.62 cu. yds. of masonry in the arch per lin. ft. of tunnel. The
bricks were laid in rowlock bond, two gangs, of three bricklayers and
six helpers each, laying about 12 lin. ft. per day. The brickwork cost
about $17 per cu. yd. The total cost of the new lining averaged about
$50 per lin. ft.

[Illustration: ~Cross Section.~

~Longitudinal Section.~

FIG. 166.--Relining Timber-Lined Tunnel, Norfolk and Western Ry.]


=Little Tom Tunnel.=--The tunnel has a total length of 1,902 ft., but
only 1,410 ft. of it were originally lined with timber. This old timber
lining consists of bents spaced 3 ft. apart, and located as shown by the
dotted lines in the cross-section, Fig. 166. Instead of renewing this
timber, it was decided to replace it with a brick lining. Although the
tunnel was constructed through rock, this rock is of a seamy
character, and in some portions of the tunnel it disintegrates on
exposure to the air. In removing the timber to make place for the new
lining some of the roof was found close to the lagging, but often also
considerable sections showed breakages in the roof extending to a height
varying from 1 ft. to 12 ft. above the upper side of the timbering. This
dangerous condition of the roof made it necessary that only a small
section of the timber lining should be removed at one time. It made it
necessary, also, that the brick arch should be built quickly to close
this opening, and finally that all details of centers, etc., should be
arranged so as to furnish ample clearance to trains. The accompanying
illustrations show the solution of the problem which was arrived at.

[Illustration: FIG. 167.--Relining Timber-Lined Tunnel, Norfolk and
Western Ry.]

Referring to the transverse and longitudinal sections shown by Fig.
166, it will be seen that two side trestles were built to carry an
adjustable centering for the roof arch. Two sections of these trestles
and centerings were used alternately, one being carried ahead and set up
to remove the timbering while the masons were at work on the other. The
manner of setting up and adjusting the trestles and centerings is shown
by Fig. 166 and also by Fig. 167, which is an enlarged detail drawing of
the set screw and rollers for the centering ribs. The following is the
bill of material required for one set of trestles and one center:

  Trestles:
    Caps and sills               8 pieces 8 ×  8 ins. × 20 ft.
    Posts                       18   „    8 ×  8  „   × 11  „
    Braces                      16   „    6 ×  4  „   ×  7  „
  Centerings:
    Ribs                        27   „    2 × 18  „   ×  7  „
    Bracing                     12   „    2 ×  8  „   ×  7  „
    Support to crown lagging     2   „    6 ×  6  „   × 10  „
    Crown lagging               20   „    3 ×  6  „   ×  2  „
    Side lagging                30   „    3 ×  6  „   × 10  „
    Side strips                  2   „    2 × 12  „   ×  9  „
    Blocking for rollers         1   „    5 ×  8  „   × 12  „

  6 screw and roller castings complete with bolts and lever; 114 bolts
  ³⁄₄-ins. in diameter; 7¹⁄₂ U. H. hexagonal nut and 2 cast washers
  each.

With this arrangement the progress made per day varied from 2 lin. ft.
to 3 lin. ft. of lining complete. By work complete is meant the entire
lining, including stone packing between the brickwork and the rock. On
Feb. 23, 1900, 363 ft. of lining had been completed, at a cost of $33.50
per lin. ft. This cost includes the cost of removing the old timber, the
loose rock above it, and all other work whatsoever.




CHAPTER XXIV.

THE VENTILATION AND LIGHTING OF TUNNELS DURING CONSTRUCTION.


VENTILATION.

In long tunnels, especially when excavated in hard rock, proper
ventilation is of great importance, because the air cannot be easily
renewed, and the amount of oxygen consumed by miners horses and lamps
during construction is very large. The gases produced by blasting also
tend to fill the head of excavation with foul air. Pure atmospheric air
contains about 21% of oxygen and only 0.04% of carbonic acid; when the
latter gas reaches 0.1% the fact is indicated by the bad odor; at 0.3%
the air is considered foul, and when it reaches 0.5% it is dangerous. It
is generally admitted that the standard of purity of the air is when it
contains 0.08% of carbonic acid.

A large quantity of carbonic acid in the air is easily detected by
observing the lamps, which then give out a dim red light and smoke
perceptibly; the workmen also suffer from headache and pains in the
eyes, and breathe with difficulty. Naturally, miners cannot easily work
in foul air and, therefore, make very slow progress. It is, therefore,
to the interest of the engineer to afford good ventilation, not only
because of his duty to care for the safety and health of his men, but
also for reasons of economy, so that the men may work with the greatest
possible ease, thus assuring the rapid progress of the work.

It would be impossible to change completely the atmosphere inside a
tunnel, as the gases developed from blasting will penetrate into all the
cavities and gather there, but the fresh air carried inside by
ventilation has a very small percentage of carbonic acid, mixes with
that which contains a greater quantity, and dilutes it until the air
reaches the standard of purity. We have not here considered the gases
developed from the decomposition of carboniferous and sulphuric rocks,
which may be met with in some tunnels, and which render ventilation
still more necessary. Tunnels may be ventilated either by natural or
artificial means.


=Natural Ventilation.=--It is well known that if two rooms of different
temperatures are put in communication with each other, e.g., by opening
a door, a draft from the colder room will enter the other from the
bottom, and a similar draft at the top, but with a contrary direction,
will carry the hot air into the colder room, thus producing perfect
ventilation, until the two rooms have the same temperature. Now, during
the construction of tunnels the temperature inside may be considered as
constant, or independent of the outside atmospheric variations; hence
during summer and winter, there will always be a draft affording
ventilation, owing to the difference of temperature inside and outside
the tunnel. In winter time the cold air outside will enter at the bottom
of the entrances and headings, or along the sides of the shafts, and the
hot air will pass out near the top of the headings or entrances or the
center of the shafts; in summer the air currents will take the contrary
direction.

Natural ventilation in tunnels is improved when the excavation of the
heading reaches a shaft, because the interior air can then communicate
with the exterior at two points, at different levels. In such cases a
force equal to the difference in weight between a column of air in the
shaft and a similar one of different density at the entrance of the
tunnel, will act upon the mass of air in the tunnel and keep it in
movement, thus producing ventilation. Consequently, during winter, when
the outside air has greater weight than that inside, the air will come
in by the headings and go out by the shaft, and in the summer it will
enter at the shaft and pass out at the entrance. Sometimes to afford
better ventilation shafts 8 or 12 in. in diameter are sunk exclusively
for the purpose of changing the air. When the inside temperature is
equal to that outside, as often happens during the spring and autumn,
there are no drafts, and consequently the air in the excavation is not
renewed and becomes foul; then fires are lighted under the shaft and a
draft is artificially produced. The hot air going out through the shaft,
as through a chimney, allows the fresh air to come in as in ordinary
ventilation.

When the head of the excavation is very far from the entrances, or when
the mountain is too high to allow excavation by shafts, it is quite
impossible to secure good natural ventilation, especially during the
spring and autumn months, and the engineer has to resort to some
artificial means by which to supply fresh air to the workmen.


=Artificial Ventilation.=--Artificial ventilation in tunnels may be
obtained in two different ways, known as the vacuum and plenum methods.
Their characteristic difference consists in this, that in the vacuum
method the air is drawn from the inside and the vacuum thus produced
causes the fresh air from the outside to rush in, while the plenum
method consists in forcing in the fresh air which dilutes the carbonic
air produced inside the tunnel by workingmen and explosives. In the
vacuum method the pressure of the atmosphere inside the tunnel is always
less than the pressure outside, while in the plenum method the pressure
within is always greater than that outside. Ventilation is the result of
this difference of pressure, as the tendency of the air toward
equilibrium produces continuous drafts. Both these methods have their
advantages and disadvantages; but in the presence of hard rock, when
explosives are continually required, the vacuum method is considered the
best, because the gases attracted to the exhaust pipes are expelled
without passing through the whole length of the tunnel, thus avoiding
the trouble that a draft of foul air will give to the workmen who are
within the tunnel. In both these methods it is necessary to separate
the fresh air from the foul one; and this is done by means of pipes
which will exhaust and expel the foul air in the vacuum method, or force
to the front a current of fresh air when the plenum method is used.
Artificial ventilation may also be obtained by compressed air which is
set free after it has driven the machines, especially in tunnels
excavated through rock, when rock drilling machines moved by compressed
air are employed.


=Vacuum Method Contrivances.=--The most common of the vacuum appliances
consists in the simple arrangement of a pipe leading from the head of
the tunnel out through the fire of a furnace. The air in the pipe is
rarefied by the heat of the furnace and then set free from the other end
of the pipe, thus creating a partial vacuum in the pipe, into which the
foul air of the head rushes, the fresh air from the entrance taking its
place, and thus ventilating the tunnel. A similar arrangement may be
used with shafts, and the foul air may be driven out by a furnace which
is placed either at the top or bottom of the shaft. Such furnaces act
the same as those commonly used for heating purposes in the houses, with
this difference, that, instead of fresh air being forced in, foul air is
expelled. Another simple arrangement for producing a vacuum is by means
of a steam jet which is thrown into the pipe, and which helps the
expulsion of the air by heating it, thus producing a different density
which originates a draft besides that mechanically originated by the
force of the steam jet, which tends to carry out the foul air of the
pipes.

Foul air may also be expelled by means of exhaust fans which are
connected with pipes near the entrance of the tunnel. The fan consists
of a box containing a kind of a paddle wheel turned by steam or water
power and arranged so as to revolve at a high speed. The air inside the
pipe is forced out by blades attached to the wheel, and thus the foul
air of the front is driven away and fresh air from the entrance rushes
in to take its place, and perfect ventilation is obtained.

The best manner of expelling foul air from tunnels, according to the
vacuum method, is by means of bell exhausters. This consists of two sets
of bells connected by an oscillating beam and balancing each other. Each
set consists of a movable bell, which covers and surrounds a fixed bell
with a water joint. In the central part of the fixed bell there are
valves which open upwards, and on the bottom of each movable bell there
are valves which open from the outside. When one bell ascends, the
valves at the bottom are closed, the air beneath is then rarefied, and a
vacuum is produced; the valves in the central part of the fixed bell
filled with water are opened, and there is an aspiratory action from the
pipe leading to the headings, and the foul air is thus carried away. The
apparatus makes about ten oscillations per minute, and the dimensions of
the bells depend upon the quantity of air to be exhausted in a minute.
In the St. Gothard tunnel, where these bell exhausters were used, they
exhausted 16,500 cu. ft. of air per minute.


=Plenum Method Contrivances.=--Fresh air may be driven into tunnels to
dilute the carbonic acid by two different ways, viz., by water blast and
by fans. Water when running at a great velocity produces a movement in
the air which may be sometimes usefully and economically employed for
ventilating tunnels. Water falling vertically is let run into a large
horizontal zinc pipe having a funnel at the outer end; into this the air
attracted by the velocity of the water is forced. By an opening at the
bottom the water is afterward withdrawn from the pipe, and there remains
only the air which is pushed forward by the air which is being
continually sucked in by the velocity of the water.

The best and most common means of ventilation by the plenum method is by
fans. There are numerous varieties of these fans in the market, but they
all consist of a kind of fan wheel which by rapid revolution forces the
fresh air into the pipe leading to the headings of the tunnel or to the
working places. Instead of a large single fan, such as is used for
mining purposes, it is better to have a number of small fans acting
independently of each other, conveying the fresh air where it is needed
through independent pipes.


=Saccardo’s System.=--A new method of ventilating tunnels was devised by
Mr. Saccardo for the ventilation of the Pracchia tunnel along the
Bologna and Lucca Railway in Italy. At the highest end of the tunnel the
mouth was contracted inward in a funnel shaped form so as to just admit
a train. Immediately at this contraction, a lateral tunnel, 50 feet
long, branched off from one side of the main tunnel. At the mouth of
this lateral tunnel was installed a fan which forced air into the tunnel
and with 70 revolutions per minute delivered 3.532 cu. ft. of air per
second at a water pressure of 1 in. This air current was directed inward
through a second contraction or funnel, parallel to the one at the
entrance and 23 ft. beyond it. In operation the action of the artificial
air current was to suck in a considerable volume of outside air, while
the air pressure was sufficient to counterbalance the movement of air
produced by a train moving at a velocity of 16.1 ft. per second. Mr.
Saccardo’s method was employed in ventilating a tunnel on the Norfolk
and Western Railway with satisfactory results.


=Compressed Air.=--In the excavation of tunnels in hard rock a number of
rock drilling machines are employed which are moved by compressed air at
a pressure of not less than five atmospheres. At each stroke about 100
cu. ins. of compressed air are set free, and at an average of 10 strokes
per minute there would be 5000 cu. ins. of air at five atmospheres or
25,000 cu. ins., or a little more than 175 cu. ft. of fresh air at
normal pressure set free every minute by each of the machines employed.
But the air exhausted from the drilling machine is foul.

Regarding ventilation by compressed air, Mr. Adolph Sutro, in a lecture
delivered to the mining students of the University of California, said:

  “I will note a curious fact which I have never seen explained, and
  which is worthy of close investigation by means of experiments. In the
  Sutro tunnel we found that the compressed air used for driving the
  machine drills, after having been compressed and expanded and
  discharged from the drills, was not wholesome to breathe, and the men
  and mules would all crowd around the end of the blower pipe to get
  fresh air. Whether the air in being compressed has parted with some of
  its oxygen or because vitiated from some other cause, I do not know,
  and I hope that this subject will at some future day be carefully
  examined into.”

In the December, 1901, number of “_Compressed Air_,” a magazine
especially devoted to the useful application of compressed air, is read:

  Compressed air wasted from power drills is so contaminated with oil
  from the cylinders that it cannot be taken into consideration as
  ventilation. It is as important to displace it with pure air as it is
  to drive out or draw off other vitiated air. The ventilation should be
  an independent supply provided by fan or blower, delivering by pipe at
  the point where miners are working.


=Quantity of Air.=--The quantity of air to be introduced into tunnels
must be in proportion to the oxygen consumed by the men, the animals,
and the explosions. It is allowed that the quantity of air required for
breathing purpose and explosions is as follows:

  1 workman with lamp needs 240 cu. yds. of fresh air in 24 hours.
  1 horse               „   850    „          „    „         „
  1 lb. gunpowder           100    „          „    „
  1 lb. dynamite            150    „          „    „

In a long tunnel excavated through hard rock the number of workmen all
together may be assumed at 400 at each end, and each workman is supposed
to be furnished with a lamp. No less than ten horses are employed, and
the average quantity of dynamite consumed is 600 lbs. per day. From the
data given the consumption of air by workmen and lamps would be: 240 ×
400 = 96,000 cu. yds.; the consumption of air by horses would be 850 ×
10 = 8500 cu. yds.; the consumption of air by dynamite would be 150 ×
600 = 90,000 cu. yds.; making a total consumption of air per day of
194,500 cu. yds., or about 8000 cu. yds. per hour.

To obtain good ventilation, then, it will be necessary to furnish every
hour a quantity of fresh air amounting to not less than 8000 cu. yds.
Since, however, a large quantity of pure air is expelled with the foul
air, it is necessary greatly to increase this quantity.

It may be observed, in closing, that the water having its particles
divided, as in a fog or mist, rapidly precipitates the gases produced by
explosions. Now, when hydraulic machines are used, there is a hollow
ball pierced by holes that are almost imperceptible, from which the
compressed water spreads in very subtile particles, and this causes the
fall of the gases from explosions. Such a method of precipitating gases
is very good, but does not have the advantage of supplying new oxygen to
replace that consumed by the men, animals, lamps, and explosions;
besides, it has the defect of increasing the quantity of water to be
removed. In tunnels the pipes used either for conveying the fresh air or
for carrying away the foul air, are of iron, having a diameter of about
8 in.; they are fixed along the side walls about 3 ft. above the
inverted arch.


LIGHTING.

The object and necessity of a perfect lighting of the tunnel-workings
during construction are so obvious that they need not be enlarged upon.
Comparatively few tunnels require lighting after completion; and these
are generally tunnels for passenger traffic under city streets, of which
the Boston Subway is a representative American example. Considering the
methods of lighting tunnels during construction, we may, for sake of
convenience, chiefly, divide the means of supplying light into (1) lamps
and lanterns usually burning oil; (2) coal-gas lighting; (3) acetylene
gas lighting; and (4) electric lighting.


=Lamps and Lanterns.=--Lamps and lanterns are commonly employed by
engineers for making surveys inside the tunnel, and to light the
instrument. For ranging in the center line, a convenient form of lamp
consists of an oil light inclosed in glass chimney covered with sheet
metal, except for a slit at the front and back through which the light
shines, and on which the observer sights his instrument. To direct the
operations of his rodmen the engineer usually employs a lantern, either
with white or colored glass, much like the ordinary railway trainman’s
lantern, which he swings according to some prearranged code of signals.

Lamps and lanterns are used by the workmen both for signaling and for
lighting the workings. For signaling purposes red lanterns are usually
placed to denote the presence of unexploded blasts or other points of
possible danger; and colored or white lights are usually placed on the
front and rear of spoil and material trains. For lighting purposes, two
forms of lamps are employed, which may be somewhat crudely designated as
lamps for individual use and lamps for general lighting. Individual
lamps are usually of small size, and burn oil; they may be carried in
front of the miner’s helmet, or be fixed to standards, which can be set
up close to the work being done by each man. Miners’ safety lamps should
be employed where there is danger from gas. A great variety of lamps for
mining and tunneling purposes are on the market, for descriptions of
which the reader is referred to the catalogues of their manufacturers.

Lamps for general lighting are always of larger size than lamps for
individual use. A common form consists of a cylinder ten or twelve
inches in diameter, provided with a hook or bail for suspension, and
filled with benzine, gasolene, or other similar oil. Connected with this
cylinder is a pipe of considerable length and small diameter through
which the benzine or gasolene vapor runs, and burns when lighted with a
brilliant flame. Lamps of this type burning gasolene were extensively
employed in building the Croton Aqueduct tunnel. Various patented forms
of lamps for burning coal-oil products are on the market, for
descriptions of which the manufacturers’ catalogues may be consulted.


=Coal-gas Lighting.=--A common method of lighting tunnel workings is by
piping coal-gas into the headings and drifts from some nearby permanent
gas plant, or from a special gas works constructed especially for the
work. Gas lighting has the great advantage over lamps and lanterns of
giving a light which is more brilliant and steady. Its great objection
is the danger of explosion caused by leaks in the pipes, by breaks
caused by flying fragments of rock, and by the carelessness of workmen
who neglect to turn off completely the burners when they extinguish the
lights. In nearly every tunnel where gas has been used for lighting, the
records of the work show the occurrence of accidents which have
sometimes been very serious, particularly when fire has been
communicated to the tunnel timbering.


=Acetylene Gas Lighting.=--The comparatively recent development of
acetylene gas manufactured from carbide of calcium has given little
opportunity for its use in tunnel lighting, and the only instance of its
use in the United States, so far as the author knows, is the water-works
tunnel conduit for the city of Washington, D. C. Col. A. M. Miller, U.
S. Engineer Corps, who is in charge of this work, describes the method
adopted in his annual report for 1899 as follows:--

  “It had been the practice to do all work underground by the light of
  miners’ lamps and torches. This means of illumination is very poor for
  mechanical work. The fumes and smoke from blasting, added to the smoke
  from torches and lamps, render the atmosphere underground, especially
  when the barometer conditions were unfavorable to ventilation, very
  offensive and discomforting to the workmen. An investigation of the
  subject of lighting the tunnel by other means, more especially at the
  locality where the mechanics were at work,--brick and stone masons,
  and the workmen on the iron lining,--resulted in the selection of
  acetylene gas as the most available and economical in this special
  emergency. Accordingly, an acetylene gas plant for 300 burners was
  erected at Champlain-Avenue shaft, and one for 60 lights at Foundry
  Branch. The engine-houses at the shafts, the head-houses, and
  localities in the tunnel, when required, are lighted by these plants.

  “Gas pipes were carried down the Champlain-Avenue shaft and along the
  tunnel both in an easterly and westerly direction, with cocks for
  burners at proper intervals every 30 feet; and this system sufficed
  for illumination from Hock Creek to Harvard University, a distance of
  over two miles. The plant erected at Foundry Branch was in like manner
  utilized for the illumination from that point in both directions.

  “By connecting with the stopcocks by means of a rubber hose, a
  movable light, chandelier, or ‘Christmas-tree’ of any required number
  of burners is used, thus concentrating the light in the immediate
  vicinity of the work, and also enabling the illumination to be carried
  into the cavities or ‘crow-nests,’ so called, behind the defective old
  lining.

  “This method of illuminating has proved very satisfactory and quite
  economical. It is especially valuable as enabling good work to be
  done, and facilitating a thorough inspection of the same.”


=Electric Lighting.=--By far the most perfect, and at present the most
commonly employed means for lighting tunnel workings, is electricity.
The light furnished by electric lamps is steady and brilliant, and does
not consume oxygen or give off offensive gases. The wires are easily
removed and extended, and the lamps are easily put in place and removed.
About the only objection to the method is the fragility of the lamps,
which are easily broken by the flying stones and the concussion produced
by blasting.




CHAPTER XXV.

THE COST OF TUNNEL EXCAVATION AND THE TIME REQUIRED FOR THE WORK.


=Cost.=--The cost of a tunnel will depend upon the cost of the two
principal operations required in its construction, viz., the excavation
of the cross section and the lining of the excavation with masonry,
metal, or timber. These two operations may in turn be subdivided, in
respect to expense, into cost of labor and cost of materials. It is a
comparatively simple matter to calculate the cost of the building
materials required to construct a tunnel; but it is very difficult to
estimate with accuracy what the cost of labor will be. The reason for
this is that it is impossible to foresee exactly what the conditions
will be; the character of the material may change greatly as the work
proceeds, increasing or decreasing the cost of excavation; water may be
encountered in quantities which will materially increase the
difficulties of the work, etc. Nevertheless, while accurate preliminary
estimates of cost are not practicable, it is always desirable to attempt
to obtain some idea of the probable expense of the work before beginning
it, and the more usual means of getting at this point will be discussed
here.

Two methods of estimating the cost of tunnel work are employed. The
first is to calculate the probable expense of the various items of work,
based upon the available data, per unit of length, and then add to this
a margin of at least 10% to allow for contingencies; the second is to
apply to the new work the unit cost of some previous tunnel built under
substantially the same conditions. In the first method it is usual to
consider the strutting and hauling as constituting a part of the work
of excavation. To estimate the cost of excavation involves the
consideration of three general items, viz., the excavation proper, the
strutting of the walls of the excavation, and the hauling of the
excavated materials and the materials of construction.

The cost of excavating the preliminary headings or drifts is greater per
unit of material removed than that of excavating the enlargement of the
section. The cost of bottom drifts is also always greater than that of
top headings, the material penetrated remaining the same. Mr. Rziha
gives the comparative unit costs of excavating drifts, headings, and
enlargement of the profile as follows:--

  Bottom drifts           $9.20 per cu. yd.
  Top headings             4.80  „   „   „
  Enlargement of profile   2.84  „   „   „

The cost of hauling increases with the length of the tunnel. This fact
and amount of this increase are indicated by the following actual prices
for the Arlberg tunnel:--

  Top heading             $6.76 per cu. yd., increasing 37 cts. per mile
  Bottom drift             7.40  „   „   „        „     26  „    „   „
  Enlargement of profile   2.70  „   „   „        „     10  „    „   „

In all the prices given above, the cost of strutting and hauling is
included in the cost of excavation.

The cost of excavation is not always the same for the same character of
materials in different tunnels. The following figures show the prices
paid for the excavation of calcareous rock in four different German
tunnels:--

  Berliner Nordhausen Wetzler R.R.     $1.24 per cu. yd.
  Ofen                                 1.30  „   „   „
  Stafflach                            2.76  „   „   „
  Gries                                1.92  „   „   „

The method of tunneling has little influence upon the cost of the work,
as shown by the following figures from tunnels excavated through
calcareous rock by different methods:--

  Ofen tunnel        Austrian method      $93.19 per lin. ft.
  Dorremberg tunnel  Belgian method        86.08  „   „    „
  Stafflach tunnel   English method        91.69  „   „    „

The Martha and Merten tunnels, excavated through soft ground by the
Austrian and German methods respectively, cost $87.95 and $87.55 per
lin. ft. respectively. In the excavation of the various sections of the
tunnel for the new Croton Aqueduct in America, the following prices were
paid:--

  Excavation of heading    $8 to $10.00 per cu. yd.
  Tunnel in soft ground     8 to   9.00  „   „   „
  Tunnel in rock            7 to   8.50  „   „   „
  Brick masonry                   10.00  „   „   „
  Timber in place             $40 per M. ft. B. M.

It is the practice in America to include the work of hauling under
excavation, but not to include the strutting, which is paid for
separately. In some cases only the market price of the timber is paid
for separately, the cost of setting up being included in the price of
excavation. The writer prefers the European practice of including the
total cost of timbering under excavation, since the two operations are
so closely connected, and since the contractor employs the same timber
over and over again. Knowing the dimensions of the several members of
the strutting, it is a simple, although somewhat tedious,

process to calculate the total quantity required. An idea of the
quantity of timber required for strutting in soft ground may be had from
the data given on page 55. The quantity will decrease as the cohesion of
the material penetrated increases, until it becomes so small in hard
rock-tunnels as to cut very little figure in the total cost.

The cost of hoisting excavated materials through shafts depends upon the
depth from which it is hoisted, and upon the character of hoisting
apparatus employed. The following table, showing the cost of hoisting
for different lifts and by different methods, is given by Rziha, the
cost being in francs per cubic meter:--

  +-------+----------+----------------------+-------------+
  | HEIGHT| WINDLASS.|     HORSE GINS.      |STEAM HOISTS.|
  |   IN  +----------+----------+-----------+-------------+
  |METRES.|          |ONE HORSE.|TWO HORSES.|             |
  |       |  Francs  |  Francs  |   Francs  |   Francs    |
  |       |per Cu. M.|per Cu. M.| per Cu. M.|  per Cu. M. |
  +-------+----------+----------+-----------+-------------+
  |   15  |   0.172  |   0.077  |   0.062   |    0.035    |
  |   30  |   0.212  |   0.087  |   0.070   |    0.045    |
  |   45  |   0.257  |   0.100  |   0.080   |    0.050    |
  |   60  |   0.305  |   0.112  |   0.092   |    0.082    |
  |   90  |   0.410  |   0.152  |   0.110   |    0.087    |
  |  120  |   0.535  |   0.195  |   0.135   |    0.092    |
  |  150  |   0.722  |   0.240  |   0.157   |    0.112    |
  +-------+----------+----------+-----------+-------------+

Mr. Séjourné, a French engineer, who has been connected with the
construction of numerous tunnels by the Belgian method where he was in
position to secure comparative figures, has given the following rules
for calculating the cost of tunnels. Assuming _A_ to represent the cost
of excavating a cu. yd. in the open air, the cost of excavating the same
quantity underground in driving headings will be from 9 _A_ to 11 _A_,
and in enlarging the profile it will be about 5 _A_. The cost of
constructing single-track tunnels varies with the thickness of the
lining, and may be calculated by the following formulas:

  Without lining,           _C_ = 5.5 _A_.
  With roof arch only,      _C_ = 6.4 + 6.4 _A_.
  With lining 18 in. thick, _C_ = 9.4 + 7 _A_.
  With lining 2 ft. thick,  _C_ = 11 + 8 _A_.

In these formulas _C_ is the cost per cu. yd. of excavation, including
the masonry. For double-track tunnels the amounts given by the above
formulas may be used by reducing them about 7¹⁄₂% or 8%.

The second method of estimating the cost of tunnel work consists in
assuming as a unit the unit cost of tunnels previously excavated under
similar conditions. Mr. La Dame gives the following unit prices for a
number of tunnels driven through different materials:

  +-------------------+--------+-------------+--------+----------------+
  |  NATURE OF SOIL.  |TUNNELS,| EXCAV. PER  |COST PER|  MAX. AND MIN. |
  |                   | NO. OF |   CU. YD.   |LIN. FT.|  PER LIN. FT.  |
  +-------------------+--------+-------------+--------+----------------+
  |Granite-gneiss     |   56   |$3.07 @ $3.85| $100.  |$61.46 @ $190.40|
  |Schist             |   39   | 1.38 @  1.53|   75.42| 43.11 @   70.68|
  |Triassic           |    3   |     ...     |   90.85| 84.75 @   93.33|
  |Jurassic           |   69   | 1.23 @  1.38|   77.86| 35.24 @  157.2 |
  |Cretaceous         |   34   | 0.61 @  0.77|   59.60| 27.37 @   92.25|
  |Tertiary and modern|   39   | 0.33 @  0.61|  105.80| 51.52 @  188.36|
  +-------------------+--------+-------------+--------+----------------+

In the following table is given a list of tunnels excavated through
different soils, from the most compact to very loose materials, and
driven according to the various methods which have been illustrated.

DOUBLE-TRACK TUNNELS.

  +--------------+-----------------+--------+-------------+
  |NAME OF       |QUALITY OF SOIL. |COST PER|  METHOD OF  |
  |TUNNELS.      |                 |LIN. FT.|  TUNNELING. |
  +--------------+-----------------+--------+-------------+
  |Mt. Cenis     |Granitic,        |$273.73 |Drift.       |
  |St. Gothard   |...              | 193.63 |Heading.     |
  |Stammerich    |Granitic,        | 157.90 |English.     |
  |Stalle        |Broken schist,   | 290.58 |Austrian.    |
  |Bothenfels    |Dolomite,        | 115.64 |English.     |
  |Dorremberg    |Calcareous,      |  86.08 |Belgian.     |
  |Stafflach     |Calcareous,      |  91.69 |English.     |
  |Ofen          |Calcareous,      |  93.19 |Austrian.    |
  |Wartha        |Grewack,         |  87.95 |Austrian.    |
  |Mertin        |Grewack,         |  87.55 |German.      |
  |Schloss Matrei|Clay schist,     |  94.25 |English.     |
  |Trietbitte    |Clay and sand,   | 229.0  |German.      |
  |Canaan        |Clay-slate,      |  69.50 |Wide heading.|
  |Church-Hill   |Clay with shells,| 178.0  |...          |
  |Bergen No. 1  |Trap rock,       | 182.31 |...          |
  +--------------+-----------------+--------+-------------+

SINGLE-TRACK TUNNELS.

  +--------------+--------------------+----------+-------------+
  |   NAME OF    |  QUALITY OF SOIL.  | COST PER |  METHOD OF  |
  |   TUNNELS.   |                    | LIN. FT. |  TUNNELING. |
  +--------------+--------------------+----------+-------------+
  |Mt. Cenis     |Gneiss,             |$82.27    |Heading.     |
  |Stalletti     |Granite and quartz, | 62.75    |Austrian.    |
  |Marein        |Clay schist,        | 64.36    |English.     |
  |Welsberg      |Gravel,             |165.07    |Austrian.    |
  |Sancina       |Clay of 1st variety,|129.40    |Belgian.     |
  |Starre        |Clay of 2d variety, |191.61    |Belgian.     |
  |Cristina      |Clay of 3d variety, |307.42    |Italian.     |
  |Burk          |...                 | 83.90    |Wide heading.|
  |Brafford Ridge|...                 | 85.33    |Wide heading.|
  |Dunbeithe     |Limestone,          | 70.47    |Wide heading.|
  |Fergusson     |Sandstone,          | 37.46[16]|Wide heading.|
  |Port Henry    |Limestone,          | 80.00[17]|Wide heading.|
  |Points        |Granite,            | 72.00[16]|Wide heading.|
  +--------------+--------------------+----------+-------------+

  [16] Are unlined.

  [17] Lined with timber.

The Habas tunnel through quicksand, between Dax and Ramoux, France,
cost $118.50 per lin. ft. The cost of the Boston subway was $342.40 per
lin. ft. The Severn and Mersey tunnels, constructed through rock under
water, cost respectively $208.38 and $263 per lin. ft. The First Thames
Tunnel, driven by Brunel’s shield, cost $1661.66 per lin. ft. The Hudson
River and St. Clair River tunnels, excavated through soft ground by
means of shields and compressed air, cost respectively $305 and $315 per
lin. ft. The Blackwall double-track tunnel under the River Thames, which
is the largest tunnel ever built by the shield system, cost $600 per
lin. ft.

In making estimates of the cost of projected tunnel work based on the
cost of tunnels previously constructed through similar materials, it is
important to keep in mind the date and location of the work used as the
basis for calculations. For example, a tunnel excavated in Italy, where
labor is very cheap, will cost less than one excavated in America, where
labor is dear, all other conditions being the same. Other reasons for
variation in cost due to difference of date and location of construction
will suggest themselves, and should be taken into full consideration in
estimating the cost of the new work.


=Time.=--The time required to excavate a tunnel depends upon the
character of the material penetrated and upon the method of work
adopted. Tunnels driven through soft ground by hand require about the
same time to construct as tunnels driven through hard rock by the aid of
machinery. Tunnels can be driven through hard rock at about as great a
speed as through soft or fissured rock, chiefly because the work of
blasting is more efficient in hard rock, and because no time is required
in timbering. The following table shows the average rate of progress in
different parts of the tunnel excavation through both hard and soft
materials in feet per month:--

  +---------------+--------------------+--------------------+-----------+
  |    QUALITY    |      HEADING.      |      EXCAVATION    |ENLARGEMENT|
  |   OF SOIL.    |                    |      OF SHAFTS.    |OF PROFILE.|
  |               +----------+---------+---------+----------+-----------+
  |               | By hand. |    By   | By hand.|    By    |  By hand. |
  |               |          | machine.|         | machine. |           |
  +---------------+----------+---------+---------+----------+-----------+
  |Very loose soil|16.7- 26.8|         | 6.6-16.7|          |  6.6- 16.7|
  |Loose soil     |33.4-100  |         |16.7-33.4|          | 16.7- 33.4|
  |Soft rock      |66.8      |233.8-334|33.4-66.8|66.8-132.6| 33.4- 50  |
  |Hard rock      |50  - 66.8|233.8-334|33.4-50  |66.8-132.6| 66.8-100  |
  |Very hard rock |33.4      |233.8-334|16.7-33.4|66.8-132.6| 66.8-100  |
  +---------------+----------+---------+---------+----------+-----------+

The following tables showing the average rate of progress have been
compiled from the actual records made in the tunnels named:

  +-------------+-------------+--------+--------------+-------------+
  |   NAME OF   |  DIMENSIONS |MONTHLY | CHARACTER OF |OBSERVATIONS.|
  |   TUNNEL.   |   IN FEET.  |PROGRESS|   MATERIAL.  |             |
  |             |             |IN FEET.|              |             |
  +-------------+-------------+--------+--------------+-------------+
  |Excavation of|             |        |              |             |
  |headings by  |             |        |              |             |
  |hand:     |  |             |        |              |             |
  | Mount Cenis |10    × 10   |  65.8  |Schist,       |Bottom drift.|
  | Sutro       | 6.7  ×  5.7 |  70.14 |Quartzose,    |...          |
  | St. Gothard | 8.4  ×  8.7 |  70.14 |Granite,      |Top heading. |
  |             |             |        |              |             |
  |Excavation of|             |        |              |             |
  |headings by  |             |        |              |             |
  |machine:  |  |             |        |              |             |
  | Mount Cenis |10    × 10   | 188.7  |Calcareous    |             |
  |             |             |        |schist,       |Bottom drift.|
  | Sutro       | 8.15 × 10   | 227.45 |Quartzose,    |...          |
  | St. Gothard | 8.4  ×  8.7 | 339.45 |Granite,      |Top heading. |
  | Trari       | 8    ×  9.35| 167    |Gneiss,       |Top heading. |
  | Arlberg     | 8.35 ×  9.35| 474.2  |Mica schist,  |Bottom drift.|
  | Palisades   |16    ×  7   | 160    |Trap rock,    |Top heading. |
  | Busk        |15    ×  7   | 126    |Granite,      |Top heading. |
  | Cascade     |16    ×  8   | 180    |Basaltic rock,|Top heading. |
  | Franklin    |15    ×  7   | 240    |...           |Top heading. |
  +-------------+-------------+--------+--------------+-------------+

The following table shows the monthly progress of completed tunnel in
feet excavated through rock:

  +---------------+--------+----------+------------+
  |NAME OF TUNNEL.|PROGRESS|MATERIAL. |  METHOD.   |
  |               |IN FEET.|          |            |
  +---------------+--------+----------+------------+
  |Cascade        |  207   |Basalt,   |Top heading.|
  |Palisades      |  186   |Trap rock,|Top heading.|
  |Busk           |  190   |Granite,  |Top heading.|
  |Tennessee Pass |  169.5 |Granite,  |Top heading.|
  +---------------+--------+----------+------------+

The average monthly progress in feet of excavating tunnels through
treacherous ground may be quite generally assumed to be for: (1) clay of
the first variety from 43.4 ft. to 60 ft.; for clay of the second
variety from 33.4 ft. to 43.4 ft.; for clay of the third variety from
23.3 ft. to 33.4 ft., and for quicksand from 30 ft. to 50 ft. The
monthly progress in feet made in sinking the shafts of the Hoosac and
Musconetcong tunnels in America was as follows:--

  +---------------+------------+---------+--------+------------+
  |NAME OF TUNNEL.| DIMENSIONS | DEPTH   |PROGRESS|CHARACTER OF|
  |               |  IN FEET.  |IN FEET. |IN FEET.| MATERIAL.  |
  +---------------+------------+---------+--------+------------+
  |Hoosac:        |            |         |        |            |
  | East shaft    |15.4  × 27.7|  1035   |  21.7  |Mica schist.|
  | West shaft    | 8    × 16  |   267   |  16.7  |Gneiss.     |
  |Musconetcong:  |            |         |        |            |
  | Vertical shaft| 8.35 × 16.7|   113.5 | 100    |Loose rock. |
  | Inclined shaft| 8.35 × 26  |   304.  |  32    |Loose rock. |
  +---------------+------------+---------+--------+------------+

The average monthly progress of sinking shafts in treacherous soils may
be assumed to be as follows: clay of first variety, 50 ft. to 75 ft;
clay of second variety, 36.75 to 50 ft; clay of third variety, 23.4 ft.
to 36.75 ft; quicksand, 16.7 ft. to 33.4 ft.

For the reason that the details change with the various conditions
encountered in every work, all the tunnel operations have been treated
in a general way, purposely avoiding to give any detail. Also the rate
of progress and items of cost of tunnels have been given in a broad
manner because they greatly vary in the different works. This
information, however, can be easily obtained by consulting the
Engineering Magazines, where are reported all the tunnel works of
America and Europe, and where are given so many details which are very
valuable to expert engineers in charge of similar works, but not to
students and people who are looking only for general knowledge.




INDEX


  Accidents and Repairs in the Belgian Method, 152
  Accidents in Tunnels:
    After Construction, 308
    Baltimore Belt Line, 165
    Chattanooga Tunnel, 311
    During Construction, 301
    General Discussion, 301
    Giovi Tunnel, 309
    Repairing of, 304
  Acetylene Gas Lighting, 334
  Air Compressors, Description of, 87
  Air Locks, 264-272
  Air Pressure, 268
  American Method:
    General Description, 172
    Excavation, 172
    Strutting, 174
    Hauling, 175
  Arrangement of Drill Holes, 90
  Artificial Ventilation, 327
  Austrian Method of Tunneling:
    Advantages and Disadvantages, 180
    Excavation, 176
    General Description, 176
    Lining, 180
    Strutting, 177
  Average Progress in Tunnels, 342

  Baltimore Belt Line Tunnel, General Description, 160
  Barlow’s Shield, 242
  Beach’s Shield, 246
  Belgian Method:
    Accidents and Repairs, 152
    Advantages and Disadvantages, 152
    Excavation, 145
    General Description, 144
    Lining, 148
    Hauling, 150
    Strutting, 146
  Bench, 131
  Bends, 268
  Blackwall’s Tunnel Shield, 248
  Blasting-cone, 33
  Blickford Match, 31
  Boston Subway:
    General Descriptions, 203
    Roof Shield, 251
  Boulder Tunnel Relined, 315
  Box-cars, 61
  Box Strutting, 51
  Brandt Drilling Machine, 28, 112
  Brown, W. L., 269
  Brunel’s Shield, 240

  Caissons, 293
  Canals and Pipe Lines, 86
  Cascade Tunnel, 98
  Center-cut, 91
  Center Line:
    Curvilinear Tunnels, 14
    Determination of, 9
    Rectilinear Tunnels, 9
    Simplon Tunnel, 106
    Submarine Tunnels, 265
    Triangulation, 12
    Transferred through Center Shafts, 13
    Transferred through Side Shafts, 14
    Value’s Device, 10
  Centers:
    For Arches, 68
    English Method, 169
    Ground Molds, 66
    Italian Method, 184
    Lagging, 71
    Leading Frames, 67
    Setting Up, 70
    Striking, 71
  Chattanooga Tunnel, Accident, 311
  City and South London Railway Shield, 250
  Classification of Tunnels, 42
  Coal-gas Lighting, 333
  Cofferdam Method of Tunneling, 281
    Van Buren Street Tunnel, Chicago, 282
  Collapse of Tunnels, 302
  Compressed Air:
    For Power, 87
    For Ventilation, 330
  Concrete Lining, 75
    Fort George Tunnel, 139
    Murray Hill Tunnel, 126
  Cost of:
    Double-track Tunnels, 340
    Hauling, 338
    Headings, 337
    Hoisting, 338
    Single-track Tunnel, 340
    Submarine Tunnels, 341
    Subways, 209-217
    Tunnels, 336
  Craven, Alfred, 39
  Craven’s Sunflower, 39
  Cross-section:
    Dimensions of, 20
    Form of, 18
    Hudson River Tunnel Pennsylvania Railroad, 277
  Crown-bar (see American Method).
    Subways, 204-211
  Croton Aqueduct Tunnel, 95
  Culverts, 80

  Detroit River Tunnel, 296
  Diamond Drilling Machine, 27
  Directing the Shield, 265
  Drift, 37
  Drift Method:
    General Discussion, 102
    Murray Hill Tunnel, 123
    Simplon Tunnel, 103
  Drilling Machines:
    Brandt, 112
    Ingersoll, 26
  Drills:
    Diamond, 27
    Hand, 23
    Mountings for, 25
    Percussion, 24
    Power, 24
    Rotary, 27
  Dumping Cars, 60

  Electric Firing, 32
  Electric Lighting, 335
  English Method:
    Advantages and Disadvantages, 171
    Centers, 169
    Excavation, 166
    General Discussion, 166
    Lining, 170
    Strutting, 167
  Enlargement of the Profile, 38
  Entrances, 81
  Erector, 272
  Excavation:
    American Method, 172
    Arrangement of Drill Holes, 90
    Austrian Method, 176
    Belgian Method, 145
    Center-cut, 91
    Enlargement of Profile, 38
    English Method, 166
    Fort George Tunnel, 136
    German Method, 155
    Headings, 37, 91
    Hudson River Tunnel of Pennsylvania Railroad, 273
    Italian Method, 182
    Murray Hill Tunnel, 124
    Quicksand Method, 189
    Pilot Method, 193
    Shield and Compressed Air Method, 267
    Simplon Tunnel, 110
  Excavating Machines:
    For Earth, 22
    For Rock, 23
  Explosions, 33
    Dynamite, 30
    Gunpowder, 28
    Nitroglycerine, 29
    Quantity of, 34
    Storage of, 30

  Failure of Tunnel Roof, 305
  Forgie, James, 269
  Fort George Tunnel, 135
  Foundations for Lining, 76
  Fox, Charles B., 103
  Frame Strutting, 49
  Fuses, 31

  Geological Survey, 3
  German Method:
    Advantages and Disadvantages, 159
    Excavation, 155
    General Description, 155
    Hauling, 158
    Strutting, 156
  Giovi Tunnel Accident, 309
  Graveholz Tunnel, 98
  Greathead’s Shield, 245

  Hand Drills, 23
  Harlem River Tunnel, 285
  Hauling:
    American Method, 175
    Belgian Method, 150
    Italian Method, 185
    German Method, 158
    Hudson River Tunnel of Pennsylvania Railroad, 278
    Motive Power, 61
    By Way of Entrances, 59
    Simplon Tunnel, 111
    By Way of Shafts, 62
  Heading and Bench Method:
    Fort George Tunnel, 135
    General Discussion, 130
    St. Gothard Tunnel, 1
  Headings, 37, 91
  Hewett, H. B., 269
  History of Tunnels, xiii
  Hoisting Machines:
    General Discussion, 62
    Elevators, 64
    Horse Gins, 63
    Windlass, 63
  Hoosac Tunnel, 93
  Hopkins, Stephen W., 135
  Hudson River Tunnel of Pennsylvania Railroad, 269
  Hydraulic Jacks, 260, 271
  Hydraulic Rams, 271

  Illumination:
    Acetylene Gas, 334
    Coal-gas, 333
    Electric, 335
    Hudson River Tunnel of Pennsylvania Railroad, 280
    Lamps and Lanterns, 330
  Inclination of Strata, 6
  Ingersoll Drilling Machine, 26
  Inverted Arch Lining, 77
  Iron and Masonry Lining, 74
  Iron Lining, 73, 261, 276
  Iron Strutting, 55
    Full Section, 56
    Headings, 56
    Shafts, 57
  Italian Method:
    Advantages and Disadvantages, 188
    Excavation, 182
    General Description, 182
    Modifications, 186
    Strutting, 183

  Jacks, 260, 271
  Joining the Caissons, 295

  Lagging, 71
  Lamps and Lanterns, 330
  Lighting (see Illumination).
  Lining:
    Austrian Method, 180
    Belgian Method, 148
    Concrete, 126, 139
    English Method, 170
    Foundations, 76
    General Observations, 78
    German Method, 158
    Hudson River Tunnel Pennsylvania Railroad, 276
    Invert, 77
    Iron, 73, 261, 276
    Iron and Masonry, 74
    Italian Method, 185
    Masonry, 74
    Quicksand Method, 191
    Roof Arch, 77
    Side Tunnels, 79, 83
    Side Walls, 77
    Subways, 207-213
    Timber, 72
    Thickness of Masonry, 78, 83
  Little Tom Tunnel Relined, 321
  Loose Soil (see Soft Ground).

  Masonry (see Centers).
  Masonry Culverts, 80
  Masonry (see Lining).
  Masonry Lining, 74
  Masonry Niches, 81
  McBean, Daniel, 285
  Mechanical Installations for Tunnel Work, 84
  Milwaukee Tunnel, 226
  Mont Cenis Tunnel, 92
  Monthly Progress of Tunnels, 342
  Mullan Tunnel Relined, 319
  Murray Hill Tunnel, 123

  Natural Ventilation, 326
  New York Rapid Transit Subway, 209
  Niagara Falls Power Tunnel, 97
  Niches, 81

  Open Cut or Tunnel, 1
  Open-cut Tunneling:
    General Discussion, 195
    Parallel Longitudinal Trenches, 197
    Single Trench, 196
    Single Narrow Trench, 197
    Transverse Trenches, 200
    Tunnels on the Surface, 200

  Palisade Tunnel, 94
  Pennsylvania Railroad Shield, 270
  Percussion Drills, 24
  Pilot Method of Tunneling, 192
  Plank Centers, 69
  Platform Cars, 59
  Plenum Method of Ventilation, 329
  Pneumatic Caissons, 287
  Polar Protractor, 39
  Portals, 81
  Power Drills, 24
  Power Plants:
    Air Compressors, 87
    Canals and Pipe Lines, 86
    Cascade Tunnel, 98
    Croton Aqueduct Tunnel, 95
    General Description, 84
    Graveholz Tunnel, 98
    Hoosac Tunnel, 93
    Hudson River Tunnel Pennsylvania Railroad, 279
    Mont Cenis Tunnel, 92
    Murray Hill Tunnel, 128
    Niagara Falls Power Tunnel, 97
    Palisades Tunnel, 94
    Receivers, 89
    Reservoirs, 86
    Simplon Tunnel, 117
    Sonnstein Tunnel, 99
    St. Clair River Tunnel, 99
    St. Gothard Tunnel, 133
    Steam, 85
    Strickler Tunnel, 96
    Turbines, 86
  Prelini’s Shield, 251
  Presence of Water, 7
  Prevention of Collapse, 303
  Progress in Sinking Shafts, 343
  Progress of Excavation, 342
  Progress of the Work, 342
  Progress in Simplon Tunnel, 122

  Quantity of Air for Ventilation, 331
  Quicksand Tunneling:
    General Discussion, 188
    Removing the Seepage Water, 191
  Quantity of Timber in Strutting, 54

  Receivers, 89
  Relining Tunnels, 315
    Boulder Tunnel, 315
    Little Tom Tunnel, 321
    Mullan Tunnel, 319
  Repairing of Accidents in Tunnels, 308
  Reservoirs, 86
  Roof Arch Lining, 77
  Roof Shield for Boston Subway, 251
  Roof of Caissons, 287-291
  Rotary Drills, 27
  Ryder, B. H., 296

  Saccardo System of Ventilation, 330
  Saunders, W. L., 88
  Seepage Water, 191
  Seine River Tunnel, 293
  Setting up Centers, 70
  Severn Tunnel, 221
  Shafts, Description of, 40
  Shaler, Ira A., 142
  Shield and Compressed Air Method, 263
  Shield Construction:
    Diaphragm, 256
    Cellular Division, 255
    Dimensions of Shields, 259
    Front End, 254
    General Form, 252
    Rear End, 257
    Shell, 253
  Shield Method:
    Barlow Shield, 242
    Beach’s Shield, 245
    Blackwall Tunnel Shield, 248
    Brunel Shield, 240
    City and South London Railway Shield, 250
    Greathead’s Shield, 245
    History, 238
    Prelini’s Shield, 251
    St. Clair River Tunnel Shield, 247
  Side Shafts, 41
  Side Tunnels Lining, 79
  Side Walls Lining, 77
  Simplon Tunnel, 103
  Soils Encountered in Tunnels, 3
  Sonnstein Tunnel, 99
  Stations of Subways, 207-216
  St. Clair River Tunnel Shield, 247
  St. Gothard Tunnel, 132
  Steam Power Plant, 85
  Stratification of the Soils, 6
  Strickler Tunnel, 96
  Striking the Centers, 71
  Strutting:
    American Method, 174
    Austrian Method, 177
    Belgian Method, 146
    Dimensions of Timber, 54
    English Method, 167
    Fort George Tunnel, 137
    Full Section, 51
    German Method, 156
    Headings, 48
    Italian Method, 183
    Murray Hill Tunnel, 125
    Pilot Method, 193
    Quantity of Timber, 54
    Shafts, 52
    Iron: Full Section, 56
      Headings, 56
      Shafts, 57
  Submarine Tunneling:
    Cofferdam Method, 281
    Compressed Air Method, 225
    Detroit River Tunnel, 296
    General Discussion, 218
    Harlem River Tunnel, 285
    Hudson River Tunnel Pennsylvania Railroad, 269
    Lining, 261
    Milwaukee Water-Works Tunnel, 226
    Pneumatic Caisson Method, 284
    Seine River Tunnel, 293
    Severn Tunnel, 221
    Shield and Compressed Air Method, 263
    Shield System, 238
    Sinking and Joining Sections Built on Land, 293
    Van Buren Street Tunnel, 282
  Subways:
    Boston, 203
    Cost of, 209-217
    Cross-sections, 204-211
    General Discussion, 195-202
    Lining, 207-213
    New York Rapid Transit Railway, 209
    Stations, 207-216
  Sutro, Adolph, 330

  Tamping, 32
  Thickness of Lining Masonry, 78, 83
  Thomson Excavating Machine, 22
  Timber Lining, 72
  Timbering (see Strutting).
  Tremies, 299
  Trussed Centers, 70
  Tunnel or Open Cut, 1
  Tunnels:
    Baltimore Belt Line, 160
    Classification of, 42
    Fort George, 135
    Murray Hill, 123
    Simplon, 103
    St. Gothard, 132
    Hard Rock, 84
      Drift Method, 102
      Comparison of Methods, 141
      Heading and Bench Method, 152
      Heading Method, 130
    Soft Ground:
      American Method, 172
      Austrian Method, 176
      Belgian Method, 144
      English Method, 166
      German Method, 155
      Italian Method, 182
      Pilot Method, 192
      Quicksand Method, 188
    Submarine:
      Detroit River Tunnel, 296
      Harlem River Tunnel, 285
      Hudson River Tunnel of Pennsylvania Railroad, 269
      Milwaukee Tunnel, 226
      Seine River Tunnel, 293
      Severn Tunnel, 221
      Van Buren Street Tunnel, Chicago, 282
    Under City Streets:
      General Description, 201
      Boston Subway, 203
  Turbines, 86

  Vacuum Method of Ventilation, 328
  Value, Beverley R., 10
  Van Buren Street Tunnel, 282
  Ventilation, 325
    Artificial, 327
    Compressed Air, 330
    Natural, 326
    Plenum Method, 329
    Quantity of Air, 331
    Saccardo’s System, 330
    Simplon Tunnel, 120
    Vacuum Method, 328
  Vernon-Harcourt, L. F., 221

  Working Platforms, 286
  Wyman, Erastus, 293




  Transcriber’s Notes


  Inconsistencies in spelling and hyphenation have been retained except
  as mentioned below; non-English words and phrases have not been
  corrected except as listed below. The (minor) differences between the
  Table of Contents and the chapter headings have not been rectified.

  Page 36/132: Figs. 14 and 61 are identical.

  Page 92/93, Sommeilier: possibly an error for Sommeiller.

  Page 134, Soummelier: possibly an error for Sommeiller.

  Page 174, Footnote 11: presumably Fig. 92, indicating the planes of
  the sections, is from the same publication.

  Page 176, Austrian method: Dresden and Leipsic, and the Oberau Tunnel,
  are (and were in 1837) in Saxony, Germany (or Prussia).

  Page 179, The short transverse beam _c_, Fig. 90: there is no short
  transverse beam visible in Fig. 90, nor is it clear which other figure
  might be intended; there is therefore no hyperlink to the
  illustration.

  Page 279, Stirtling boiler: possibly an error for Stirling boiler.

  Pages 337 and 342, Arlberg: possibly an error for Aarlberg.

  Page 340, Wartha: possibly an error for Martha; Mertin: possibly an
  error for Merten.


  Changes made

  Footnotes, tables and illustrations have been moved out of text
  paragraphs; some table data have been re-arranged for better
  legibility. In some of the formulas brackets have been added for
  clarity.

  Several obvious minor typographical and punctuation errors have been
  corrected silently.

  Page 12, footnote 3: Chapter IX. changed to Chapter X.

  Page 35: on page 155 changed to on page 135

  Page 36: on page 34 changed to on page 35

  Page 53: The lagging plank may be ... changed to The lagging planks
  may be ...

  Page 113: (1) changed to (_I_) (2×)

  Page 117: ... and it in this clearing ... changed to ... and it is in
  this clearing ...

  Page 130: as indicated by Fig. 58 changed to as indicated by Fig. 61

  Page 136: as indicated in the Fig. 63 changed to as indicated in the
  Fig. 65

  Page 146: as shown by Fig. 63 changed to as shown by Fig. 69

  Page 149: underpining changed to underpinning

  Page 150: Since the roof arch rests for some time ... changed to Since
  the roof arch rests are for some time ...; as shown by Fig. 66 changed
  to as shown by Fig. 72

  Page 172: illustrated in Fig. 12 changed to illustrated in Fig. 11

  Page 175: page 127 changed to page 123

  Page 179: as at _b_, Fig. 90 changed to as at _b_, Fig. 97

  Page 204: The third type of section is shown by Fig. 116 changed to
  The third type of section is shown by Fig. 117

  Page 218: Malinö changed to Malmö

  Page 261: Fig. 118 shows the hydraulic jacks changed to Fig. 136 shows
  the hydraulic jacks

  Page 282: shown by Fig. 119 changed to shown by Fig. 141

  Page 297: towed down to the tunnel side changed to towed down to the
  tunnel site

  Page 315: shown in Figs. 141 and 142 changed to shown in Figs. 159 and
  160

  Page 324: shown by Fig. 148 changed to shown by Fig. 166

  Page 338: given on page 50 changed to given on page 55

  Page 340: Scloss Matrei changed to Schloss Matrei

  Page 341: _Time._ changed to =Time.=

  Page 348, entry Ryder: page number 296 added; Sounstein changed to
  Sonnstein (2×).