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This is Paper 41 from the _Smithsonian Institution United States
National Museum Bulletin 240_, comprising Papers 34-44, which will
also be available as a complete e-book.

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Smithsonian Institution

United States National Museum

Bulletin 240




[Illustration: Smithsonian Press]




Museum of History and Technology


Contributions from the Museum of History and Technology


_Papers 34-44_

_On Science and Technology_


Smithsonian Institution · Washington, D.C. 1966


       *       *       *       *       *


_Publications of the United States National Museum_

The scholarly and scientific publications of the United States National
Museum include two series, _Proceedings of the United States National
Museum_ and _United States National Museum Bulletin_.

In these series, the Museum publishes original articles and monographs
dealing with the collections and work of its constituent museums--The
Museum of Natural History and the Museum of History and
Technology--setting forth newly acquired facts in the fields of
anthropology, biology, history, geology, and technology. Copies of
each publication are distributed to libraries, to cultural and
scientific organizations, and to specialists and others interested in
the different subjects.

The _Proceedings_, begun in 1878, are intended for the publication,
in separate form, of shorter papers from the Museum of Natural History.
These are gathered in volumes, octavo in size, with the publication
date of each paper recorded in the table of contents of the volume.

In the _Bulletin_ series, the first of which was issued in 1875, appear
longer, separate publications consisting of monographs (occasionally in
several parts) and volumes in which are collected works on related
subjects. _Bulletins_ are either octavo or quarto in size, depending
on the needs of the presentation. Since 1902 papers relating to the
botanical collections of the Museum of Natural History have been
published in the _Bulletin_ series under the heading _Contributions from
the United States National Herbarium_, and since 1959, in _Bulletins_
titled "Contributions from the Museum of History and Technology," have
been gathered shorter papers relating to the collections and research of
that Museum.

The present collection of Contributions, Papers 34-44, comprises
Bulletin 240. Each of these papers has been previously published in
separate form. The year of publication is shown on the last page of
each paper.

  FRANK A. TAYLOR
  _Director, United States National Museum_


       *       *       *       *       *




CONTRIBUTIONS FROM

THE MUSEUM OF HISTORY AND TECHNOLOGY.

PAPER 41




TUNNEL ENGINEERING--A MUSEUM TREATMENT


_Robert M. Vogel_




                                       INTRODUCTION      203

                                     ROCK TUNNELING      206

                              SOFT-GROUND TUNNELING      215

                                       BIBLIOGRAPHY      239

                                          FOOTNOTES

                                              INDEX




[Illustration: Figure 1.--MINING BY EARLY EUROPEAN CIVILIZATIONS,
using fire setting and hand chiseling to break out ore and rock.
MHT model--3/4" scale. (Smithsonian photo 49260-H.)]




_Robert M. Vogel_

TUNNEL ENGINEERING--A MUSEUM TREATMENT


     _During the years from 1830 to 1900, extensive developments took
     place in the field of tunneling, which today is an important,
     firmly established branch of civil engineering. This paper offers
     a picture of its growth from the historical standpoint, based on
     a series of models constructed for the Hall of Civil Engineering
     in the new Museum of History and Technology. The eight models
     described highlight the fundamental advances which have occurred
     between primitive man's first systematic use of fire for excavating
     rock in mining, and the use in combination of compressed air, an
     iron lining, and a movable shield in a subaqueous tunnel at the end
     of the 19th century._

     THE AUTHOR: _Robert M. Vogel is curator of heavy machinery and
     civil engineering, in the Smithsonian Institution's Museum of
     History and Technology._




Introduction


With few exceptions, civil engineering is a field in which the ultimate
goal is the assemblage of materials into a useful structural form
according to a scientifically derived plan which is based on various
natural and man-imposed conditions. This is true whether the result be,
for example, a dam, a building, a bridge, or even the fixed plant of a
railroad. However, one principal branch of the field is based upon an
entirely different concept. In the engineering of tunnels the utility of
the "structure" is derived not from the bringing together of elements
but from the separation of one portion of naturally existing material
from another to permit passage through a former barrier.

In tunneling hard, firm rock, this is practically the entire compass
of the work: breaking away the rock from the mother mass, and,
coincidently, removing it from the workings. The opposite extreme in
conditions is met in the soft-ground tunnel, driven through material
incapable of supporting itself above the tunnel opening. Here, the
excavation of the tunneled substance is of relatively small concern,
eclipsed by the problem of preventing the surrounding material from
collapsing into the bore.

[Illustration: Figure 2.--HOOSAC TUNNEL. METHOD OF WORKING EARLY
SECTIONS of the project; blast holes drilled by hand jacking.
MHT model--1/2" scale. (Smithsonian photo 49260-L.)]

In one other principal respect does tunnel engineering differ widely
from its collateral branches of civil engineering. Few other physical
undertakings are approached with anything like the uncertainty
attending a tunnel work. This is even more true in mountain tunnels,
for which test borings frequently cannot be made to determine the
nature of the material and the geologic conditions which will be
encountered.

The course of tunnel work is not subject to an overall preliminary
survey; the engineer is faced with not only the inability to
anticipate general contingencies common to all engineering work, but
with the peculiar and often overwhelming unpredictability of the very
basis of his work.

Subaqueous and soft-ground work on the other hand, while still subject
to many indeterminates, is now far more predictable than during its
early history, simply because the nature of the adverse condition
prevailing eventually was understood to be quite predictable. The
steady pressures of earth and water to refill the excavated area are
today overcome with relative ease and consistency by the tunneler.

In tunneling as in no other branch of civil engineering did empiricism
so long resist the advance of scientific theory; in no other did the
"practical engineer" remain to such an extent the key figure in
establishing the success or failure of a project. The Hoosac Tunnel,
after 25 years of legislative, financial, and technical difficulties,
in 1875 was finally driven to successful completion only by the
efforts of a group who, while in the majority were trained civil
engineers, were to an even greater extent men of vast practical
ability, more at home in field than office.

DeWitt C. Haskin (see p. 234), during the inquest that followed the
death of a number of men in a blowout of his pneumatically driven
Hudson River Tunnel in 1880, stated in his own defense: "I am not a
scientific engineer, but a practical one ... I know nothing of
mathematics; in my experience I have grasped such matters as a whole;
I believe that the study of mathematics in that kind of work
[tunneling] has a tendency to dwarf the mind rather than enlighten
it...." An extreme attitude perhaps, and one which by no means adds to
Haskin's stature, but a not unusual one in tunnel work at the time. It
would not of course be fair to imply that such men as Herman Haupt,
Brunel the elder, and Greathead were not accomplished theoretical
engineers. But it was their innate ability to evaluate and control the
overlying physical conditions of the site and work that made possible
their significant contributions to the development of tunnel
engineering.

Tunneling remained largely independent of the realm of mathematical
analysis long after the time when all but the most insignificant
engineering works were designed by that means. Thus, as structural
engineering has advanced as the result of a flow of new theoretical
concepts, new, improved, and strengthened materials, and new methods
of fastening, the progress of tunnel engineering has been due more to
the continual refinement of constructional techniques.


A NEW HALL OF CIVIL ENGINEERING

In the Museum of History and Technology has recently been established
a Hall of Civil Engineering in which the engineering of tunnels is
comprehensively treated from the historical standpoint--something not
previously done in an American museum. The guiding precept of the
exhibit has not been to outline exhaustively the entire history of
tunneling, but rather to show the fundamental advances which have
occurred between primitive man's first systematic use of fire for
excavating rock in mining, and the use in combination of compressed
air, iron lining, and a movable shield in a subaqueous tunnel at the
end of the 19th century. This termination date was selected because it
was during the period from about 1830 to 1900 that the most
concentrated development took place, and during which tunneling became
a firmly established and important branch of civil engineering and
indeed, of modern civilization. The techniques of present-day
tunneling are so fully related in current writing that it was deemed
far more useful to devote the exhibit entirely to a segment of the
field's history which is less commonly treated.

[Illustration: Figure 3.--HOOSAC TUNNEL. WORKING OF LATER STAGES with
Burleigh pneumatic drills mounted on carriages. The bottom heading is
being drilled in preparation for blasting out with nitroglycerine.
MHT model--1/2" scale. (Smithsonian photo 49260-M.)]

The major advances, which have already been spoken of as being ones of
technique rather than theory, devolve quite naturally into two basic
classifications: the one of supporting a mass of loose, unstable,
pressure-exerting material--soft-ground tunneling; and the
diametrically opposite problem of separating rock from the basic mass
when it is so firm and solid that it can support its own overbearing
weight as an opening is forced through it--rock, or hard-ground
tunneling.

To exhibit the sequence in a thorough manner, inviting and capable of
easy and correct interpretation by the nonprofessional viewer, models
offered the only logical means of presentation. Six tunnels were
selected, all driven in the 19th century. Each represents either a
fundamental, new concept of tunneling technique, or an important,
early application of one. Models of these works form the basis of the
exhibit. No effort was made to restrict the work to projects on
American soil. This would, in fact, have been quite impossible if an
accurate picture of tunnel technology was to be drawn; for as in
virtually all other areas of technology, the overall development
in this field has been international. The art of mining was first
developed highly in the Middle Ages in the Germanic states; the tunnel
shield was invented by a Frenchman residing in England, and the use of
compressed air to exclude the water from subaqueous tunnels was first
introduced on a major work by an American. In addition, the two main
subdivisions, rock and soft-ground tunneling, are each introduced by
a model not of an actual working, but of one typifying early classical
methods which were in use for centuries until the comparatively recent
development of more efficient systems of earth support and rock
breaking. Particular attention is given to accuracy of detail
throughout the series of eight models; original sources of descriptive
and graphic information were used in their construction wherever
possible. In all cases except the introductory model in the
rock-tunneling series, representing copper mining by early
civilizations, these sources were contemporary accounts.

The plan to use a uniform scale of reduction throughout, in order
to facilitate the viewers' interpretation, unfortunately proved
impractical, due to the great difference in the amount of area to be
encompassed in different models, and the necessity that the cases
holding them be of uniform height. The related models of the Broadway
and Tower Subways represent short sections of tunnels only 8 feet or
so in diameter enabling a relatively large scale, 1-1/2 inches to the
foot, to be used. Conversely, in order that the model of Brunel's
Thames Tunnel be most effective, it was necessary to include one of
the vertical terminal shafts used in its construction. These were
about 60 feet in depth, and thus the much smaller scale of 1/4 inch
to the foot was used. This variation is not as confusing as might be
thought, for the human figures in each model provide an immediate and
positive sense of proportion and scale.

Careful thought was devoted to the internal lighting of the models, as
this was one of the critical factors in establishing, so far as is
possible in a model, an atmosphere convincingly representative of work
conducted solely by artificial light. Remarkable realism was achieved
by use of plastic rods to conduct light to the tiny sources of tunnel
illumination, such as the candles on the miners' hats in the Hoosac
Tunnel, and the gas lights in the Thames Tunnel. No overscaled
miniature bulbs, generally applied in such cases, were used. At
several points where the general lighting within the tunnel proper has
been kept at a low level to simulate the natural atmosphere of the
work, hidden lamps can be operated by push-button in order to bring
out detail which otherwise would be unseen.

The remainder of the material in the Museum's tunneling section
further extends the two major aspects of tunneling. Space limitations
did not permit treatment of the many interesting ancillary matters
vital to tunnel engineering, such as the unique problems of
subterranean surveying, and the extreme accuracy required in the
triangulation and subsequent guidance of the boring in long mountain
tunnels; nor the difficult problems of ventilating long workings, both
during driving and in service; nor the several major methods developed
through the years for driving or constructing tunnels in other than
the conventional manner.[1]




Rock Tunneling


While the art of tunneling soft ground is of relatively recent origin,
that of rock tunneling is deeply rooted in antiquity. However,
the line of its development is not absolutely direct, but is
more logically followed through a closely related branch of
technology--mining. The development of mining techniques is a
practically unbroken one, whereas there appears little continuity or
relationship between the few works undertaken before about the 18th
century for passage through the earth.

The Egyptians were the first people in recorded history to have driven
openings, often of considerable magnitude, through solid rock. As is
true of all major works of that nation, the capability of such grand
proportion was due solely to the inexhaustible supply of human power
and the casual evaluation of life. The tombs and temples won from the
rock masses of the Nile Valley are monuments of perseverance rather
than technical skill. Neither the Egyptians nor any other peoples
before the Middle Ages have left any consistent evidence that they
were able to pierce ground that would not support itself above the
opening as would firm rock. In Egypt were established the methods of
rock breaking that were to remain classical until the first use of
gun-powder blasting in the 17th century which formed the basis of the
ensuing technology of mining.

Notwithstanding the religious motives which inspired the earliest rock
excavations, more constant and universal throughout history has been
the incentive to obtain the useful and decorative minerals hidden
beneath the earth's surface. It was the miner who developed the
methods introduced by the early civilizations to break rock away from
the primary mass, and who added the refinements of subterranean
surveying and ventilating, all of which were later to be assimilated
into the new art of driving tunnels of large diameter. The connection
is the more evident from the fact that tunnelmen are still known as
miners.


COPPER MINING, B.C.

Therefore, the first model of the sequence, reflecting elemental
rock-breaking techniques, depicts a hard-rock copper mine (fig. 1).
Due to the absence of specific information about such works during the
pre-Christian eras, this model is based on no particular period or
locale, but represents in a general way, a mine in the Rio Tinto area
of Spain where copper has been extracted since at least 1000 B.C.
Similar workings existed in the Tirol as early as about 1600 B.C. Two
means of breaking away the rock are shown: to the left is the most
primitive of all methods, the hammer and chisel, which require no
further description. At the right side, the two figures are shown
utilizing the first rock-breaking method in which a force beyond that
of human muscles was employed, the age-old "fire-setting" method. The
rock was thoroughly heated by a fierce fire built against its face and
then suddenly cooled by dashing water against it. The thermal shock
disintegrated the rock or ore into bits easily removable by hand.

[Illustration: Figure 4.--HOOSAC TUNNEL. Bottom of the central shaft
showing elevator car and rock skip; pumps at far right. In the center,
the top bench is being drilled by a single column-mounted Burleigh
drill. MHT model--1/2" scale. (Smithsonian photo 49260-N.)]

The practice of this method below ground, of course, produced a
fearfully vitiated atmosphere. It is difficult to imagine whether the
smoke, the steam, or the toxic fumes from the roasting ore was
the more distressing to the miners. Even when performed by labor
considered more or less expendable, the method could be employed only
where there was ventilation of some sort: natural chimneys and
convection currents were the chief sources of air circulation. Despite
the drawbacks of the fire system, its simplicity and efficacy weighed
so heavily in its favor that its history of use is unbroken almost to
the present day. Fire setting was of greatest importance during the
years of intensive mining in Europe before the advent of explosive
blasting, but its use in many remote areas hardly slackened until the
early 20th century because of its low cost when compared to powder.
For this same reason, it did have limited application in actual tunnel
work until about 1900.

Direct handwork with pick, chisel and hammer, and fire setting were
the principal means of rock removal for centuries. Although various
wedging systems were also in favor in some situations, their
importance was so slight that they were not shown in the model.


HOOSAC TUNNEL

It was possible in the model series, without neglecting any major
advancement in the art of rock tunneling, to complete the sequence of
development with only a single additional model. Many of the greatest
works of civil engineering have been those concerned directly with
transport, and hence are the product of the present era, beginning in
the early 19th century. The development of the ancient arts of route
location, bridge construction, and tunnel driving received a powerful
stimulation after 1800 under the impetus of the modern canal, highway,
and, especially, the railroad.

The Hoosac Tunnel, driven through Hoosac Mountain in the very
northwest corner of Massachusetts between 1851 and 1875, was the first
major tunneling work in the United States. Its importance is due not
so much to this as to its being literally the fountainhead of modern
rock-tunneling technology. The remarkable thing is that the work was
begun using methods of driving almost unchanged during centuries
previous, and was completed twenty years later by techniques which
were, for the day, almost totally mechanized. The basic pattern of
operation set at Hoosac, using pneumatic rock drills and efficient
explosives, remains practically unchanged today.

The general history of the Hoosac project is so thoroughly recorded
that the briefest outline of its political aspects will suffice here.
Hoosac Mountain was the chief obstacle in the path of a railroad
projected between Greenfield, Massachusetts, and Troy, New York.
The line was launched by a group of Boston merchants to provide a
direct route to the rapidly developing West, in competition with the
coastal routes via New York. The only route economically reasonable
included a tunnel of nearly five miles through the mountain--a
length absolutely without precedent, and an immense undertaking in
view of the relatively primitive rock-working methods then available.

[Illustration: Figure 5.--BURLEIGH ROCK DRILL, improved model of about
1870, mounted on frame for surface work. (Catalog and price list: The
Burleigh Rock Drill Company, 1876.)]

The bore's great length and the desire for rapid exploitation inspired
innovation from the outset of the work. The earliest attempts at
mechanization, although ineffectual and without influence on tunnel
engineering until many years later, are of interest. These took the
form of several experimental machines of the "full area" type,
intended to excavate the entire face of the work in a single operation
by cutting one or more concentric grooves in the rock. The rock
remaining between the grooves was to be blasted out. The first such
machine tested succeeded in boring a 24-foot diameter opening for 10
feet before its total failure. Several later machines proved of equal
merit.[2] It was the Baltimore and Ohio's eminent chief engineer,
Benjamin H. Latrobe, who in his _Report on the Hoosac Tunnel_
(Baltimore, Oct. 1, 1862, p. 125) stated that such apparatus contained
in its own structure the elements of failure, "... as they require
the machines to do too much and the powder too little of the work,
thus contradicting the fundamental principles upon which all
labor-saving machinery is framed ... I could only look upon it as a
misapplication of mechanical genius."

[Illustration: Figure 6.--HOOSAC TUNNEL. Flash-powder photograph of
Burleigh drills at the working face. (_Photo courtesy of State
Library, Commonwealth of Massachusetts._)]

Latrobe stated the basic philosophy of rock-tunnel work. No mechanical
agent has ever been able to improve upon the efficiency of explosives
for the shattering of rock. For this reason, the logical application
of machinery to tunneling was not in replacing or altering the
fundamental process itself, but in enabling it to be conducted with
greater speed by mechanically drilling the blasting holes to receive
the explosive.

Actual work on the Hoosac Tunnel began at both ends of the tunnel in
about 1854, but without much useful effect until 1858 when a contract
was let to the renowned civil engineer and railroad builder, Herman
Haupt of Philadelphia. Haupt immediately resumed investigations of
improved tunneling methods, both full-area machines and mechanical
rock drills. At this time mechanical rock-drill technology was in a
state beyond, but not far beyond, initial experimentation. There
existed one workable American machine, the Fowle drill, invented in
1851. It was steam-driven, and had been used in quarry work, although
apparently not to any commercial extent. However, it was far too
large and cumbersome to find any possible application in tunneling.
Nevertheless, it contained in its operating principle, the seed of a
practical rock drill in that the drill rod was attached directly to
and reciprocated by a double-acting steam piston. A point of great
importance was the independence of its operation on gravity,
permitting drilling in any direction.

While experimenting, Haupt drove the work onward by the classical
methods, shown in the left-hand section of the model (fig. 2). At
the far right an advance heading or adit is being formed by pick and
hammer work; this is then deepened into a top heading with enough
height to permit hammer drilling, actually the basic tunneling
operation. A team is shown "double jacking," i.e., using two-handed
hammers, the steel held by a third man. This was the most efficient of
the several hand-drilling methods. The top-heading plan was followed
so that the bulk of the rock could be removed in the form of a bottom
bench, and the majority of drilling would be downward, obviously the
most effective direction. Blasting was with black powder and its
commercial variants. Some liberty was taken in depicting these steps
so that both operations might be shown within the scope of the model:
in practice the heading was kept between 400 and 600 feet in advance
of the bench so that heading blasts would not interfere with the bench
work. The bench carriage simply facilitated handling of the blasted
rock. It was rolled back during blasts.

[Illustration: Figure 7.--HOOSAC TUNNEL. GROUP OF MINERS descending
the west shaft with a Burleigh drill. (_Photo courtesy of State
Library, Commonwealth of Massachusetts._)]

The experiments conducted by Haupt with machine drills produced no
immediate useful results. A drill designed by Haupt and his associate,
Stuart Gwynn, in 1858 bored hard granite at the rate of 5/8 inch per
minute, but was not substantial enough to bear up in service. Haupt
left the work in 1861, victim of intense political pressures and
totally unjust accusations of corruption and mismanagement. The
work was suspended until taken over by a state commission in 1862.
Despite frightful ineptitude and very real corruption, this period
was exceedingly important in the long history both of Hoosac Tunnel
and of rock tunneling in general.

The merely routine criticism of the project had by this time become
violent due to the inordinate length of time already elapsed and the
immense cost, compared to the small portion of work completed. This
served to generate in the commission a strong sense of urgency to
hurry the project along. Charles S. Storrow, a competent engineer, was
sent to Europe to report on the progress of tunneling there, and in
particular on mechanization at the Mont Cenis Tunnel then under
construction between France and Italy. Germain Sommeiller, its chief
engineer, had, after experimentation similar to Haupt's, invented a
reasonably efficient drilling machine which had gone into service at
Mont Cenis in March 1861. It was a distinct improvement over hand
drilling, almost doubling the drilling rate, but was complex and
highly unreliable. Two hundred drills were required to keep 16 drills
at work. But the vital point in this was the fact that Sommeiller
drove his drills not with steam, but air, compressed at the tunnel
portals and piped to the work face. It was this single factor, one of
application rather than invention, that made the mechanical drill
feasible for tunneling.

All previous effort in the field of machine drilling, on both sides
of the Atlantic, had been directed toward steam as the motive power.
In deep tunnels, with ventilation already an inherent problem, the
exhaust of a steam drill into the atmosphere was inadmissible.
Further, steam could not be piped over great distances due to serious
losses of energy from radiation of heat, and condensation. Steam
generation within the tunnel itself was obviously out of the question.
It was the combination of a practical drill, and the parallel
invention by Sommeiller of a practical air compressor that resulted in
the first workable application of machine rock drilling to tunneling.

[Illustration: Figures 8 & 9.--HOOSAC TUNNEL. CONTEMPORARY
ENGRAVINGS. As such large general areas could not be sufficiently
illuminated for photography, the Museum model was based primarily on
artists' versions of the work. (_Science Record_, 1872; _Leslie's
Weekly_, 1873.)]

The Sommeiller drills greatly impressed Storrow, and his report of
November 1862 strongly favored their adoption at Hoosac. It is curious
however, that not a single one was brought to the U.S., even on trial.
Storrow does speak of Sommeiller's intent to keep the details of the
machine to himself until it had been further improved, with a view to
its eventual exploitation. The fact is, that although workable, the
Sommeiller drill proved to be a dead end in rock-drill development
because of its many basic deficiencies. It did exert the indirect
influence of inspiration which, coupled with a pressing need for
haste, led to renewed trials of drilling machinery at Hoosac. Thomas
Doane, chief engineer under the state commission, carried this program
forth with intensity, seeking and encouraging inventors, and himself
working on the problem. The pattern of the Sommeiller drill was
generally followed; that is, the drill was designed as a separate,
relatively light mechanical element, adapted for transportation by
several miners, and attachable to a movable frame or carriage during
operation. Air was of course the presumed power. To be effective, it
was necessary that a drill automatically feed the drill rod as the
hole deepened, and also rotate the rod automatically to maintain a
round, smooth hole. Extreme durability was essential, and usually
proved the source of a machine's failure. The combination of these
characteristics into a machine capable of driving the drill rod into
the rock with great force, perhaps five times per second, was a
severe test of ingenuity and materials. Doane in 1864 had three
different experimental drills in hand, as well as various steam
and water-powered compressors.

Success finally came in 1865 with the invention of a drill by Charles
Burleigh, a mechanical engineer at the well-known Putnam Machine Works
of Fitchburg, Massachusetts. The drills were first applied in the east
heading in June of 1866. Although working well, their initial success
was limited by lack of reliability and a resulting high expense for
repairs. They were described as having "several weakest points." In
November, these drills were replaced by an improved Burleigh drill
which was used with total success to the end of the work. The era of
modern rock tunneling was thus launched by Sommeiller's insight in
initially applying pneumatic power to a machine drill, by Doane's
persistence in searching for a thoroughly practical drill, and by
Burleigh's mechanical talent in producing one. The desperate need to
complete the Hoosac Tunnel may reasonably be considered the greatest
single spur to the development of a successful drill.

The significance of this invention was far reaching. Burleigh's was
the first practical mechanical rock drill in America and, in view of
its dependability, efficiency, and simplicity when compared to the
Sommeiller drill, perhaps in the world. The Burleigh drill achieved
success almost immediately. It was placed in production by Putnam for
the Burleigh Rock Drill Company before completion of Hoosac in 1876,
and its use spread throughout the western mining regions and other
tunnel works. For a major invention, its adoption was, in relative
terms, instantaneous. It was the prototype of all succeeding
piston-type drills, which came to be known generically as "burleighs,"
regardless of manufacture. Walter Shanley, the Canadian contractor who
ultimately completed the Hoosac, reported in 1870, after the drills
had been in service for a sufficient time that the techniques for
their most efficient use were fully understood and effectively
applied, that the Burleigh drills saved about half the drilling costs
over hand drilling. The per-inch cost of machine drilling averaged 5.5
cents, all inclusive, vs. 11.2 cents for handwork. The more important
point, that of speed, is shown by the reports of average monthly
progress of the tunnel itself, before and after use of the air drills.

  _Year_     _Average monthly
              progress in feet_

  1865              55
  1866              48
  1867              99
  1868              --
  1869             138
  1870             126
  1871             145
  1872             124

[Illustration: Figure 10.--TRINITROGLYCERINE BLAST at Hoosac Tunnel.
(_Leslie's Weekly_, 1873.)]

The right portion of the model (fig. 3) represents the workings during
the final period. The bottom heading system was generally used after
the Burleigh drills had been introduced. Four to six drills were
mounted on a carriage designed by Doane. These drove the holes for
the first blast in the center of the heading in about six hours. The
full width of the heading, the 24-foot width of the tunnel, was then
drilled and blasted out in two more stages. As in the early section,
the benches to the rear were later removed to the full-tunnel height
of about 20 feet. This operation is shown by a single drill (fig. 4)
mounted on a screw column. Three 8-hour shifts carried the work
forward: drilling occupied half the time and half was spent in running
the carriage back, blasting, and mucking (clearing the broken rock).

[Illustration: Figure 11.--HOOSAC TUNNEL survey crew at engineering
office. The highest accuracy of the aboveground and underground survey
work was required to insure proper vertical and horizontal alignment
and meeting of the several separately driven sections. (_Photo
courtesy of State Library, Commonwealth of Massachusetts._)]

The tunnel's 1028-foot central shaft, completed under the Shanley
contract in 1870 to provide two additional work faces as well as a
ventilation shaft is shown at the far right side of this half of the
model. Completed so near the end of the project, only 15 percent of
the tunnel was driven from the shaft.

The enormous increase in rate of progress was not due entirely to
machine drilling. From the outset of his jurisdiction, Doane undertook
experiments with explosives as well as drills, seeking an agent more
effective than black powder. In this case, the need for speed was not
the sole stimulus. As the east and west headings advanced further and
further from the portals, the problem of ventilation grew more acute,
and it became increasingly difficult to exhaust the toxic fumes
produced by the black powder blasts.

In 1866, Doane imported from Europe a sample of trinitroglycerine,
the liquid explosive newly introduced by Nobel, known in Europe as
"glonoïn oil" and in the United States as "nitroglycerine." It already
had acquired a fearsome reputation from its tendency to decompose with
heat and age and to explode with or without the slightest provocation.
Nevertheless, its tremendous power and characteristic of almost
complete smokelessness led Doane to employ the chemist George W.
Mowbray, who had blasted for Drake in the Pennsylvania oil fields, to
develop techniques for the bulk manufacture of the new agent and for
its safe employment in the tunnel.

Mowbray established a works on the mountain and shortly developed
a completely new blasting practice based on the explosive. Its
stability was greatly increased by maintaining absolute purity in the
manufacturing process. Freezing the liquid to reduce its sensitivity
during transport to the headings, and extreme caution in its handling
further reduced the hazard of its use. At the heading, the liquid was
poured into cylindrical cartridges for placement in the holes. As with
the Burleigh drill, the general adoption of nitroglycerine was
immediate once its qualities had been demonstrated. The effect on the
work was notable. Its explosive characteristics permitted fewer blast
holes over a given frontal area of working face, and at the same time
it was capable of effectively blowing from a deeper drill hole, 42
inches against 30 inches for black powder, so that under ideal
conditions 40 percent more tunnel length was advanced per cycle of
operations. A new fuse and a system of electric ignition were
developed which permitted simultaneous detonation and resulted
in a degree of effectiveness impossible with the powder train and
cord fusing used with the black powder. Over a million pounds of
nitroglycerine were produced by Mowbray between 1866 and completion
of the tunnel.

[Illustration: Figure 12.--WORKS AT THE CENTRAL SHAFT, HOOSAC TUNNEL,
for hoisting, pumping and air compressing machinery, and general
repair, 1871. (_Photo courtesy of State Library, Commonwealth of
Massachusetts._)]

[Illustration: Figure 13.--HOOSAC TUNNEL. AIR-COMPRESSOR BUILDING on
Hoosac River near North Adams. The compressors were driven partially
by waterpower, derived from the river. (_Photo courtesy of State
Library, Commonwealth of Massachusetts._)]

[Illustration: Figure 14.--WEST PORTAL OF HOOSAC TUNNEL before
completion, 1868, showing six rings of lining brick. (_Photo
courtesy of State Library, Commonwealth of Massachusetts._)]

When the Shanleys took the work over in 1868, following political
difficulties attending operation by the State, the period of
experimentation was over. The tunnel was being advanced by totally
modern methods, and to the present day the overall concepts have
remained fundamentally unaltered: the Burleigh piston drill has been
replaced by the lighter hammer drill; the Doane drill carriage by the
more flexible "jumbo"; nitroglycerine by its more stable descendant
dynamite and its alternatives; and static-electric blasting machines
by more dependable magnetoelectric. But these are all in the nature of
improvements, not innovations.

Unlike the preceding model, there was good documentation for this one.
Also, the Hoosac was apparently the first American tunnel to be well
recorded photographically. Early flashlight views exist of the drills
working at the heading (fig. 6) as well as of the portals, the winding
and pumping works at the central shaft, and much of the machinery and
associated aspects of the project. These and copies of drawings of
much of Doane's experimental apparatus, a rare technological record,
are preserved at the Massachusetts State Library.




Soft-Ground Tunneling


So great is the difference between hard-rock and soft-ground tunneling
that they constitute two almost separate branches of the field. In
penetrating ground lacking the firmness or cohesion to support itself
above an opening, the miner's chief concern is not that of removing
the material, but of preventing its collapse into his excavation. The
primitive methods depending upon brute strength and direct application
of fire and human force were suitable for assault on rock, but lacked
the artifice needed for delving into less stable material. Roman
engineers were accomplished in spanning subterranean ways with masonry
arches, but apparently most of their work was done by cut-and-cover
methods rather than by actual mining.

Not until the Middle Ages did the skill of effectively working
openings in soft ground develop, and not until the Renaissance
was this development so consistently successful that it could
be considered a science.


RENAISSANCE MINING

From the earliest periods of rock working, the quest for minerals
and metals was the primary force that drove men underground. It was
the technology of mining, the product of slow evolution over the
centuries, that became the technology of the early tunnel, with no
significant modification except in size of workings.

Every aspect of 16th century mining is definitively detailed in
Georgius Agricola's remarkable _De re Metallica_, first published in
Basel in 1556. During its time of active influence, which extended
for two centuries, it served as the authoritative work on the subject.
It remains today an unparalleled early record of an entire branch
of technology. The superb woodcuts of mine workings and tools in
themselves constitute a precise description of the techniques of the
period, and provided an ideal source of information upon which to base
the first model in the soft-ground series.

[Illustration: Figure 15.--CENTERING FOR PLACEMENT OF FINISHED
STONEWORK at west portal, 1874. At top-right are the sheds where
the lining brick was produced. (_Photo courtesy of State Library,
Commonwealth of Massachusetts._)]

The model, representing a typical European mine, demonstrates the
early use of timber frames or "sets" to support the soft material of
the walls and roof. In areas of only moderate instability, the sets
alone were sufficient to counteract the earth pressure, and were
spaced according to the degree of support required. In more extreme
conditions, a solid lagging of small poles or boards was set outside
the frames, as shown in the model, to provide absolute support of the
ground. Details of the framing, the windlass, and all tools and
appliances were supplied by Agricola, with no need for interpretation
or interpolation.

The basic framing pattern of sill, side posts and cap piece, all
morticed together, with lagging used where needed, was translated
unaltered into tunneling practice, particularly in small exploratory
drifts. It remained in this application until well into the 20th
century.

The pressure exerted upon tunnels of large area was countered during
construction by timbering systems of greater elaboration, evolved
from the basic one. By the time that tunnels of section large
enough to accommodate canals and railways were being undertaken as
matter-of-course civil engineering works, a series of nationally
distinguishable systems had emerged, each possessing characteristic
points of favor and fault. As might be suspected, the English system
of tunnel timbering, for instance, was rarely applied on the
Continent, nor were the German, Austrian or Belgian systems normally
seen in Great Britain. All were used at one time or another in this
country, until the American system was introduced in about 1855.
While the timbering commonly remained in place in mines, it would be
followed up by permanent masonry arching and lining in tunnel work.

Overhead in the museum Hall of Civil Engineering are frames
representing the English, Austrian and American systems. Nearby, a
series of small relief models (fig. 19) is used to show the sequence
of enlargement in a soft-ground railroad tunnel of about 1855, using
the Austrian system. Temporary timber support of tunnels fell from use
gradually after the advent of shield tunneling in conjunction with
cast-iron lining. This formed a perfect support immediately behind the
shield, as well as the permanent lining of the tunnel.


BRUNEL'S THAMES TUNNEL

The interior surfaces of tunnels through ground merely unstable are
amenable to support by various systems of timbering and arching. This
becomes less true as the fluidity of the ground increases. The soft
material which normally comprises the beds of rivers can approach an
almost liquid condition resulting in a hydraulic head from the
overbearing water sufficient to prevent the driving of even the most
carefully worked drift, supported by simple timbering. The basic
defect of the timbering systems used in mining and tunneling was
that there was inevitably a certain amount of the face or ceiling
unsupported just previous to setting a frame, or placing over it the
necessary section of lagging. In mine work, runny soil could, and did,
break through such gaps, filling the working. For this reason, there
were no serious attempts made before 1825 to drive subaqueous tunnels.

In that year, work was started on a tunnel under the Thames between
the Rotherhithe and Wapping sections of London, under guidance of the
already famous engineer Marc Isambard Brunel (1769-1849), father of
I. K. Brunel. The undertaking is of great interest in that Brunel
employed an entirely novel apparatus of his own invention to provide
continuous and reliable support of the soft water-bearing clay which
formed the riverbed. By means of this "shield," Brunel was able to
drive the world's first subaqueous tunnel.[3]

[Illustration: Figure 16.--WEST PORTAL UPON COMPLETION, 1876.
(_Photo courtesy of New-York Historical Society._)]

The shield was of cast-iron, rectangular in elevation, and was
propelled forward by jackscrews. Shelves at top, bottom, and sides
supported the tunnel roof, floor, and walls until the permanent brick
lining was placed. The working face, the critical area, was supported
by a large number of small "breasting boards," held against the ground
by small individual screws bearing against the shield framework. The
shield itself was formed of 12 separate frames, each of which could be
advanced independently of the others. The height was 22 feet 3 inches:
the width 37 feet 6 inches.

The progress was piecemeal. In operation the miners would remove one
breasting board at a time, excavate in front of it, and then replace
it in the advanced position--about 6 inches forward. This was repeated
with the next board above or below, and the sequence continued until
the ground for the entire height of one of the 12 sections had been
removed. The board screws for that section were shifted to bear on the
adjacent frames, relieving the frame of longitudinal pressure. It
could then be screwed forward by the amount of advance, the screws
bearing to the rear on the completed masonry. Thus, step by step the
tunnel progressed slowly, the greatest weekly advance being 14 feet.

In the left-hand portion of the model is the shaft sunk to begin
operations; here also is shown the bucket hoist for removing the
spoil. The V-type steam engine powering the hoist was designed by
Brunel. At the right of the main model is an enlarged detail of the
shield, actually an improved version built in 1835.

The work continued despite setbacks of every sort. The financial ones
need no recounting here. Technically, although the shield principle
proved workable, the support afforded was not infallible. Four or five
times the river broke through the thin cover of silt and flooded
the workings, despite the utmost caution in excavating. When this
occurred, masses of clay, sandbags, and mats were dumped over the
opening in the riverbed to seal it, and the tunnel pumped out. I. K.
Brunel acted as superintendent and nearly lost his life on a number of
occasions. After several suspensions of work resulting from withdrawal
or exhaustion of support, one lasting seven years, the work was
completed in 1843.

Despite the fact that Brunel had, for the first time, demonstrated a
practical method for tunneling in firm and water-bearing ground, the
enormous cost of the work and the almost overwhelming problems
encountered had a discouraging effect rather than otherwise. Not for
another quarter of a century was a similar project undertaken.

The Thames Tunnel was used for foot and light highway traffic until
about 1870 when it was incorporated into the London Underground
railway system, which it continues to serve today. The roofed-over
top sections of the two shafts may still be seen from the river.

A number of contemporary popular accounts of the tunnel exist, but one
of the most thorough and interesting expositions on a single tunnel
work of any period is Henry Law's _A Memoir of the Thames Tunnel_,
published in 1845-1846 by John Weale. Law, an eminent civil engineer,
covers the work in incredible detail from its inception until the
major suspension in late 1828 when slightly more than half completed.
The most valuable aspect of his record is a series of plates of
engineering drawings of the shield and its components, which, so
far as is known, exist nowhere else. These formed the basis of the
enlarged section of the shield, shown to the right of the model of the
tunnel itself. A vertical section through the shield is reproduced
here from Law for comparison with the model (figs. 21 and 23).

[Illustration: Figure 17.--SOFT-GROUND TUNNELING. The support of
walls and roof of mine shaft by simple timbering; 16th century.
MHT model--3/4" scale. (Smithsonian photo 49260-J.)]

[Illustration: Figure 18.--SOFT-GROUND TUNNELING. The model of a 16th
century mine in the Museum of History and Technology was constructed
from illustrations in such works as G. E. von Löhneyss' _Bericht vom
Bergwerck_, 1690, as well as the better known ones from _De re
Metallica_.]

[Illustration: Figure 19.--THE SUCCESSIVE STAGES in the enlargement
of a mid-19th century railroad tunnel, using the Austrian system of
timbering. MHT model.]

[Illustration: Figure 20.--M. I. BRUNEL'S THAMES TUNNEL, 1825-1843,
the first driven beneath a body of water. MHT model--1/4" scale.
(Smithsonian photo 49260-F.)]


THE TOWER SUBWAY

Various inventors attempted to improve upon the Brunel shield, aware
of the fundamental soundness of the shield principle. Almost all
bypassed the rectangular sectional construction used in the Thames
Tunnel, and took as a starting point a sectional shield of circular
cross section, advanced by Brunel in his original patent of 1818.
James Henry Greathead (1844-1896), rightfully called the father of
modern subaqueous tunneling, surmised in later years that Brunel had
chosen a rectangular configuration for actual use, as one better
adapted to the sectional type of shield. The English civil engineer,
Peter W. Barlow, in 1864 and 1868 patented a circular shield, of one
piece, which was the basis of one used by him in constructing a small
subway of 1350 feet beneath the Thames in 1869, the first work to
follow the lead of Brunel. Greathead, acting as Barlow's contractor,
was the designer of the shield actually used in the work, but it was
obviously inspired by Barlow's patents.

The reduction of the multiplicity of parts in the Brunel shield to
a single rigid unit was of immense advantage and an advance perhaps
equal to the shield concept of tunneling itself. The Barlow-Greathead
shield was like the cap of a telescope with a sharpened circular ring
on the front to assist in penetrating the ground. The diaphragm
functioned, as did Brunel's breasting boards, to resist the
longitudinal earth pressure of the face, and the cylindrical portion
behind the diaphragm bore the radial pressure of roof and walls. Here
also for the first time, a permanent lining formed of cast-iron
segments was used, a second major advancement in soft-ground tunneling
practice. Not only could the segments be placed and bolted together
far more rapidly than masonry lining could be laid up, but unlike the
green masonry, they could immediately bear the full force of the
shield-propelling screws.

Barlow, capitalizing on Brunel's error in burrowing so close to the
riverbed, maintained an average cover of 30 feet over the tunnel,
driving through a solid stratum of firm London clay which was
virtually impervious to water. As the result of this, combined with
the advantages of the solid shield and the rapidly placed iron lining,
the work moved forward at a pace and with a facility in startling
contrast to that of the Thames Tunnel, although in fairness it must be
recalled that the face area was far less.

The clay was found sufficiently sound that it could be readily
excavated without the support of the diaphragm, and normally three
miners worked in front of the shield, digging out the clay and passing
it back through a doorway in the plate. This could be closed in case
of a sudden settlement or break in. Following excavation, the shield
was advanced 18 inches into the excavated area by means of 6 screws,
and a ring of lining segments 18 inches in length bolted to the
previous ring under cover of the overlapping rear skirt of the shield.
The small annular space left between the outside of the lining and the
clay by the thickness and clearance of the skirt--about an inch--was
filled with thin cement grout. The tunnel was advanced 18 inches
during each 8-hour shift. The work continued around the clock, and the
900-foot river section was completed in only 14 weeks.[4] The entire
work was completed almost without incident in just under a year, a
remarkable performance for the world's second subaqueous tunnel.

[Illustration: Figure 21.--ENLARGED DETAIL of Brunel's tunneling
shield, vertical section. The first two and part of the third of the
twelve frames are shown. To the left is the tunnel's completed brick
lining and to the right, the individual breasting boards and screws
for supporting the face. The propelling screws are seen at top and
bottom, bearing against the lining. Three miners worked in each frame,
one above the other. MHT model--3/4" scale. (Smithsonian photo
49260-G.)]

[Illustration: Figure 22.--BROADSIDE PUBLISHED AFTER COMMENCEMENT
OF WORK on the Thames Tunnel, 1827. (MHT collections.)

  OPEN TO THE PUBLIC EVERY DAY (_Sundays excepted_) _from Seven in the
  Morning, until Eight in the Evening_,

  THE THAMES TUNNEL.

  Fig. 1 shows a transverse section of the Thames, and beneath it a
  longitudinal section of the Tunnel, as it will be when completed;
  with the ascents in the inclinations in which they will be finished.

  Fig. 2 shows the two arched entrances of the Tunnel from the shaft.

  Fig. 3 is a representation of the iron shield, and shows a workman
  in each of the compartments.

  The Entrance to the Tunnel is near to Rotherhithe Church, and nearly
  opposite to the London-Docks. The nearest landing place from the river
  is Church Stairs. The Greenwich and Deptford coaches which go the
  lower road, start hourly from Charing-cross, and Gracechurch-street,
  and pass close by the works at Rotherhithe.

  Books relative to the Tunnel may be had at the works.

  The Public may view the Tunnel every day (Sundays excepted) from
  Seven in the morning until Eight in the Evening, upon payment of
  One Shilling each Person.

  The extreme northern end of the Tunnel is for the present secured
  by a strong wall; but visitors will find a dry, warm, and gravelled
  promenade, as far as to almost the centre of the river, and
  brilliantly lighted with oil gas.

  The entrance is from Rotherhithe Street, and by a safe, commodious,
  and easy stair case.

    H. Teape & Son, Printers, Tower-hill, London.]

[Illustration: Figure 23.--VERTICAL SECTION THROUGH BRUNEL'S SHIELD.
The long lever, x, supported the wood centering for turning the
masonry arches of the lining. (LAW, _A Memoir of the Thames
Tunnel._)]

[Illustration: Figure 24.--THAMES TUNNEL. SECTION THROUGH riverbed and
tunnel following one of the break-throughs of the river. Inspection of
the damage with a diving bell. (BEAMISH, _A Memoir of the Life of Sir
Marc Isambard Brunel_.)]

The Tower Subway at first operated with cylindrical cars that nearly
filled the 7-foot bore; the cars were drawn by cables powered by small
steam engines in the shafts. This mode of power had previously been
used in passenger service only on the Greenwich Street elevated
railway in New York. Later the cars were abandoned as unprofitable and
the tunnel turned into a footway (fig. 32). This small tunnel, the
successful driving due entirely to Greathead's skill, was the
forerunner of the modern subaqueous tunnel. In it, two of the three
elements essential to such work thereafter were first applied: the
one-piece movable shield of circular section, and the segmental
cast-iron lining.

The documentation of this work is far thinner than for the Thames
Tunnel. The most accurate source of technical information is a brief
historical account in Copperthwaite's classic _Tunnel Shields and the
Use of Compressed Air in Subaqueous Works_, published in 1906.
Copperthwaite, a successful tunnel engineer, laments the fact that he
was able to turn up no drawing or original data on this first shield
of Greathead's, but he presents a sketch of it prepared in the
Greathead office in 1895, which is presumably a fair representation
(fig. 33). The Tower Subway model was built on the basis of this and
several woodcuts of the working area that appeared contemporaneously
in the illustrated press. In this and the adjacent model of Beach's
Broadway Subway, the tunnel axis has been placed on an angle to the
viewer, projecting the bore into the case so that the complete circle
of the working face is included for a more suggestive effect. This was
possible because of the short length of the work included.

Henry S. Drinker, also a tunnel engineer and author of the most
comprehensive work on tunneling ever published, treats rock tunneling
in exhaustive detail up to 1878. His notice of what he terms
"submarine tunneling" is extremely brief. He does, however, draw a
most interesting comparison between the first Thames Tunnel, built by
Brunel, and the second, built by Greathead 26 years later:

    FIRST THAMES TUNNEL             SECOND THAMES TUNNEL
                                     (TOWER SUBWAY)

  Brickwork lining, 38 feet       Cast-iron lining of 8 feet
  wide by 22-1/2 feet high.       outside diameter.

  120-ton cast-iron shield,       2-1/2-ton, wrought-iron shield,
  accommodating 36 miners.        accommodating at most 3 men.

  Workings filled by irruption   "Water encountered at almost
  of river five times.            any time could have been
                                  gathered in a stable pail."

  Eighteen years elapsed between  Work completed in about
  start and finish of work.       eleven months.

  Cost: $3,000,000.               Cost: $100,000.

[Illustration: Figure 25.--TRANSVERSE SECTION THROUGH SHIELD, after
inundation. Such disasters, as well as the inconsistency of the
riverbed's composition, seriously disturbed the alignment of the
shield's individual sections. (LAW, _A Memoir of the Thames Tunnel_.)]

[Illustration: Figure 26.--LONGITUDINAL SECTION THROUGH THAMES TUNNEL
after sandbagging to close a break in the riverbed. The tunnel is
filled with silt and water. (LAW, _A Memoir of the Thames Tunnel_.)]

[Illustration: Figure 27.--INTERIOR OF THE THAMES TUNNEL shortly after
completion in 1843. (_Photo courtesy of New York Public Library
Picture Collection._)]

[Illustration: Figure 28.--THAMES TUNNEL in use by London Underground
railway. (_Illustrated London News_, 1869?)]

[Illustration: Figure 29.--PLACING A segment of cast-iron lining in
Greathead's Tower Subway, 1869. To the rear is the shield's diaphragm
or bulkhead. MHT model--1-1/2" scale. (Smithsonian photo 49260-B.)]


BEACH'S BROADWAY SUBWAY

Almost simultaneously with the construction of the Tower Subway,
the first American shield tunnel was driven by Alfred Ely Beach
(1826-1896). Beach, as editor of the _Scientific American_ and
inventor of, among other things, a successful typewriter as early as
1856, was well known and respected in technical circles. He was not
a civil engineer, but had become concerned with New York's pressing
traffic problem (even then) and as a solution, developed plans for a
rapid-transit subway to extend the length of Broadway. He invented a
shield as an adjunct to this system, solely to permit driving of the
tunnel without disturbing the overlying streets.

An active patent attorney as well, Beach must certainly have known of
and studied the existing patents for tunneling shields, which were,
without exception, British. In certain aspects his shield resembled
the one patented by Barlow in 1864, but never built. However, work on
the Beach tunnel started in 1869, so close in time to that on the
Tower Subway, that it is unlikely that there was any influence from
that source. Beach had himself patented a shield, in June 1869, a
two-piece, sectional design that bore no resemblance to the one used.
His subway plan had been first introduced at the 1867 fair of the
American Institute in the form of a short plywood tube through which
a small, close-fitting car was blown by a fan. The car carried 12
passengers. Sensing opposition to the subway scheme from Tammany, in
1868 Beach obtained a charter to place a small tube beneath Broadway
for transporting mail and small packages pneumatically, a plan he
advocated independently of the passenger subway.

[Illustration: Figure 30.--CONTEMPORARY ILLUSTRATIONS of Tower
Subway works used as basis of the model in the Museum of History
and Technology. (_Illustrated London News_, 1869.)

  ADVANCING THE SHIELD. FITTING THE CASTINGS.]

[Illustration: Figure 31.--EXCAVATION IN FRONT OF SHIELD, Tower
Subway. This was possible because of the stiffness of the clay
encountered. MHT model--front of model shown in fig. 29.
(Smithsonian photo 49260-A.)]

Under this thin pretense of legal authorization, the sub-rosa
excavation began from the basement of a clothing store on Warren
Street near Broadway. The 8-foot-diameter tunnel ran eastward a short
distance, made a 90-degree turn, and thence southward under Broadway
to stop a block away under the south side of Murray Street. The total
distance was about 312 feet. Work was carried on at night in total
secrecy, the actual tunneling taking 58 nights. At the Warren Street
terminal, a waiting room was excavated and a large Roots blower
installed for propulsion of the single passenger car. The plan was
similar to that used with the model in 1867: the cylindrical car
fitted the circular tunnel with only slight circumferential clearance.
The blower created a plenum within the waiting room and tunnel area
behind the car of about 0.25 pounds per square inch, resulting in a
thrust on the car of almost a ton, not accounting for blowby. The car
was thus blown along its course, and was returned by reversing the
blower's suction and discharge ducts to produce an equivalent vacuum
within the tunnel.

[Illustration: Figure 32.--INTERIOR OF COMPLETED TOWER SUBWAY.
(THORNBURY, _Old and New London, 1887, vol. 1, p. 126_.)]

The system opened in February of 1870 and remained in operation for
about a year. Beach was ultimately subdued by the hostile influences
of Boss Tweed, and the project was completely abandoned. Within a very
few more years the first commercially operated elevated line was
built, but the subway did not achieve legitimate status in New York
until the opening of the Interborough line in 1904. Ironically, its
route traversed Broadway for almost the length of the island.

[Illustration: Figure 33.--VERTICAL SECTION through the Greathead
shield used at the Tower Subway, 1869. The first one-piece shield of
circular section. (COPPERTHWAITE, _Tunnel Shields and the Use of
Compressed Air in Subaqueous Works_.)]

The Beach shield operated with perfect success in this brief trial,
although the loose sandy soil encountered was admittedly not a severe
test of its qualities. No diaphragm was used; instead a series of 8
horizontal shelves with sharpened leading edges extended across the
front opening of the shield. The outstanding feature of the machine
was the substitution for the propelling screws used by Brunel and
Greathead of 18 hydraulic rams, set around its circumference. These
were fed by a single hand-operated pump, seen in the center of figure
34. By this means the course of the shield's forward movement could be
controlled with a convenience and precision not attainable with
screws. Vertical and horizontal deflection was achieved by throttling
the supply of water to certain of the rams, which could be
individually controlled, causing greater pressure on one portion of
the shield than another. This system has not changed in the ensuing
time, except, of course, in the substitution of mechanically produced
hydraulic pressure for hand.

[Illustration: Figure 34.--BEACH'S Broadway Subway. Advancing the
shield by hydraulic rams, 1869. MHT model--1-1/2" scale. (Smithsonian
photo 49260-E.)]

[Illustration: Figure 35.--VERTICAL SECTION through the Beach shield
used on the Broadway Subway, showing the horizontal shelves (C), iron
cutting ring (B), hydraulic rams (D), hydraulic pump (F), and rear
protective skirt (H). (_Scientific American_, March 5, 1870.)]

Unlike the driving of the Tower Subway, no excavation was done in
front of the shield. Rather, the shield was forced by the rams into
the soil for the length of their stroke, the material which entered
being supported by the shelves. This was removed from the shelves and
hauled off. The ram plungers then were withdrawn and a 16-inch length
of the permanent lining built up within the shelter of the shield's
tail ring. Against this, the rams bore for the next advance. Masonry
lining was used in the straight section; cast-iron in the curved. The
juncture is shown in the model.

[Illustration: Figure 36.--INTERIOR of Beach Subway showing iron
lining on curved section and the pneumatically powered passenger car.
View from waiting room. (_Scientific American_, March 5, 1870.)]

Enlarged versions of the Beach shield were used in a few tunnels in
the Midwest in the early 1870's, but from then until 1886 the shield
method, for no clear reason, again entered a period of disuse finding
no application on either side of the Atlantic despite its virtually
unqualified proof at the hands of Greathead and Beach. Little precise
information remains on this work. The Beach system of pneumatic
transit is described fully in a well-illustrated booklet published
by him in January 1868, in which the American Institute model is
shown, and many projected systems of pneumatic propulsion as well
as of subterranean and subaqueous tunneling described. Beach again
(presumably) is author of the sole contemporary account of the
Broadway Subway, which appeared in _Scientific American_ following its
opening early in 1870. Included are good views of the tunnel and car,
of the shield in operation, and, most important, a vertical sectional
view through the shield (fig. 35).

It is interesting to note that optical surveys for maintenance of the
course apparently were not used. The article illustrated and described
the driving each night of a jointed iron rod up through the tunnel
roof to the street, twenty or so feet above, for "testing the
position."


THE FIRST HUDSON RIVER TUNNEL

Despite the ultimate success of Brunel's Thames Tunnel in 1843, the
shield in that case afforded only moderately reliable protection
because of the fluidity of the soil driven through, and its tendency
to enter the works through the smallest opening in the shield's
defense. An English doctor who had made physiological studies of the
effects on workmen of the high air pressure within diving bells is
said to have recommended to Brunel in 1828 that he introduce an
atmosphere of compressed air into the tunnel to exclude the water
and support the work face.

This plan was first formally described by Sir Thomas Cochrane
(1775-1860) in a British patent of 1830. Conscious of Brunel's
problems, he proposed a system of shaft sinking, mining, and tunneling
in water-bearing materials by filling the excavated area with air
sufficiently above atmospheric pressure to prevent the water from
entering and to support the earth. In this, and his description of air
locks for passage of men and materials between the atmosphere and the
pressurized area, Cochrane fully outlined the essential features of
pneumatic excavation as developed since.

[Illustration: Figure 37.--THE GIANT ROOTS LOBE-TYPE BLOWER used for
propelling the car.]

In 1839, a French engineer first used the system in sinking a mine
shaft through a watery stratum. From then on, the sinking of shafts,
and somewhat later the construction of bridge pier foundations, by the
pneumatic method became almost commonplace engineering practice in
Europe and America. Not until 1879 however, was the system tried in
tunneling work, and then, as with the shield ten years earlier, almost
simultaneously here and abroad. The first application was in a small
river tunnel in Antwerp, only 5 feet in height. This project was
successfully completed relying on compressed air alone to support the
earth, no shield being used. The importance of the work cannot be
considered great due to its lack of scope.

[Illustration: Figure 38.--TESTING ALIGNMENT of the Broadway Subway at
night by driving a jointed rod up to street level. (_Scientific
American_, March 5, 1870.)]

In 1871 Dewitt C. Haskin (1822-1900), a west coast mine and railroad
builder, became interested in the pneumatic caissons then being used
to found the river piers of Eads' Mississippi River bridge at St.
Louis. In apparent total ignorance of the Cochrane patent, he evolved
a similar system for tunneling water-bearing media, and in 1873
proposed construction of a tunnel through the silt beneath the Hudson
to provide rail connection between New Jersey and New York City.

[Illustration: Figure 39.--HASKIN'S pneumatically driven tunnel
under the Hudson River, 1880. In the engine room at top left was the
machinery for hoisting, generating electricity for lighting, and air
compressing. The air lock is seen in the wall of the brick shaft.
MHT model--0.3" scale. (Smithsonian photo 49260.)]

[Illustration: Figure 40.--ARTIST'S CONCEPTION OF MINERS escaping
into the air lock during the blowout in Haskin's tunnel.]

It would be difficult to imagine a site more in need of such
communication. All lines from the south terminated along the west
shore of the river and the immense traffic--cars, freight and
passengers--was carried across to Manhattan Island by ferry and barge
with staggering inconvenience and at enormous cost. A bridge would
have been, and still is, almost out of the question due not only to
the width of the crossing, but to the flatness of both banks. To
provide sufficient navigational clearance (without a drawspan),
impracticably long approaches would have been necessary to obtain
a permissibly gentle grade.

Haskin formed a tunneling company and began work with the sinking of
a shaft in Hoboken on the New Jersey side. In a month it was halted
because of an injunction by, curiously, the D L & W Railroad, who
feared for their vast investment in terminal and marine facilities.
Not until November of 1879 was the injunction lifted and work again
commenced. The shaft was completed and an air lock located in one wall
from which the tunnel proper was to be carried forward. It was
Haskin's plan to use no shield, relying solely on the pressure of
compressed air to maintain the work faces and prevent the entry of
water. The air was admitted in late December, and the first
large-scale pneumatic tunneling operation launched. A single 26-foot,
double-track bore was at first undertaken, but a work face of such
diameter proved unmanageable and two oval tubes 18 feet high by 16
feet wide were substituted, each to carry a single track. Work went
forward with reasonable facility, considering the lack of precedent.
A temporary entrance was formed of sheet-iron rings from the air lock
down to the tunnel grade, at which point the permanent work of the
north tube was started. Immediately behind the excavation at the face,
a lining of thin wrought-iron plates was built up, to provide form for
the 2-foot, permanent brick lining that followed. The three stages are
shown in the model in about their proper relationship of progress. The
work is shown passing beneath an old timber-crib bulkhead, used for
stabilizing the shoreline.

The silt of the riverbed was about the consistency of putty and under
good conditions formed a secure barrier between the excavation and the
river above. It was easily excavated, and for removal was mixed with
water and blown out through a pipe into the shaft by the higher
pressure in the tunnel. About half was left in the bore for removal
later. The basic scheme was workable, but in operation an extreme
precision was required in regulating the air pressure in the work
area.[5] It was soon found that there existed an 11-psi difference
between the pressure of water on the top and the bottom of the working
face, due to the 22-foot height of the unlined opening. Thus, it was
impossible to maintain perfect pneumatic balance of the external
pressure over the entire face. It was necessary to strike an average
with the result that some water entered at the bottom of the face
where the water pressure was greatest, and some air leaked out at the
top where the water pressure was below the air pressure. Constant
attention was essential: several men did nothing but watch the
behavior of the leaks and adjusted the pressure as the ground density
changed with advance. Air was supplied by several steam-driven
compressors at the surface.

The air lock permitted passage back and forth of men and supplies
between the atmosphere and the work area, without disturbing the
pressure differential. This principle is demonstrated by an animated
model set into the main model, to the left of the shaft (fig. 39). The
variation of pressure within the lock chamber to match the atmosphere
or the pressurized area, depending on the direction of passage, is
clearly shown by simplified valves and gauges, and by the use of light
in varying color density. In the Haskin tunnel, 5 to 10 minutes were
taken to pass the miners through the lock so as to avoid too abrupt a
physiological change.

Despite caution, a blowout occurred in July 1880 due to air leakage
not at the face, but around the temporary entrance. One door of the
air lock jammed and twenty men drowned, resulting in an inquiry which
brought forth much of the distrust with which Haskin was regarded by
the engineering profession. His ability and qualifications were
subjected to the bitterest attack in and by the technical press. There
is some indication that, although the project began with a staff of
competent engineers, they were alienated by Haskin in the course of
work and at least one withdrew. Haskin's remarks in his own defense
indicate that some of the denunciation was undoubtedly justified. And
yet, despite this reaction, the fundamental merit of the pneumatic
tunneling method had been demonstrated by Haskin and was immediately
recognized and freely acknowledged. It was apparent at the same time,
however, that air by itself did not provide a sufficiently reliable
support for large-area tunnel works in unstable ground, and this
remains the only major subaqueous tunnel work driven with air alone.

[Illustration: Figure 41.--LOCATION OF HUDSON RIVER TUNNEL. (_Leslie's
Weekly_, 1879.)]

After the accident, work continued under Haskin until 1882 when funds
ran out. About 1600 feet of the north tube and 600 feet of the south
tube had been completed. Greathead resumed operations with a shield
for a British company in 1889, but exhaustion of funds again caused
stoppage in 1891. The tunnel was finally completed in 1904, and is
now in use as part of the Hudson and Manhattan rapid-transit system,
never providing the sought-after rail link. A splendid document of the
Haskin portion of the work is S. D. V. Burr's _Tunneling Under the
Hudson River_ published in 1885. It is based entirely upon firsthand
material and contains drawings of most of the work, including the
auxiliary apparatus. It is interesting to note that electric
illumination (arc, not incandescent, lights) and telephones were used,
unquestionably the first employment of either in tunnel work.

[Illustration: Figure 42.--ST. CLAIR TUNNEL. View of front of shield
showing method of excavation in firm strata. Incandescent electric
illumination was used. 1889-90. MHT model--1" scale. (Smithsonian
photo 49260-D.)]


THE ST. CLAIR TUNNEL

The final model of the soft-ground series reflects, as did the Hoosac
Tunnel model for hard-rock tunneling, final emergence into the modern
period. Although the St. Clair Tunnel was completed over 70 years ago,
it typifies in its method of construction, the basic procedures of
subaqueous work in the present day. The Thames Tunnel of Brunel, and
Haskin's efforts beneath the Hudson, had clearly shown that by
themselves, both the shield and pneumatic systems of driving through
fluid ground were defective in practice for tunnels of large area.
Note that the earliest successful works by each method had been of
very small area, so that the influence of adverse conditions was
greatly diminished.

The first man to perceive and seize upon the benefits to be gained by
combining the two systems was, most fittingly, Greathead. Although he
had projected the technique earlier, in driving the underground City
and South London Railway in 1886, he brought together for the first
time the three fundamental elements essential for the practical
tunneling of soft, water-bearing ground: compressed-air support of the
work during construction, the movable shield, and cast-iron, permanent
lining. The marriage was a happy one indeed; the limitations of each
system were almost perfectly overcome by the qualities of the others.

The conditions prevailing in 1882 at the Sarnia, Ontario, terminal of
the Grand Trunk Railway, both operational and physical, were almost
precisely the same as those which inspired the undertaking of the
Hudson River Tunnel. The heavy traffic at this vital U.S.--Canada rail
interchange was ferried inconveniently across the wide St. Clair
River, and the bank and river conditions precluded construction of a
bridge. A tunnel was projected by the railway in that year, the time
when Haskin's tribulations were at their height. Perhaps because of
this lack of precedent for a work of such size, nothing was done
immediately. In 1884 the railway organized a tunnel company; in 1886
test borings were made in the riverbed and small exploratory drifts
were started across from both banks by normal methods of mine
timbering. The natural gas, quicksand, and water encountered soon
stopped the work.

[Illustration: Figure 43.--REAR VIEW OF ST. CLAIR SHIELD showing the
erector arm placing a cast-iron lining segment. The three motions of
the arm--axial, radial, and rotational, were manually powered.
(Smithsonian photo 49260-C.)]

It was at this time that the railway's president visited Greathead's
City and South London workings. The obvious answer to the St. Clair
problem lay in the successful conduct of this subway. Joseph Hobson,
chief engineer of the Grand Trunk and of the tunnel project, in
designing a shield, is said to have searched for drawings of the
shields used in the Broadway and Tower Subways of 1868-9, but unable
to locate any, he relied to a limited extent on the small drawings of
those in Drinker's volume. There is no explanation as to why he did
not have drawings of the City and South London shield at that moment
in use, unless one considers the rather unlikely possibility that
Greathead maintained its design in secrecy.

[Illustration: Figure 44.--OPENING OF THE ST. CLAIR TUNNEL, 1891.
(_Photo courtesy of Detroit Library, Burton Historical Collection._)]

The Hobson shield followed Greathead's as closely as any other, in
having a diaphragm with closable doors, but a modification of Beach's
sharpened horizontal shelves was also used. However, these functioned
more as working platforms than supports for the earth. The machine was
21-1/2 feet in diameter, an unprecedented size and almost twice that
of Greathead's current one. It was driven by 24 hydraulic rams.
Throughout the entire preliminary consideration of the project there
was a marked sense of caution that amounted to what seems an almost
total lack of confidence in success. Commencement of the work from
vertical shafts was planned so that if the tunnel itself failed, no
expenditure would have been made for approach work. In April 1888,
the shafts were started near both riverbanks, but before reaching
proper depth the almost fluid clay and silt flowed up faster than it
could be excavated and this plan was abandoned. After this second
inauspicious start, long open approach cuts were made and the work
finally began. The portals were established in the cuts, several
thousand feet back from each bank and there the tunneling itself
began. The portions under the shore were driven without air. When the
banks were reached, brick bulkheads containing air locks were built
across the opening and the section beneath the river, about 3,710 feet
long, driven under air pressure of 10 to 28 pounds above atmosphere.
For most of the way, the clay was firm and there was little air
leakage. It was found that horses could not survive in the compressed
air, and so mules were used under the river.

In the firm clay, excavation was carried on several feet in front of
the shield, as shown in the model (fig. 42). About twelve miners
worked at the face. However, in certain strata the clay encountered
was so fluid that the shield could be simply driven forward by the
rams, causing the muck to flow in at the door openings without
excavation. After each advance, the rams were retracted and a ring of
iron lining segments built up, as in the Tower Subway. Here, for the
first time, an "erector arm" was used for placing the segments, which
weighed about half a ton. In all respects, the work advanced with
wonderful facility and lack of operational difficulty. Considering
the large area, no subaqueous tunnel had ever been driven with such
speed. The average monthly progress for the American and Canadian
headings totaled 455 feet, and at top efficiency 10 rings or a length
of 15.3 feet could be set in a 24-hour day in each heading. The 6,000
feet of tunnel was driven in just a year; the two shields met
vis-a-vis in August of 1890.

The transition was complete. The work had been closely followed by the
technical journals and the reports of its successful accomplishment
thus were brought to the attention of the entire civil engineering
profession. As the first major subaqueous tunnel completed in America
and the first in the world of a size able to accommodate full-scale
rail traffic, the St. Clair Tunnel served to dispel the doubts
surrounding such work, and established the pattern for a mode of
tunneling which has since changed only in matters of detail.

Of the eight models, only this one was built under the positive
guidance of original documents. In the possession of the Canadian
National Railways are drawings not only of all elements of the shield
and lining, but of much of the auxiliary apparatus used in
construction. Such materials rarely survive, and do so in this case
only because of the foresight of the railway which, to avoid paying a
high profit margin to a private contractor as compensation for the
risk and uncertainty involved, carried the contract itself and,
therefore, preserved all original drawing records.

While the engineering of tunnels has been comprehensively treated in
this paper from the historical standpoint, it is well to still reflect
that the advances made in tunneling have not perceptibly removed the
elements of uncertainty but have only provided more positive and
effective means of countering their forces. Still to be faced are the
surprises of hidden streams, geologic faults, shifts of strata,
unstable materials, and areas of extreme pressure and temperature.




BIBLIOGRAPHY


  AGRICOLA, GEORGIUS. _De re Metallica._ [English transl. H. C. and L.
     H. Hoover (_The Mining Magazine_, London, 1912).] Basel: Froben,
     1556.

  BEACH, ALFRED ELY. _The pneumatic dispatch._ New York: The American
     News Company, 1868.

  BEAMISH, RICHARD. _A memoir of the life of Sir Marc Isambard
     Brunel._ London: Longmans, Green, Longmans and Roberts, 1862.

  BURR, S. D. V. _Tunneling under the Hudson River._ New York: John
     Wiley and Sons, 1885.

  COPPERTHWAITE, WILLIAM CHARLES. _Tunnel shields and the use of
     compressed air in subaqueous works._ New York: D. Van Nostrand
     Company, 1906.

  DRINKER, HENRY STURGESS. _Tunneling, explosive compounds and rock
     drills._ New York: John Wiley and Sons, 1878.

  LATROBE, BENJAMIN H. Report on the Hoosac Tunnel (Baltimore, October
     1, 1862). Pp. 125-139, app. 2, in _Report of the commissioners upon
     the Troy and Greenfield Railroad and Hoosac Tunnel_. Boston, 1863.

  LAW, HENRY. A memoir of the Thames Tunnel. _Weale's Quarterly Papers
     on Engineering_ (London, 1845-46), vol. 3, pp. 1-25 and vol. 5,
     pp. 1-86.

  The pneumatic tunnel under Broadway, N.Y. _Scientific American_
     (March 5, 1870), pp. 154-156.

  _Report of the commissioners upon the Troy and Greenfield Railroad
     and Hoosac Tunnel to his excellency the governor and the honorable
     the executive council of the state of Massachusetts, February 28,
     1863._ Boston, 1863.

  STORROW, CHARLES S. Report on European tunnels (Boston, November 28,
     1862). Pp. 5-122, app. 1, in _Report of the commissioners upon the
     Troy and Greenfield Railroad and Hoosac Tunnel...._ Boston, 1863.

  The St. Clair Tunnel. _Engineering News_ (in series running October
     4 to December 27, 1890).




FOOTNOTES

  [1] There are two important secondary techniques for opening
      subterranean and subaqueous ways, neither a method truly of
      tunneling. One of these, of ancient origin, used mainly in the
      construction of shallow subways and utility ways, is the "cut and
      cover" system, whereby an open trench is excavated and then roofed
      over. The result is, in effect, a tunnel. The concept of the other
      method was propounded in the early 19th century but only used
      practically in recent years. This is the "trench" method, a sort
      of subaqueous equivalent of cut and cover. A trench is dredged in
      the bed of a body of water, into which prefabricated sections of
      large diameter tube are lowered, in a continuous line. The joints
      are then sealed by divers, the trench is backfilled over the tube,
      the ends are brought up to dryland portals, the water is pumped
      out, and a subterranean passage results. The Chesapeake Bay Bridge
      Tunnel (1960-1964) is a recent major work of this character.

  [2] In 1952 a successful machine was developed on this plan, with
      hardened rollers on a revolving cutting head for disintegrating
      the rock. The idea is basically sound, possessing advantages in
      certain situations over conventional drilling and blasting
      systems.

  [3] In 1807 the noted Cornish engineer Trevithick commenced a small
      timbered drift beneath the Thames, 5 feet by 3 feet, as an
      exploratory passage for a larger vehicular tunnel. Due to the
      small frontal area, he was able to successfully probe about 1000
      feet, but the river then broke in and halted the work. Mine
      tunnels had also reached beneath the Irish Sea and various rivers
      in the coal regions of Newcastle, but these were so far below the
      surface as to be in perfectly solid ground and can hardly be
      considered subaqueous workings.

  [4] Unlike the Brunel tunnel, this was driven from both ends
      simultaneously, the total overall progress thus being 3 feet per
      shift rather than 18 inches. A top speed of 9 feet per day could
      be advanced by each shield under ideal conditions.

  [5] Ideally, the pressure of air within the work area of a
      pneumatically driven tunnel should just balance the hydrostatic
      head of the water without, which is a function of its total height
      above the opening. If the air pressure is not high enough, water
      will, of course, enter, and if very low, there is danger of
      complete collapse of the unsupported ground areas. If too high,
      the air pressure will overcome that due to the water and the air
      will force its way out through the ground, through increasingly
      larger openings, until it all rushes out suddenly in a "blowout."
      The pressurized atmosphere gone, the water then is able to pour
      in through the same opening, flooding the workings.




INDEX


  Agricola, Georgius, 215, 216


  Barlow, Peter W., 221, 227
  Beach, Alfred Ely, 224, 227-229, 231, 237
  Brunel, Marc Isambard (the elder), 204, 205, 217, 218, 221,
    224, 229, 231, 236
  Burleigh, Charles, 212, 213
  Burleigh Rock Drill Company, 212
  Burr, S. D. V., 236

  Cochrane, Sir Thomas, 231, 232
  Copperthwaite, William Charles, 224

  Doane, Thomas, 210, 212, 213, 215
  Drinker, Henry S., 224, 237

  Greathead, James Henry, 204, 218, 221, 224, 229, 231, 235-237
  Gwynn, Stuart, 210

  Haskin, DeWitt C., 204, 232, 234-236
  Haupt, Herman, 204, 209, 210
  Hobson, Joseph, 237

  Latrobe, Benjamin H., 208, 209
  Law, Henry, 218

  Mowbray, George W., 213, 215

  Nobel, Alfred B., 213

  Putnam Machine Works, 212

  Shanley, Walter, 212
  Shanley Bros., 215
  Sommeiller, Germain, 210
  Storrow, Charles S., 210

  Tweed, William Marcy (Boss), 229

  Weale, John, 218


       *       *       *       *       *


Transcriber's Notes

All obvious typographical errors corrected. Formatting inconsistancies
and spelling were standardized. Paragraphs split by illustrations were
rejoined. The text in the reproduced handbill for the Thames Tunnel
was transcribed with a slight modification to the figure description
portion. The Index was extracted from the full publication Index.