A permanent shaft, similar to the river shafts in Long Island City, was
constructed at Manhattan over each pair of tunnels. Each shaft was
located across two lines, with its longer axis transverse to the
tunnels. Plate XIII shows their relative positions. They were divided
equally by a reinforced concrete partition wall transverse to the line
of the tunnels. On completion, the western portions were turned over to
the contractor for the cross-town tunnels for his exclusive use.
_South Shaft._--Work on the south shaft was started on June 9th, 1904,
with the sinking of a 16 by 16-ft. test pit in the center of the south
half of the south shaft, which reached disintegrated rock at a depth of
about 20 ft.
Starting in August, the full shaft area, 74 by 40 ft., was taken out in
an open untimbered cut to the rock, and a 20 by 50-ft. shaft was sunk
through the rock to tunnel grade, leaving a 10 or 12-ft. berm around it.
(Fig. 1, Plate LXX.)
The erection of the caisson was started, about the middle of January, on
the rock berm surrounding the 20 by 50-ft. shaft and about 15 ft. below
the surface. Fig. 3, Plate LXIV, shows the cutting edge of the caisson
assembled. The excavation of the small shaft had shown that hard rock
and only a very small quantity of water would be encountered, and that
the caisson need be sunk only a short distance below the rock surface.
Therefore, no working-chamber roof was provided, the caisson was built
to a height of only 40 ft., and the circular openings were permanently
closed.
The assembling of the caisson took 2-1/2 months, and on April 2d
lowering was started. Inverted brackets were bolted temporarily to the
cutting-edge stiffening brackets, and the sinking was carried on by
methods similar to those used at Long Island. The jacks and blocking
supporting the caisson are shown in Fig. 4, Plate LXIV. As soon as the
cutting edge entered the rock, which was drilled about 6 in. outside of
the neat lines, the space surrounding the caisson was back-filled with
clay and muck to steady it and provide skin friction. As the friction
increased, the walls were filled with concrete, and as the caisson
slowly settled, it was checked and guided by blocking. The cutting edge
finally came to rest 31 ft. below mean high water, the sinking having
been accomplished in about seven weeks, at an average rate of 0.50 ft.
per day.
The final position of the cutting edge in relation to its designed
position is shown in Table 1.
A berm about 4 ft. wide was left at the foot of the caisson below which
the rock was somewhat fissured and required timbering. The cutting edge
of the caisson was sealed to the rock with grout on the outside and a
concrete base to the caisson walls on the inside, the latter resting on
the 4-ft. berm. Following the completion of the shaft, the permanent
sump was excavated to grade for use during construction.
_North Shaft._--The north shaft had to be sunk in a very restricted
area. The east side of the caisson cleared an adjoining building at one
point by only 1 ft., while the northwest corner was within the same
distance of the east line of First Avenue. As in the case of the Long
Island shafts, the steelwork for only the lower 40 ft. was ordered at
the start. This height was completely assembled before sinking was
begun. The caisson was lowered in about the same manner as those
previously described. The bearing brackets for the hydraulic jacks were
attached, as at the south shaft, to the inside of the cutting-edge
brackets. The east side of the caisson was in contact with the
foundations of the neighboring building, while the west side was in much
softer material. As a consequence, the west side tended to settle more
rapidly and thus throw the caisson out of level and position. To
counteract that tendency, it was necessary to load the east wall heavily
with cast-iron tunnel sections, in addition to the concrete filling in
the walls.
Soon after sinking was begun, a small test shaft was sunk to a point
below the elevation of the top of the tunnels. The rock was found to be
sound, hard, and nearly dry. It was then decided to stop the caisson as
soon as a foundation could be secured on sound rock. The latter was
found at a depth of 38 ft. below mean high water. With the cutting edge
seated at that depth, the top of the caisson was only 2 ft. above mean
high water, and as this was insufficient protection against high tides,
a 10-ft. extension was ordered for the top. Work, however, went on
without delay on the remainder of the excavation. The junction between
the cutting edge and the rock was sealed with concrete and grout. The
caisson was lowered at an average rate of 0.53 ft. per day. The size of
the shaft below the cutting edge was 62 ft. 7 in. by 32 ft. The average
rate of excavation during the sinking in soft material was 84 cu. yd.
per day. The average rate of rock excavation below the final position of
the cutting edge was 125 cu. yd. per day. There were night and day
shifts, each working 10 hours. Excavation in earth cost $3.96 per cu.
yd., of which $1.45 was for labor and $2.51 for top charges, etc. The
excavation of rock cost $8.93 per cu. yd., $2.83 being for labor and
$6.10 for top charges.
The final elevations of the four corners of the cutting edge, together
with their displacement from the desired positions, are shown in Table
1.