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AMERICAN SOCIETY OF CIVIL ENGINEERS

INSTITUTED 1852


TRANSACTIONS

Paper No. 1158


THE NEW YORK TUNNEL EXTENSION OF THE
PENNSYLVANIA RAILROAD.
THE CROSS-TOWN TUNNELS.[A]

BY JAMES H. BRACE AND FRANCIS MASON, MEMBERS, AM. SOC. C. E.




In this paper, it is proposed to describe the construction of the
tunnels extending eastward from the easterly extension of the Terminal
Station to the permanent shafts east of First Avenue.

They were located under 32d and 33d Streets from the station to Second
Avenue, and thence, curving to the left, passed under private property
and First Avenue to the shafts, as described in a preceding paper.
Typical cross-sections of the tunnels are shown on Plate XII.[B]

On May 29th, 1905, a contract was entered into with the United
Engineering and Contracting Company for the performance of this work.
This contract provided that work on each pair of tunnels should be
carried on from two shafts. The first, here referred to as the First
Avenue Shafts, were located just east of that avenue and directly over
the line of the tunnels; the other two, called the Intermediate Shafts,
were located on private property to the north of each pair of tunnels in
the blocks between Fourth and Madison Avenues. It was originally
intended to do all the work of construction from these four shafts.
Workings were started both east and west from the Intermediate Shafts,
and those to the west were to be continued to the Terminal Station.
After the change of plans, described in a previous paper, it was decided
to sink a third shaft on each line. These were known as the West Shafts,
and were located between Sixth and Seventh Avenues. Finally, it was
found necessary to build a portion of the tunnels on each line west of
Sixth Avenue in open cut. The locations of the shafts are shown on Plate
XIV.[C]

The First Avenue shafts were built by S. Pearson and Son, Inc., for the
joint use of the two contractors, as described in the paper on the
tunnels under the East River. While the shafts were being sunk, the
full-sized tunnels were excavated westward by the contractor for the
river tunnels for a distance of 50 ft., and top headings for 50 ft.
farther. By this means, injury to the caissons and to the contractor's
plant in the shafts by the subsequent work in the Cross-Town Tunnels was
avoided. The west half of the shaft was for the exclusive use of the
contractor for the Cross-Town Tunnels.


CONTRACTOR'S PLANT.

The method of handling the work adopted by the contractor was, broadly
speaking, as follows: Excavation was usually carried on by modifications
of the top-heading and bench method, the bench being carried as close to
the face as possible in order to allow the muck from the heading to be
blasted over the bench into the full section. The spoil was loaded into
3-yd. buckets (designed by the contractor and hereinafter described), by
steam shovels operated by compressed air, and hauled to the shafts by
electric locomotives. Electrically-operated telphers, suspended from a
timber trestle, hoisted the buckets, and, traveling on a mono-rail
track, deposited them on wagons for transportation to the dock. Arriving
at the dock, the buckets were lifted by electrically-operated stiff-leg
derricks and their contents deposited on scows for final disposal. The
spoil was thus transported from the heading to the scow without breaking
bulk.

When concreting was in progress, the spoil buckets were returned to the
shafts loaded with sand and stone. The concrete materials were deposited
in storage bins placed in the shafts, from which they were fed to the
mixers located at the foot of the shaft about on a level with the crown
of the tunnels. The concrete was transported to the forms in side-dump,
steel, concrete cars, hauled by the electric locomotives.

Electrical power was adopted largely on account of the restricted area
at the shaft sites, where a steam plant would have occupied considerable
space of great value for other purposes. The installation of a steam
plant at the Intermediate Shafts, which were located in a high-class
residential district, would have been highly objectionable to the
neighboring property owners, on account of the attendant noise, smoke,
and dirt, and, in addition, the cost of the transportation of fuel would
have been a serious burden. Except for the forges and, toward the last,
the steam locomotives, not a pound of coal was burned on the work. The
use of the bucket and telpher also eliminated most of the objectionable
noise incident to the transfer of spoil from tunnel cars to ordinary
wagons at the shaft sites. Power plants were installed at the North
Shaft near First Avenue and at the rear of the 33d Street Intermediate
Shaft.

_First Avenue Plant._--Fig. 1, Plate LVIII, is a general view of the
First Avenue plant. The power-house at the corner of 34th Street and
First Avenue supplied compressed air for operating drills, shovels,
pumps, and hoists in the tunnels driven from the river shafts, and in it
three Laidlaw-Dunn-Gordon compressors were installed. The largest was a
32 by 20 by 30-in., two-stage, cross-compound, direct-connected to a
Fort Wayne 480 h.p., 230-volt, direct-current, constant-speed motor run
at 100 rev. per min. This compressor was rated at 2,870 cu. ft. of free
air per minute at a pressure of 100 lb. It was governed by throttling
the suction, the governor being controlled by the pressure in the air
receiver and the motor running continuously at a constant speed. The two
others were of similar type, one was 22-1/2 by 14 by 18-in., rated at
1,250 cu. ft. of free air at a pressure of 100 lb., the other was 16 by
10 by 18-in., rated at 630 cu. ft. They were fitted with 9-ft.
fly-wheels, and were driven at 150 rev. per min. by 105-h.p., General
Electric, 220-volt, compound-wound, direct-current motors running at 655
rev. per min. The larger of these two compressors was driven by two of
the motors belted in tandem, and the smaller was belt-connected to a
third motor. The compressors were water-jacketed and had small
inter-coolers, the water supply for which was itself cooled in a Wheeler
Condenser and Engineering Company's water-cooling tower. The pump and
the blower operating it were electrically driven.

The telphers, used for hoisting muck from the tunnels and for lowering
supplies, were each hung from single rails on a timber trestle, about
40 ft. high, spanning and connecting the two shafts. One machine was
provided for each shaft, and where their tracks crossed 33d Street they
were separated sufficiently to permit the machines to pass each other.
At this point, and covering the street, a large platform was provided,
on which the trucks were loaded and unloaded (Fig. 2, Plate LVIII), and
from which they descended by an incline on First Avenue leading south to
32d Street. The platform also covered practically all the yard at the
South Shaft and materially increased the available working area. The
telphers were built by the Dodge Cold Storage Company, and were operated
by a 75-h.p. General Electric motor for hoisting and a 15-h.p. Northern
Electric Company motor for propulsion. Their rated lifting capacity was
10,000 lb. at a speed of 200 ft. per min.

The carpenter shop and machine-shop, both of which served the entire
work, were conveniently located in small buildings on the loading
platform. In the former the saws were each run independently by small
electric motors suspended under the platform. The heavy forms and form
carriages used in lining the tunnels with concrete were fabricated and
stored on the platform outside. The machine-shop lathes, etc., were all
belted to one shaft driven by an 8-h.p. General Electric motor. Above
the machine-shop was a locker-room and below it on the street level was
the main blacksmith shop for the work. Subsidiary blacksmith shops were
located at each of the other shafts. The storeroom and additional
locker-rooms were located above the power-plant in the North Shaft yard,
and isolated from the other structures was a small oil-house. Additional
storage space was provided by the contractor on 32d Street just west of
First Avenue by renting three old buildings and the yards in the rear of
them and of the Railroad Company's cement warehouse adjacent. Here
electric conduits, pipe, castings, and other heavy and bulky supplies
were stored.

During excavation the headings were supplied with forced ventilation
through 12-in. and 14-in. No. 16, spiral-riveted, asphalted pressure
pipes, canvas extensions being used beyond the ends of the pipes. A No.
4 American Blower, located at the top of each shaft and driven by a
15-h.p. General Electric motor, supplied the air.

[Illustration: PLATE LVIII, FIG. 1.--VIEW OF FIRST AVENUE PLANT.]

[Illustration: PLATE LVIII, FIG. 2.--TELPHER STRUCTURE AND LOADING
PLATFORM, FIRST AVENUE SHAFT.]

[Illustration: PLATE LVIII, FIG. 3.--HEADWORKS AT 33D STREET:
INTERMEDIATE SHAFT.]

[Illustration: PLATE LVIII, FIG. 4.--LOADING SPOIL ON BARGES, 35TH
STREET PIER.]

A concrete-mixing plant was placed in each shaft, the mixer being
located high enough to discharge into cars at about the level of the
springing line of the arch. Above the mixers were the measuring
hoppers set in the floor of a platform which was large enough to carry
half a day's supply of cement. At the South Shaft the cement was
delivered to this floor from the loading platform through a spiral steel
chute; at the North Shaft it was lowered in buckets by the telpher. The
sand and stone were drawn into the hoppers through short chutes from the
base of the storage bins which occupied the remaining height of the
shaft--about 50 ft. At the South Shaft the bins were of concrete and
steel, about 6 by 12 ft. in section, and attached to the central wall of
the caisson. Sand and stone were delivered into them from dump-wagons on
the loading platform. At the North Shaft steel-plate bins were used, and
were supplied with material by the buckets handled by the telpher. The
mixers were No. 5 Smith, belt-connected to 25-h.p. motors, and about 0.8
cu. yd. of concrete was mixed at a batch. The concrete cars were steel
side-dumpers of the Wiener or Koppel type.

In order to be able to continue concreting during the winter, when
neither sand nor stone could be obtained by water, practically all the
space under the loading platforms in the South Shaft yards not occupied
by the blacksmith shop was filled with these materials, which were
placed in storage in the late fall.

_Intermediate-Shaft Plant._--The air-compressing plant was located at
the rear of the 33d Street Intermediate Shaft, and supplied air for
driving the tunnels east and west from the Intermediate Shafts on both
32d and 33d Streets. Two compressors, the same as the large
Laidlaw-Dunn-Gordon machine at First Avenue, were installed here, with a
similar water-cooling tower.

Both shafts were on private property, owned by the Railroad Company, on
the north side of the streets, and each was equipped with two telphers
supported on timber trestles, similar to those at First Avenue. Here,
however, the buckets were placed on wagons standing at the curb, as
shown by Fig. 3, Plate LVIII.

Blowers for ventilation were installed at each shaft, as at First
Avenue, and, after the excavation had proceeded some distance, small
blacksmith shops, for sharpening drill steel and making minor repairs,
were located in the tunnels near the shafts.

The concrete plant in each shaft was similar in arrangement to those at
First Avenue, but the storage bins had wooden walls made of 2 by 4-in.
and 2 by 6-in. scantling nailed flat on each other.

The contractor's office on 33d Street backed up against the 32d Street
shaft site, and the basement was used as a storeroom for supplies for
both shafts.

After the decision to do part of the work between Sixth and Seventh
Avenues in open cut, an 8-in. air main was laid in 33d Street to the
West Shafts, and air was supplied from the Intermediate Shaft for work
on both streets in that neighborhood.

_West-Shaft Plant._--West of Sixth Avenue, between 32d and 33d Streets
and adjacent to the open-cut sections, the Railroad Company obtained
from the Hudson and Manhattan Railroad Company the use of a large area
from which the buildings had recently been removed, and gave the use of
it to the contractor. This was of great value in prosecuting the west
end of the work. The two West Shafts were located in the streets and
were supplied with short timber trestles similar to those at the
Intermediate Shafts. One telpher was taken from each of the Intermediate
Shafts to operate at each of the West Shafts. In addition, a number of
stiff-leg derricks were set up along the open-cut section, and were
operated by Lidgerwood or Lambert air hoisting engines, or by electric
motors, as circumstances dictated. A 15-ton Bay City locomotive crane
was also used along part of the open-cut work on 32d Street.

Several concrete plants were installed at points along the open-cut
section, and were moved from place to place, the same general
arrangement being adopted as at the plants already described. No. 3 and
No. 4 Ransome mixers were used, and were generally set up at about the
level of the top of the arch. The sand and stone storage bins were made
of scantlings spiked together, and were necessarily rather shallow on
account of the proximity of the tunnels to the street surface.

_Thirty-fifth Street Pier._--For the receipt and disposal of materials
at the 35th Street pier, four stiff-leg derricks, operated by electric
hoisting engines, were installed. Two were used in lifting the muck
buckets from the wagons and dumping their contents on the scows for
final disposal (Fig. 4, Plate LVIII); and the other two were fitted with
clam-shell buckets for unloading sand and broken stone from barges and
depositing the materials in large hoppers, from which they were drawn
into wagons for transportation to the various concrete plants. A large
part of the cement (all of which was supplied by the Railroad Company)
was also unloaded at the 35th Street pier and hauled directly to the
work, the surplus being stored temporarily in the Company's cement
warehouses on 32d, 33d and 35th Streets, near First Avenue, from which
it was drawn as required. On the dock was located the main powder
magazine, a small concrete structure. Considerable use was also made of
neighboring piers for unloading electric conduits, lumber, steel, etc.

[Illustration: FIG. 1. SPECIAL STEEL BUCKET
    PLAN OF BUCKET
    END VIEW
    SIDE VIEW OF BUCKET
    SECTION AT A-A]

_Tunnel Plant._--The spoil buckets, designed by D. L. Hough and George
Perrine, Members, Am. Soc. C. E., were a novel feature of the work.
These buckets are shown in detail in Fig. 1 and various photographs.
They were of 3 cu. yd. capacity and were split longitudinally, the two
halves being pinned at the apices of the ends. For lifting, they were
suspended from eyes at that point, and, when dumping, trip ropes were
hooked into eyes at the bottom of each side; lifting the trip ropes or
lowering the hoisting rope split the bucket, as shown in Fig. 4, Plate
LVIII, and dumped the contents. They were transported in the tunnel on
flat cars, and in the street on wagons, both cars and wagons being
provided with cradles shaped to receive the bottom of the bucket.

In the tunnels the loading was done with air-operated steam shovels,
four (Model 20) Marion shovels being used at various points of the work.
In Fig. 1, Plate LIX, one of these is shown loading the bucket. The cars
were hauled by General Electric, standard, 10-ton, mine locomotives, the
current for which was taken at 220 volts from a pair of No. 00 copper
trolley wires suspended from the roof of the tunnel. The collector was a
small four-wheeled buggy riding on the wires and connected to the
locomotive by several hundred feet of cable wound on a reel for use
beyond the end of the trolley wire. Two 8-1/2-ton, Davenport, steam
locomotives were also used in 32d Street, toward the end of the work,
after the headings had been holed through and the tunnels would quickly
clear themselves of gas and smoke. The steam shovels were supplemented
by two Browning, 15-ton, locomotive cranes, which handled the spoil in
places where timbering interfered with the operation of the shovels. All
tracks were of 3-ft. gauge throughout and laid with 40-lb. rails.

Practically all the heavy drilling was done with Ingersoll drills (Model
E 52), the trimming being largely done with jap and baby drills. A large
number of pumps were used at various points on the work, and practically
all were of Cameron make, the largest ones at the shaft being 10 by 5 by
13-in. The grout machines were of the vertical-cylinder, air-stirring
type.


SHAFT SINKING.

The sinking of the Intermediate Shafts was the first work undertaken by
the contractor.

The 33d Street Shaft was 34.5 ft. long, 21 ft. wide, and 83 ft. deep.
The rock surface averaged 5 ft. below the ground surface. Sinking was
started on July 10th, 1905, and was completed on October 3d, 1905, the
rock being hard and dry. The average daily rate was 0.73 ft. and an
average of 17.1 cu. yd. were excavated per day, with two shifts of 8 hr.
each. The first shift started at 6 A. M. and the second at 2.30 P. M.,
ending at 11 P. M. These hours were adopted in order to avoid undue
disturbance during the night.

[Illustration: PLATE LIX, FIG. 1.--AIR-OPERATED STEAM SHOVEL USED IN
TUNNEL.]

[Illustration: PLATE LIX, FIG. 2.--TIMBERING IN TOP HEADINGS ABOVE
I-BEAMS.]

[Illustration: PLATE LIX, FIG. 3.--FIRST SECTION OF CONCRETE LINING AT
FIFTH AVENUE.]

[Illustration: PLATE LIX, FIG. 4.--TIMBERING AND RUBBLE MASONRY OVER
I-BEAMS.]

Before blasting the first lift of rock, channel cuts 5 or 6 ft. deep
were made along the sides of the shaft, in order to avoid damage to the
walls of neighboring buildings. Timbering was required for a depth of
only 10 ft. below the surface of the ground.

A drift, 30.6 ft. long, 17 ft. wide, and 27 ft. high, connected the
south end of the shaft with the tunnels. The drift was excavated in
three stages, a top heading and a bench in two lifts. While blasting the
cut in the top heading, there was enough concussion to break glass in
the neighboring buildings. The use of a radialax machine reduced the
concussion somewhat, but it was very quickly abandoned on account of the
length of time required for the drilling.

The construction of the 32d Street Shaft was quite similar to the one on
33d Street. It was 31.5 ft. long, 20.5 ft. wide, and 71 ft. deep. The
depth of earth excavation averaged 19.5 ft. The rock in this shaft was
seamy and not quite as hard or dry as that in 33d Street, and timbering
was required for practically the full depth to the crown of the drift.
Sinking was started on May 15th, 1905, and was completed on October
26th, 1905. The daily average rate was 0.30 ft. in earth and 0.52 ft. in
rock. The drift was excavated in much the same manner as the one in 33d
Street, but the rock being softer the radialax machine was not used.


TUNNEL EXCAVATION.

During the early part of the work, the contractor devoted his entire
attention to the work of excavation. Nearly all the excavation east of
Fifth Avenue was done before any of the lining was placed. At a number
of points west of Fifth Avenue and at a few points to the east the
nature of the rock was such that the two operations had to be done
simultaneously.

_Single-Tunnel Method._--For an average distance of 350 ft. west from
the First Avenue Shafts there were four single tunnels. The rock was
sound and comparatively dry. A top heading of the full size of the
tunnel and about 8 ft. high was first driven. It was drilled by four
drills mounted on two columns, and was blasted in the ordinary way. The
bench was about 13 ft. high. Tripod drills, standing on the bench,
drilled the usual holes, but, owing to the lack of head-room, steels
long enough to reach the bottom of the bench could not be used. Tripod
drills were set as low as possible at the foot of the bench and drilled
lifting holes. These holes were inclined downward from 10° to 15° to the
horizontal, and were spaced to converge at the location of the drainage
ditches. The heading was usually driven from 10 to 20 ft. in advance of
the bench. At this distance a large part of the muck from the heading
was shot backward over the bench. In the single tunnels the muck was
loaded by hand.

_Twin-Tunnel Methods._--From the end of the single-track tunnel westward
to Fifth Avenue on 33d Street, and to Madison Avenue on 32d Street, with
some exceptions, each pair of tunnels was excavated for the entire width
at one operation. Three different methods of work were extensively used.
They were the double-heading method, the center-heading method, and the
full-sized-heading method, and these differed only in the manner of
drilling and blasting. The bench was usually within 10 or 15 ft. of the
face of the heading, and was drilled and fired in the same way as in the
single tunnels. After the installation of the permanent plant, most of
the muck was handled by steam shovels.

In the double-heading method, shown on Plate LVII, the top headings for
each tunnel of the pair were driven separately, leaving a short rock
core-wall between them. The headings were drilled from columns in the
manner described for the single tunnels. The temporary rock dividing
wall between the headings was drilled by a tripod drill on the bench of
one of the headings, and was fired with the bench.

In the center-heading method, also shown on Plate LVII, only one heading
was driven. It was rectangular in shape, about 8 ft. high and 14 ft.
wide. It was located on the center line between the tunnels. In general,
the face was from 6 to 12 ft., or the length of one or two rounds, in
advance of the remainder of the face at the top. The center heading was
drilled by four drills mounted on two columns. By turning these drills
to the side, they were used for holes at right angles to the line of the
tunnels, by which the remainder of the face of the heading was blasted.
By turning the drills downward, the bench holes under the center heading
were also drilled. The center heading explored the rock in advance of
the full-width heading, and gave a good idea as to the care needed in
firing.

For the full-width-heading method, Fig. 2, ten drills were mounted on
five columns set abreast across the face. Holes were drilled to form a
cut near the center line between the tunnels. The remainder of the holes
were located so that they would draw into the cut. The bench was
frequently drilled from the same set-up of columns by turning the
drills downward. In sound rock this method proved to be the most rapid
of any.

Practically all trimming was left until immediately before the
concreting. It was then taken up as a separate operation, but proved to
be costly and tedious, and a hindrance to the placing of the lining.

_Materials Encountered._--All the rock encountered was the familiar
Hudson schist, but it varied widely in its mineral constituents and in
its physical characteristics. In many places where the rock surface was
penetrated, a fine sand was found that was probably quicksand. The
material above the rock in the open-cut sections was mostly sand.

[Illustration: FIG. 2. METHOD OF EXCAVATING WITH FULL-WIDTH HEADING
CROSS-TOWN TUNNELS, MANHATTAN
    SIDE ELEVATION
    FRONT ELEVATION
    PLAN SHOWING POSITION OF COLUMNS FOR DRILLING FACE]

The concurrence of the watercourse, shown on General Viele's map of
Manhattan Island (Plate IX[D]), with the points where difficulties in
the construction of the tunnels were encountered has been noted in a
previous paper.

In all cases where the course of this ancient stream was crossed (except
at its final intersection of 33d Street), the rock was found to be very
soft and disintegrated, a large quantity of water was encountered, and
heavy timbering was required. The construction at these localities will
be taken up later. In addition, disintegrated rock, but of a less
troublesome character, was invariably met under the depressions in the
rock surface developed by the borings from the streets and test holes
from the tunnels. Many of these places required timbering, and no
timbering was elsewhere necessary except at the portals. These
coincident conditions were especially marked in 32d Street, which for a
long distance closely adjoins the course of the former creek.

_Disposal of Spoil._--The materials excavated from the tunnels were
dumped at the 35th Street pier on barges furnished by the Railroad
Company under another contract, and were towed to points near the
Bayonne peninsula where the spoil was used principally in the
construction of the Greenville Freight Yards and the line across the
Hackensack Meadows to the tunnels. Details of this work will be given in
a subsequent paper. After December, 1907, when the excavation was about
85% completed, the contractor furnished the barges and effected the
complete disposal of the spoil.

_Difficulties of Excavation._--As stated in a previous paper, the
excavation of the Twin Tunnel in 33d Street was continued westward to
the west line of Fifth Avenue on the original grade. At that point the
contractor started three drifts in the three-track section. The relation
of the drifts to each other and to the cross-section are shown by Fig. 3.
The center heading was driven a little in advance of those on the
sides. At a distance of 65 ft. west of Fifth Avenue the rock surface was
broken through in the top of the heading, and a very fine sand was
encountered. For some distance east of this point the rock was badly
disintegrated, and the heading required timbering. Through the soft
material, tight lagging was placed on the sides and roof of the heading,
and the face was protected by breast boards. There was a moderate flow
of water through the cracks, and, in spite of every effort, some of the
fine sand was constantly carried into the heading.

In one or two instances considerable ground was lost at the face. On the
evening of December 14th, 1906, as a heavy coal wagon was passing along
33d Street above the heading, the rear wheels dropped through the
asphalt pavement. An examination disclosed a cavity under the pavement
about 14 ft. long, 12 ft. wide and 14 ft. deep. Evidently, the fine sand
had gradually settled into the voids caused by the loss of material at
the face, and the settlement broke the brick sewer over the heading. The
sewer was temporarily repaired, and the hole in the street was filled
before morning. A tight bulkhead was built across the heading, and work
was abandoned at that point. The north drift was advanced to a point
108 ft. west of Fifth Avenue where sand was also encountered and a
considerable run occurred. After that time all work on the three-track
section was discontinued.

The Company then took up the consideration of changes in plan. To
determine the difficulties of driving a Twin Tunnel at a lower
elevation, an exploration drift, 8 ft. high and 12 ft. wide, was driven
on the center line of the street as a top heading on the proposed new
grade. Test holes were drilled above this heading and to the sides. The
results indicated that there was sufficient rock cover of fair quality
to enable the Twin Tunnel to be driven without great risk. The new plan
(continuing the Twin Tunnel westward at a lower grade) was adopted in
March, 1907, and work was immediately resumed at Fifth Avenue.

The relation between the cross-sections under the old and new plans at
that point is shown by Fig. 3. Before the new section was excavated it
was necessary to support the timber work in the old headings. The plan
adopted is also shown by Fig. 3. The rock was excavated under the center
heading, as shown in cross-section, for a length of about 3 ft. A girder
composed of two 18-in. I-beams was then put in position over each line
and supported on the sides by posts. The ends at the center lines
between the tunnels were supported on short posts bearing on the rock
bench. The support of the timbering in the headings was then transferred
to the girders by additional posts. Blocking was also inserted between
the tops of the beams and the rock walls between the headings. Fig. 2,
Plate LIX, gives a good idea of the timber work in the top headings
above the I-beams. When the roof had been made secure, the removal of
the bench was begun. As the work advanced it was necessary to replace
the short posts at the center of the tunnel by others of full height,
and there was considerable settlement in the I-beams during this
operation. When the bench had been removed to a point 61 ft. west of
Fifth Avenue, settlement was detected in the street surface above. Bench
excavation was suspended and a section of the permanent lining, 35 ft.
long, was placed. The space between the lining and the beams and between
the beams and the roof was filled with rubble masonry. Grout pipes were
built into the masonry and later all voids were filled with grout. Fig.
3, Plate LIX, shows the first section of the concrete lining completed
and part of the rubble in place; and Fig. 4, Plate LIX, shows details of
the work above the tunnels. A second section of bench was next removed
and more lining was placed. Work was continued in this way until all the
roof at the old three-track headings had been secured. In this portion
of the work the posts were embedded in the concrete.

Between Fifth and Sixth Avenues there were two more sections of bad rock
where it was necessary to support the roof with steel beams. At these
latter points there were no complications with the excavation for the
Three-Track Tunnel, and the work was much simpler. To avoid leaving the
center posts in the permanent work, two rows of temporary posts were
placed, as shown by Fig. 1, Plate LX, the center wall and skewback were
built, and the posts were removed, as shown by Fig. 2, Plate LX, before
placing the remainder of the lining.

In 32d Street the normal progress of the excavation was frequently
interrupted by encountering soft and unsound rock. In the excavation
between the East River and the Intermediate Shafts it was possible to
overcome these conditions by temporarily narrowing the excavation on one
side and supporting the roof on 16 by 16-in. transverse timbers caught
in niches in the rock at the sides, leaving sufficient room for the
steam shovel to work through. In order to save time, the height of the
excavation was not increased before placing these timbers, so that,
previous to the concreting, they all required to be raised to clear the
masonry lining and were then supported on posts on the center line
between the tunnels. This permitted the remainder of the excavation to
be made, and such additional timbering as was required was placed. At
most of these sections a brick arch and water-proofing were used, on
account of the presence of water. In certain places the center line
posts were buried in the core-wall, and, in order to permit the placing
of the water-proofing, were then cut off one by one flush with its top
as the load was transferred to the completed masonry. In other cases the
load was transferred to posts clear of the masonry and the center line
posts were entirely removed. Under such conditions the normal concrete
methods, to be described later, could not be used, and special forms
were substituted.

[Illustration: FIG. 3. CONSTRUCTION OF TWIN TUNNELS, THROUGH EXCAVATION
STARTED FOR THREE-TRACK TUNNEL IN 33D STREET NEAR 5TH AVENUE]

In this section of the work the most serious difficulties were
encountered near Fourth Avenue a short distance east of the Intermediate
Shaft, and beneath the site of the old pond shown on General Viele's
map. The rock cover was known from the boring to be very thin, and the
presence of the subway overhead caused some anxiety. The excavation was
at first taken out to practically full width and timbered, but the rock
became so treacherous that the heading was narrowed to a width
sufficient for one tunnel only. With this span the rock in the roof held
without timbering. As the masonry lining approached, sufficient trimming
was done to permit the placing of the core-wall and one arch. Above the
completed core-wall and brick arch the voids were filled solid with
rubble masonry to give an unyielding support to the roof. The excavation
of the remaining width of tunnel was then undertaken. Near the west side
of Fourth Avenue, the excavation broke out of rock at the top, and fine
sand and gravel with a large quantity of water were encountered. The
work of excavation was arduous, and proceeded very slowly, on account of
the care with which it was executed. Only a small amount of sand entered
the tunnel, but the lining was placed as soon as the excavation was
completed. Rubble masonry packing and grout ejected through pipes built
into the arch were used to fill the voids above the roof. As a further
precaution against the settlement of the subway, 2-in. pipes were washed
down from the street above the point where soft ground was exposed in
the roof of the tunnel, and through them grout was forced into the
ground at various depths. Careful levels show that no settlement of the
subway has taken place.

West of the Intermediate Shaft the tunnel was excavated for full width
until bad rock was encountered about 60 ft. west of Madison Avenue. (See
General Viele's map, Plate IX.) Timbering was used for a short distance,
and then the heading and bench were narrowed to 18 ft., and steam-shovel
excavation was abandoned. As the heading advanced the rock grew steadily
softer, the difficult conditions in this locality culminating when a
slushy disintegrated feldspar was met, requiring poling and breasting.
Thereafter the rock improved markedly, but near the east side of Fifth
Avenue its thickness above the roof was found to be only 1-1/2 ft.,
and the advance was stopped, pending a decision as to a change of plan.

[Illustration: PLATE LX, FIG. 1.--DOUBLE ROW OF POSTS UNDER I-BEAMS,
SUPPORTING ROOF IN BAD ROCK SECTION.]

[Illustration: PLATE LX, FIG. 2.--CENTER WALL AND SKEWBACK UNDER I-BEAMS,
AFTER REMOVAL OF DOUBLE ROW OF POSTS.]

[Illustration: PLATE LX, FIG. 3.--TIMBERING IN FULL-WIDTH HEADING OF
THREE-TRACK TUNNEL.]

[Illustration: PLATE LX, FIG. 4.--UNDERPINNING WALLS IN OPEN-CUT
SECTION.]

After some delay, an exploration drift, similar to the one already
described, was driven through to Sixth Avenue, and a change in plan was
made, substantially the same as for the 33d Street tunnels. Enlargement
to full size was at once started, but, for 400 ft. the rock was very
soft and poor, and required extremely careful handling. The exploration
drift was widened out to the full Twin-Tunnel width, and I-beams were
placed and supported, in much the same manner as in 33d Street. The rock
was so soft that it was frequently necessary to drive poling boards
ahead as the face was mined out with picks and shovels. The load was
very heavy, and the work the most difficult encountered in the tunnels.

After this stage of the enlargement was reached, the excavation of the
bench and the placing of the lining proceeded alternately, with the
I-beams temporarily supported on long posts while the concrete core-wall
was being built. Considerable settlement took place while shifting the
posts, and eventually showed on the street surface and in the adjacent
sidewalk vaults, but no damage was done to the structural portions of
the buildings.

While the above work had been going on westward from Fifth Avenue, the
excavation of the Twin Tunnel eastward from the end of the open-cut
section at Sixth Avenue had been proceeding rapidly, and, toward the end
of the difficult Fifth Avenue work, it was being attacked from both
directions.


PROGRESS OF EXCAVATION.

Owing to the numerous sections of poor rock, interspersed throughout the
work with stretches of sound rock, the progress of the excavation was
very irregular, especially in 32d Street. The rate of excavation in good
ground is shown in Table 1. In the sections of bad ground, the
operations of excavation, timbering, and lining were often carried on
alternately, and it is impracticable to include them in the table.

TABLE 1.--PROGRESS AND METHODS OF EXCAVATION IN GOOD GROUND.

THIRTY-THIRD STREET.

============================================================
           1                 |   2    |          3         |
-----------------------------+--------+--------------------+
                             |        |                    |
                             |        |                    |
   Type of excavation.       |Tunnels.|    Worked from:    |
                             |        |                    |
                             |        |                    |
-----------------------------+--------+--------------------+
Full-sized single tunnel     |   B    |  1st Ave. shaft.   |
                             |        |                    |
Full-sized single tunnel     |   A    |  1st Ave. shaft.   |
                             |        |                    |
Full-sized twin tunnel       |A and B |  1st Ave. shaft.   |
                             |        |                    |
                             |        |                    |
                             |        |                    |
Full-sized twin tunnel       |A and B |Intermediate shaft. |
                             |        |  (West of shaft.)  |
                             |        |                    |
                             |        |                    |
Full-sized twin tunnel       |A and B |Intermediate shaft. |
                             |        |  (East of shaft.)  |
                             |        |                    |
                             |        |                    |
Full-sized twin tunnel       |A and B |Intermediate shaft. |
                             |        |  (East of shaft.)  |
                             |        |                    |
                             |        |                    |
Exploration drift            |A and B |Intermediate shaft. |
                             |        |  (West of shaft.)  |
                             |        |                    |
Twin tunnel. Enlargement     |A and B |     West shaft.    |
  of exploration drift       |        |  (East of shaft.)  |
=============================+========+=====================

======================================================================
           4                      |    5   |      6     |      7     |
----------------------------------+--------+------------+------------+
                                  |        |   Length   |   Average  |
             DATES.               |  Time  |   tunnel   |   advance  |
----------------------------------|elapsed,| excavated, |   per day, |
               |                  |   in   |     in     |     in     |
     From      |        To        |  days. |linear feet.|linear feet.|
----------------------------------+--------+------------+------------+
Feb. 28, 1906. |May  12, 1906.    |    74  |     346    |     4.7    |
               |                  |        |            |            |
Feb. 28, 1906. |Apr. 30, 1906.    |    62  |     255    |     4.1    |
               |                  |        |            |            |
Aug. 23, 1906. |Jan.  5, 1907.    |   136  |     789    |     5.8    |
               |                  |        |            |            |
               |                  |        |            |            |
               |                  |        |            |            |
Apr.  4, 1906. |Oct. 31, 1906.    |   210  |     730    |     3.5    |
               |                  |        |            |            |
               |                  |        |            |            |
               |                  |        |            |            |
Apr.  4, 1906. |Oct. 31, 1906.    |   210  |     783    |     3.7    |
               |                  |        |            |            |
               |                  |        |            |            |
               |                  |        |            |            |
Nov.  1, 1906. |Dec. 26, 1906.    |    56  |     311    |     5.5    |
               |                  |        |            |            |
               |                  |        |            |            |
               |                  |        |            |            |
Mar.  1, 1907. |July 23, 1907.    |   145  |     947    |     6.5    |
               |                  |        |            |            |
               |                  |        |            |            |
Sept. 6, 1907. |Dec.  4, 1907.    |    89  |     603    |     6.8    |
               |                  |        |            |            |
===============+==================+========+============+=============

=====================================================
                        8
-----------------------------------------------------


            Methods and conditions.


-----------------------------------------------------
Top heading and bench. Muck loaded by hand.

 "     "     "    "      "    "    "    "

Top full-width heading and bench. Muck loaded
  by steam shovel. Working exclusively on this
  heading.

Top center heading and bench. Muck loaded by
  steam shovel. Working alternately in headings
  east and west of the shaft.

Top center heading and bench. Muck loaded by
  steam shovel. Working alternately in headings
  east and west of the shaft.

Top full-width heading and bench. Muck loaded
  by steam shovel working exclusively on this
  heading.

Exploration drift about 9 ft. by 12 ft. Mucking
  by hand. Fourteen timber bents were placed in
  March, and seven in April, 1907.

Drift excavated to full width and bench. Muck
  loaded by steam shovel.
=====================================================


THIRTY-SECOND STREET.

============================================================
           1                 |   2    |          3         |
-----------------------------+--------+--------------------+
                             |        |                    |
                             |        |                    |
   Type of excavation.       |Tunnels.|    Worked from:    |
                             |        |                    |
                             |        |                    |
-----------------------------+--------+--------------------+
Full-sized single tunnel     |   C    |  1st Ave. shaft.   |
                             |        |                    |
Full-sized single tunnel     |   D    |  1st Ave. shaft.   |
                             |        |                    |
Full-sized twin tunnel       |C and D |  1st Ave. shaft.   |
                             |        |                    |
                             |        |                    |
                             |        |                    |
Narrowed twin tunnel         |   C    |Intermediate shaft. |
                             |        |  (East of shaft.)  |
                             |        |                    |
                             |        |                    |
Narrowed twin tunnel         |   C    |Intermediate shaft. |
                             |        |  (East of shaft.)  |
                             |        |                    |
                             |        |                    |
                             |        |                    |
                             |        |                    |
Full-sized twin tunnel       |C and D |Intermediate shaft. |
                             |        |  (West of shaft.)  |
                             |        |                    |
Exploration drift            |C and D |Intermediate shaft. |
                             |        |  (West of shaft.)  |
                             |        |                    |
Twin tunnel. Enlargement    }|C and D |{  Eastward from    |
  of exploration drift      }|        |{     open cut.     |
                             |        |                    |
Twin tunnel. Enlargement    }|C and D |{  Eastward from    |
  of exploration drift      }|        |{     open cut.     |
                             |        |                    |
=============================+========+=====================



==================================+========+============+=============
           4                      |    5   |      6     |      7     |
----------------------------------+--------+------------+------------+
                                  |        |   Length   |   Average  |
            DATES.                |  Time  |   tunnel   |   advance  |
----------------------------------|elapsed,| excavated, |   per day, |
               |                  |   in   |     in     |     in     |
     From      |        To        |  days. |linear feet.|linear feet.|
----------------------------------+--------+------------+------------+
 Jan. 25, 1906.|Apr. 30, 1906.    |   95   |     367    |     3.9    |
               |                  |        |            |            |
 Jan. 27, 1906.|Apr. 30, 1906.    |   93   |     354    |     3.8    |
               |                  |        |            |            |
{May. 22, 1906.|July 24, 1906.[E]}|  173   |     810    |     4.7    |
{Aug. 11, 1906.|Nov. 29, 1906.   }|        |            |            |
               |                  |        |            |            |
               |                  |        |            |            |
 Mar. 19, 1906.|May  28, 1906.    |   70   |      58    |     0.8    |
               |                  |        |            |            |
               |                  |        |            |            |
               |                  |        |            |            |
{May  29, 1906.|July  3, 1906.[E]}|  208   |   1,206    |     5.8    |
{July 18, 1906.|July 31, 1906.   }|        |            |            |
{Aug. 12, 1906.|Nov. 23, 1906.   }|        |            |            |
{Jan. 15, 1907.|Feb.  5, 1907.   }|        |            |            |
{Feb. 17, 1907.|Mar. 21, 1907.   }|        |            |            |
               |                  |        |            |            |
 Dec.  1, 1905.|May. 10, 1906.    |  161   |     225    |     1.4    |
               |                  |        |            |            |
               |                  |        |            |            |
 Feb.  1, 1907.|Sept.13, 1907.    |  225   |   1,033    |     4.6    |
               |                  |        |            |            |
               |                  |        |            |            |
}Feb.  1, 1908.|Feb. 14, 1908.    |   14   |      65    |     4.6    |
}              |                  |        |            |            |
               |                  |        |            |            |
}Feb. 15, 1908.|Apr. 14, 1908.    |   59   |     524    |     8.9    |
}              |                  |        |            |            |
               |                  |        |            |            |
======================================================================

[Footnote E: Time and distance omitted while working
through timbered stretches.]

=======================================================
                        8
-------------------------------------------------------
            Methods and conditions.
-------------------------------------------------------
Top heading and bench. Muck loaded by hand.

 "     "     "    "      "    "    "    "

Double heading and bench. Muck loaded by steam
  shovel. Stretches aggregating 200 ft. narrowed
  to about 25 ft. and later enlarged are included.

Excavation about 30 ft. wide. Top full-width heading
  and bench. Muck loaded by hand. Steam shovel
  not installed.

Excavation about 30 to 35 ft. wide. Top full-width
  by hand and part by steam shovel.




Double heading and bench. Part of the muck handled
  by hand and part by steam shovel.

Exploration drift about 10 ft. by 13 in. Muck loaded by
  hand. 14 ft. timbered.

At portal of twin tunnels. Drift excavated to full
  width and bench. Muck loaded by hand. 12 ft.
  timbered.

Drift excavated to full width and bench. Muck loaded
  by steam shovel. Full-width tunnel timbered for 26
  ft. independently of the main excavation.
=======================================================


THREE-TRACK TUNNEL EXCAVATION.

When it became evident that the work through the Fifth Avenue section
would be extremely slow, shafts were sunk in each street between Sixth
and Seventh Avenues. The shafts, as shown on Plate XIV, were located in
the streets, but in such a way as to block only half of the roadway. At
the same time it was decided to construct in open cut about 200 ft. of
the Three-Track Tunnel at the west end of the contract in 32d Street,
where the rock surface was below the top of the tunnel. It was hoped
that the remainder of the work could be built without opening the
street, but further investigation showed that this was impracticable,
and eventually all the Three-Track Tunnel in 32d Street, except 120 ft.
east of the shaft, was built in open cut.

_Thirty-second Street Work in Tunnel._--Following the sinking of the
shaft, a drift was driven across the street at the crown of the tunnel,
and a top heading on the south side was excavated in both directions.
Frequent cross-drifts to the north side showed that the rock was nowhere
very sound and that, except for a short distance east of the shaft, it
was distinctly unfavorable for the wide Three-Track excavation. In this
stretch the north ends of these cross-cuts were connected by a second
heading, and wall-plates and sets of three-segment arch timbering were
set up to support the roof of the drifts. The cross-cuttings were
gradually widened and timbered until the entire excavation had been made
down to the level of the wall-plates, as shown in Fig. 3, Plate LX. The
bench was then excavated in two lifts, leaving the wall-plates supported
on narrow longitudinal berms, which were removed in short sections to
permit the placing of posts under the wall-plates.

_Thirty-second Street Open-Cut Work._--Before actual open-cut excavation
was started, all buildings facing it were underpinned to rock. For this
purpose, a trench was dug along the face of the buildings and of the
same depth as their cellars. Holes were cut in the front foundation
walls through which long needle-beams (Fig. 4, Plate LX) were inserted
and jacked up on blocking placed on the cellar floor and in the trench,
until the weight of the building had been taken off its foundations. A
close-sheeted trench was then sunk to rock under the front building
walls, and a light rubble masonry retaining wall was built in it to
support the building permanently. Frequently, the excavation for the
underpinning wall, which was taken out in sections from 30 to 40 ft.
long, and in places was carried to a depth of 40 ft., was very
troublesome on account of the large quantity of water encountered and
the fineness of the sand, which exhibited a tendency to flow when
saturated.

The Elevated Railroad columns in Sixth Avenue, near the north and south
lines of 32d Street, were underpinned in a manner similar to the
building foundations, while those on the center line of the street were
supported by girders riveted to them close under the track level. The
girders in turn were supported on posts footed on the new underpinning
of the adjacent columns. On the completion of the tunnels, concrete
piers were built up from the roof of the tunnel to form a permanent
foundation for the center-line columns. The area to be excavated under
Sixth Avenue was enclosed by a rubble masonry retaining wall constructed
in a trench.

Open-cut excavation was started by planking over the street on stringers
resting on transverse 12 by 12-in. caps. The caps were gradually
undermined and supported on temporary posts which were then replaced by
short posts resting on 12 by 12-in. sills about 7 ft. below the cap. The
operation was then repeated and the sill was supported on another set of
short posts resting on a second sill. When the excavation had been
carried down in this manner to the level of the top of the tunnel,
diagonal 3 by 10-in. timbers were cut in between the posts and sills to
form a species of double A-frame, the legs of which rested in niches cut
in the rock and on posts carried up the face of the underpinning wall,
and the whole was stiffened with vertical tie-rods. This construction is
shown by Fig. 3, Plate LXII. The brick sewer was replaced temporarily by
one of riveted steel pipe. This pipe and the water and gas pipes and
electric conduits were suspended from the timbers as the pipes were
uncovered.

Excavation in rock was made by sinking a pit to sub-grade for the full
width of the tunnel and advancing the face of the pit in several lifts,
the muck being blown over the slope and loaded into buckets at its foot.

The work was attacked at several places simultaneously, and the spoil
was hoisted by derricks located at convenient points along the side of
the cut.

_Thirty-third Street Work in Tunnel and Open Cut._--The West 33d Street
Shaft was similar to the one in 32d Street, and was sunk during
February, March, and April, 1907, through 10 ft. of earth, 21 ft. of
soft rock, and 29 ft. of fairly hard rock. It was necessary to timber
heavily the upper 30 ft. of the shaft. The timber later showed evidences
of severe strain, and had to be reinforced.

[Illustration: Plate LXI.--EXCAVATION AND TIMBERING IN HEAVY GROUND
OF THREE-TRACK TUNNEL OF 33D ST.]

As soon as the shaft excavation was deep enough, a drift was driven
part way across the tunnels, and top headings were started both east and
west to explore the rock. The heading to the west was divided into two
drifts, as shown on Plate LXI. These two drifts were continued to the
west end of the contract, and were then enlarged to a full-sized heading
and timbered, as shown on Plate LXI and Fig. 3, Plate LX. The rock near
the shaft contained many wet rusty seams, and settlement was detected in
the segmental tunnel timbering soon after the widening of the heading
was completed. Short props were placed under the timbers, and the street
surface was opened with a view of stripping the earth down to the rock
and thus lightening the load on the timbering. Street traffic was
maintained on a timber structure with posts eventually carried down to
the rock surface, and the walls of the buildings on the north side of
the street were underpinned to rock. The settlement of the tunnel
timbering was checked for a time, and the bench was excavated as shown
on Plate LXI. In this work the cut in the center was first made, and the
short props were replaced by struts, as shown; after this the berms were
removed and the side posts were placed. While building the brick arches,
holes were left in the masonry around the struts. After the masonry had
hardened, piers were built on the arches to support the segmental
timbers. The struts were then removed and the openings filled with
masonry. The voids above the arch were packed with rock and afterward
thoroughly grouted.

The timbers near the shaft continued to settle, and, although they had
been placed from 9 to 12 in. above the level of the top of the masonry,
by October 1st, they encroached 9 in. within the line of masonry. It was
then decided to remove the rock for a distance of 48 ft. west of the
shaft, and build this portion of the tunnel in open cut. The posts
supporting the deck forming the street surface were replaced by an
A-frame structure similar to that developed for the 32d Street open cut,
without interruption of the street traffic.

After making the open cut to the westward of the shaft, there was a slip
in the rock north of and adjoining the shaft. Fortunately, the timbers
did not give way entirely, and no damage was done. The open cut was
extended eastward for a distance of 46 ft., making the total length of
tunnel built in open cut on this street 94 ft.

East of the shaft, for a distance of about 125 ft., the rock was broken
and could not be excavated to full size without timbering the roof, but
between this section of poor rock and those already mentioned in
connection with the work at Fifth Avenue, there was a stretch of 600 ft.
of good rock where all the spoil was handled with a steam shovel.


TWIN-TUNNEL LINING.

The masonry lining for the tunnels was not started until the late fall
of 1906, after excavation had been in progress for a year and a half. At
that time concreting was started in the single tunnels westward from the
First Avenue Shafts, and by spring was in full swing in the Twin
Tunnels.

The plans contemplated the use of a complete concrete lining except
where large quantities of water were encountered; in which case the
arches, beginning at a point 15° above the springing line, were to be
built of vitrified paving brick. By reference to Plate XII it will be
seen that the water-proofing, which in the concrete-roof tunnels
extended the full height of the sides to the 15° line, was carried in
the brick-roof tunnels completely around the extrados of the arch. The
cross-sections also show the location of the electric conduits which
were buried in the mass of the side and core-walls and which limited the
height to which the concrete could be carried in one operation.

The same general scheme of operations was used wherever possible
throughout the Twin-Tunnel work, but was subject to minor modifications
as circumstances dictated. Concrete was first deposited in the bottom,
to the grade of the flow line of the drains; after it had set,
collapsible box forms, of 2-in. plank with 3-in. plank tops, were laid
on it to form the ditch and the shoulders for the flagstone covers. The
track, which had previously been blocked up on the rock between the
ditches, was raised and supported on the ditch boxes above the finished
floor level. At the same time, light forms were braced from the ditch
boxes to the grade of the base of the low-tension and telephone-duct
bank. After depositing the concrete to this level, the telephone ducts
were laid.

The forms for the water-proofing or sand-wall up to the 15° line and for
the main side-walls and core-walls were built in 30-ft. panels and were
supported on carriages, which, traveling on a broad-gauge track above
the ditches, moved along the tunnel, section by section, as the work
advanced. The panels were hung loosely from joists carrying a platform
on the top chord of the carriage trusses, and were adjusted
transversely by bracing and wedging them out from the carriage. The
small forms for the refuge niches, ladders, etc., were collapsible, and
were spiked to the main panel forms just previous to the deposition of
the concrete. The concrete was deposited from the platform on top of the
carriage, to which the cars were elevated in various ways. Plate LXI
shows the details of the carriages, and is self-explanatory.

The concrete for the sand-walls and the core-wall, to the level of the
sidewalk, was deposited at the same time; two carriages in each tunnel,
placed opposite each other, forming a 60-ft. length, were used at each
setting. The floor section of the 4-in. tile drains had been laid with
the floor concrete, and, as the sand-wall concrete was deposited, the
drains were brought up simultaneously, broken stone being deposited
between the tile and the rock to form a blind drain and afford access to
the open joints of the tile for the water entering the tunnel through
seams in the rock. The drains were spaced at intervals not exceeding 25
ft., depending on the wetness of the rock, and were placed at similar
intervals in the core-wall under the lowest projecting points of the
rock on the center line between the tunnels. A small ditch lined with
loose 6-in. vitrified half pipe was provided in the top of the sand-wall
to collect the water from the extrados of the arch and lead it to the
top of the drains. Great difficulty was experienced in maintaining these
drains clear, and, on completion of the work, a large amount of labor
was expended in removing obstructions from the floor sections, the only
portion then accessible.

After water-proofing the sand-walls and laying the low-tension ducts, a
second pair of carriages, with panels on one side only, for 60 ft. of
side-wall and skewback to the 15° line, were set and braced against the
core-wall. These forms are shown in connection with the carriage on
Plate LXI. They were concreted to the base of the high-tension duct
bank, and, after the concrete had hardened and the bank of ducts had
been laid, the concreting was completed in a second operation.

In places where the roof was supported temporarily by posts and heavy
timbering, such as at Fifth Avenue, the form carriages could not be
used, and special methods were devised to suit the local conditions.
Usually, the panels were stripped from the carriages and moved from
section to section by hand, and, when in position, were braced to the
timbering.

The arch centers were built up of two 5 by 3 by 3/8-in. steel angles,
and, when set, were blocked up on the sidewalks opposite each other in
the two tunnels. A temporary platform was laid on the bottom chord
angles of the ribs, on which the concrete was dumped, the same as on the
form carriages. The lagging used was 3 by 3-in. dressed pine or spruce
16 ft. long, and was placed as the concreting of the arch proceeded
above the 15° line on the side-wall and above the sidewalk on the
core-wall. After the arch had reached such a height that the concrete
could not be passed over the lagging directly from the main platform, it
was cast on a small platform on the upper horizontal bracing of the
centers, shown in Fig. 3, Plate LIX, and was thence shoveled into the
work. In the upper part of the arch the face of the concrete was kept on
a radial plane, and, when only 3 ft. remained to be placed, it was keyed
in from one end, the key lagging being set in about 5-ft. lengths. The
arches were concreted usually in 60-ft. lengths.

Where brick arches were used, the core-wall skewback was concreted
behind special forms set up on the sidewalks, or the arch ribs and
lagging were used for forms, and the brick arch was not started until
after the concrete had set. In laying the brick in the arch, the five
courses of the ring were carried up as high as the void between the
extrados and the rock would permit and still leave a working space in
which to place the water-proofing. This was usually not more than 3 ft.,
except on the core-wall side. The felt and pitch water-proofing was then
laid for that height, joined to the previous water-proofing on the
side-walls, and was followed by the brick armor course over the
water-proofing and by the rock packing, after which another lift of
brick was laid and the operations were repeated. The large void (Fig. 1,
Plate LXII) above the core-wall gave convenient access for working on
top of the adjacent sides of the roof, and the keying of the arches and
the water-proofing and rock packing above the core-wall were usually
carried on from that point, the work progressing from one end.

The concrete for all work above the floor was dumped on the platform of
the carriages, to which it was transported in the early part of the work
in cars running on a high-level track laid on long ties, resting on the
finished sidewalks. This arrangement, although requiring a large amount
of timber for the track, permitted the muck to be carried out on the
low-level track without interference. Later, when the advance of the
heading had ceased and the heavy mucking was over, all concrete was
transported on the floor level, and the cars were lifted to the carriage
platforms by elevators and were hauled by hoisting engines up a movable
incline. The latter method is shown by Fig. 3, Plate LIX.

_Water-Proofing._--The water-proofing referred to above was in all cases
felt and pitch laid with six thicknesses of felt and seven of pitch. The
sub-contractor for the work was the Sicilian Asphalt Paving Company. All
joints were lapped at least 1 ft., and, where work was suspended for a
time and a bevel lap could not be made, the edges of the felt were left
unpitched for 1 ft. and the newer work was interlaced with the old. This
method was not always successful, however, on account of the softening
of the unpitched felt on long-continued exposure to the water. The felt
used was mainly "Tunaloid," together with some "Hydrex." It weighed
about 12 lb. per 100 sq. ft. when saturated and coated on one side only,
and contained about 25% of wool. The coal-tar pitch used had a melting
point of 100° Fahr.

After the completion of the tunnel, the concrete arch showed some
leakage and in places unsightly lime deposits. It was determined to
attempt to stop these leaks by the application of a water-proof cement
coating on the intrados of the arch. Extended experimental application
of two varieties of materials used for this purpose--"Hydrolithic"
cement and the U. S. Water-proofing Company's compound--have been made
with apparent success up to the present time, and the results after the
lapse of a considerable period are awaited with interest.

_Duct Laying._--The position of the electric conduits, buried in the
heart of the concrete walls, interfered greatly with the economical and
speedy placing of the lining, and their laying proved to be one of the
most troublesome features of the work. The power conduits were
single-way, with the bank for high-tension cables separated in the
side-walls from the low-tension bank, as shown on Plate XII. The
conduits for telephone and telegraph service were four-way, and were
located in the core-wall. All ducts had 3/4-in. walls and a minimum
clear opening of 3-3/8 in. square, with corners rounded. They were laid
with joints broken in all directions, and in about 1/4-in. beds of
1:2-1/2 mortar. Flat steel bond-irons, 2 by 1/8 in., with split and bent
ends, were placed in the joints at intervals of 3 ft. and projected into
the concrete 3 in. on each side, tying together the concrete on opposite
sides of the ducts. The joints were wrapped with a 6-in. strip of
10-oz. duck saturated with neat-cement grout, and, in addition, the
power conduits were completely covered with a 1/2-in. coat of mortar to
prevent the intrusion of cement and sand from the fluid concrete. The
four-way conduits were plastered only over the wraps. Splicing chambers
were provided at intervals of 400 ft.

[Illustration: PLATE LXII, FIG. 1.--WATER-PROOFING OVER BRICK
ARCHES.]

[Illustration: PLATE LXII, FIG. 2.--TRESTLE USED IN CONCRETING IN
THREE-TRACK TUNNEL.]

[Illustration: PLATE LXII, FIG. 3.--METHOD OF STREET SUPPORT OVER
OPEN-CUT EXCAVATION.]

[Illustration: PLATE LXII, FIG. 4.--JUNCTION OF TWIN AND THREE-TRACK
TUNNELS.]


THREE-TRACK TUNNEL LINING.

In the Three-Track Tunnels, a heavy brick arch was used for those
portions constructed in tunnel, while, in the open-cut sections, the
roof was of concrete. Both were completely water-proofed on the roof and
sides, and in the tunnel sections the space above the brick roof was
filled with rock packing. On account of the unstable nature of the rock
encountered throughout, the voids in the packing were afterward filled
with grout.

By reference to the cross-sections, Plate XII, it will be seen that the
haunches of the arch were tied together by steel I-beams anchored in the
concrete, with the object of making the structure self-supporting in the
event of the removal of the adjacent rock for deep cellar excavations.
This construction materially influenced the contractor's method of
placing the masonry lining.

After depositing the floor concrete, by the same method that was used in
the Twin Tunnels, a timber trestle (Fig. 2, Plate LXII) was erected to
the height of the underside of the I-beam ties, the posts being footed
in holes, about 3 in. deep, left in the concrete floor to prevent
slipping. In the open-cut sections the sand-wall forms were of undressed
plank tacked to the studding and braced from the trestle; in the tunnel
section they were spiked to the face of the posts supporting the
timbering.

The side-wall forms were made up in panels about 3 by 10 ft., and were
clamped to studs by U-shaped irons passing around the stud and bolted to
the cleats on the back of the panels, the studs being braced from the
trestle. The side-wall concrete was deposited in three sections. The
first was brought up just above the sidewalk and formed the bench for
the high-tension ducts; the second carried the wall up to the springing
line. Before placing the third section the I-beam ties were set in
position (Fig. 3, Plate LXII) on top of the trestle, and the reinforcing
rods in the haunch of the arch were hung from them. The concrete was
carried up to a skewback for the arch, as shown in the brick-roof
cross-section (Plate XII) and embedded the ends of the ties.

The centers for the arches stood on the I-beam ties, and the tops of the
hangers, for the permanent support of the ties near their center, were
inserted through the lagging. The brick arch, water-proofing, and rock
packing were laid up in lifts, in the same manner as in the Twin Tunnel,
with grout pipes built in at intervals of about 8 ft. The concrete arch
was placed in sections, from 25 to 50 ft. in length, with a rather wet
mixture and a back form on the steep slope of the extrados.

The concrete for the sand-walls and lower part of side-walls was handled
on tracks and platforms laid on cantilever beams at mid-height of the
trestle, as shown by Fig. 3, Plate LXII. For the walls above the
springing line, the tracks were laid on top of the I-beam ties, and some
of the arch concrete, also, was delivered from the mixer at that level
and hauled up an incline to the level of the top of the arch. By far the
greater part, however, was turned out from mixers set on the completed
arch, and was transported on tracks hung in part from the street
timbering.

_Completion._--Except in the heavily-timbered portions, such as at Fifth
Avenue, where the load had to be transferred from posts to the completed
masonry section by section, the lining of the tunnels presented no
special difficulty. The large number of small forms to be set, and the
mutual interference of the concreting and duct-laying operations proved
to be the most troublesome features of the work.

The restoration of the streets, public utilities, etc., at the open-cut
sections was a slow and tedious operation, but the tunnels themselves
were completed in March, 1909, 3 years and 10 months after the inception
of the work. The finished tunnels are shown by the photograph, Fig. 4,
Plate LXII, taken at the junction of the twin and three-track types.

FOOTNOTES:

[Footnote A: Presented at the meeting of December 1st, 1909.]

[Footnote B: Of the paper by Mr. Noble.]

[Footnote C: Of the paper by Mr. Noble.]

[Footnote D: Of the paper by Mr. Noble.]