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.