The river shafts were designed to serve both as working shafts and as
permanent openings to the tunnels, and were larger and more substantial
than would have been required for construction purposes. Plate X of Mr.
Noble's paper shows their design. They consist of two steel caissons,
each 40 by 74 ft. in plan, with walls 5 ft. thick filled with concrete.
A wall 6 ft. thick separated each shaft into two wells 29 by 30 ft.,
each directly over a tunnel. Circular openings for the tunnel, 25 ft. in
diameter, were provided in the sides of the caissons. During the sinking
these were closed by bulkheads of steel plates backed by horizontal
steel girders. The shafts were sunk as pneumatic caissons to a depth of
78 ft. below mean high water. There have been a few caissons which were
larger and were sunk deeper than these, but most large caissons have
been for foundations, such as bridge piers, and have been stopped at or
a little below the surface of the rock. The unusual feature of the
caissons for the Long Island shaft is that they were sunk 54 ft. through
rock.
It had been hoped that the rock would prove sound enough to permit
stopping the caissons at or a little below the surface and continuing
the excavation without sinking them further; for this reason only the
steel for the lower 40 ft. of the caissons was ordered at first.
The roof of the working chamber was placed 7 ft. above the cutting edge.
It was a steel floor, designed by the contractors, and consisted of
five steel girders, 6 ft. deep, 29 ft. long, and spaced at 5-ft.
centers. Between were plates curved upward to a radius of 4 ft. Each
working chamber had two shafts, 3 ft. by 5 ft. in cross-section, with a
diaphragm dividing it into two passages, the smaller for men and the
larger for muck buckets. On top of these shafts were Moran locks.
Mounted on top of the caisson was a 5-ton Wilson crane, which would
reach each shaft and also the muck cars standing on tracks on the ground
level beside the caissons. Circular steel buckets, 2 ft. 6 in. in
diameter and 3 ft. high, were used for handling all muck. These were
taken from the bottom of the working chamber, dumped in cars, and
returned to the bottom without unhooking. Work was carried on by three
8-hour shifts per day. The earth excavation was done at the rate of
about 67 cu. yd. per day from one caisson. The rock excavation,
amounting to about 6,200 cu. yd. in each caisson, was done at the rate
of about 44.5 cu. yd. per day. The average rate of lowering, when the
cutting edge of the south caisson was passing through earth, was 0.7 ft.
per day. In rock, the rate was 0.48 ft. per day in the south caisson,
and 0.39 ft. per day in the north caisson.
At the beginning all lowering was done with sixteen hydraulic jacks.
Temporary brackets were fastened to the outside of the caisson. A
100-ton hydraulic jack was placed under each alternate bracket and under
each of the others there was blocking. The jacks were connected to a
high-pressure pump in the power-house. As the jacks lifted the caisson,
the blocking was set for a lower position, to which the caisson settled
as the jacks were exhausted. After the caisson had penetrated the earth
about 10 ft., the outside brackets were removed and the lowering was
regulated by blocking placed under brackets in the working chamber. The
caisson usually rested on three sets of blockings on each side and two
on each end. The blocking was about 4 ft. inside the cutting edge. In
the rock, as the cutting edge was cleared for a lowering of about 2 ft.,
6 by 8-in. oak posts were placed under the cutting-edge angle. When a
sufficient number of posts had been placed, the blocking on which the
caisson had rested was knocked or blasted out, and the rock underneath
was excavated. The blocking was then re-set at a lower elevation. The
posts under the cutting edge were then chopped part way through and the
air pressure was lowered about 10 lb., which increased the net weight to
more than 4,000,000 lb. The posts then gradually crushed and the
caissons settled to the new blocking. The tilt or level of the caisson
was controlled by chopping the posts more on the side which was desired
to move first.
The caisson nearly always carried a very large net weight, usually about
870 tons. The concrete in the walls, which was added as the caisson was
being sunk, was kept at about the elevation of the ground. There was
generally a depth of from 5 to 20 ft. of water ballast on top of the
roof of the working chamber. The air pressure in the working chamber was
usually much less than the hydrostatic head outside the caisson. For
example, the average air pressure in the south caisson during January,
1906, was 16-1/2 lb., while the average head was 62.5 ft., equivalent to
27 lb. per sq. in. Under these conditions, there was a continued but
small leakage into the caisson of from 15,000 to 20,000 gal. per day.
In the rock the excavation was always carried from 2 to 5 in. outside
the cutting edge. As soon as the cutting edge was cleared, bags of clay
were placed under it in a well-tiered, solid pile, so that when the
caisson was lowered the bags were cut through and most of the clay, bags
and all, was squeezed back of the cutting edge between the rock and the
caisson.
Table 1 shows the relation of the final position of the caissons to that
designed.
The cost of rock excavation in the caisson was $4.48 per cu. yd. for
labor and $10.54 for top charges.
The bottom of the shaft is an inverted concrete arch, 4 ft. thick,
water-proofed with 6-ply felt and pitch. As soon as the caisson was down
to its final position and the excavation was completed, concrete was
deposited on the uneven rock surfaces, brought up to the line of the
water-proofing, and given a smooth 1-in. mortar coat. The felt was stuck
together in 3-ply mats on the surface with hot coal-tar pitch. These
were rolled and sent down into the working chamber, where they were put
down with cold pitch liquid at 60 deg. Fahr. Each sheet of felt overlapped
the one below 6 in. The water-proofing was covered by a 1-in. mortar
plaster coat, after which the concrete of the 4-ft. inverted arch was
placed. While the water-proofing and concreting were being done, the air
pressure was kept at from 30 to 33 lb. per sq. in., the full hydrostatic
head at the cutting edge. After standing for ten days, the air pressure
was taken off, and the removal of the roof of the working chamber was
begun. The water-proofing was done by the Union Construction and
Waterproofing Company.
TABLE 1.--RELATION OF THE FINAL POSITION OF THE CAISSONS TO THAT
DESIGNED.
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LOCATION.| LONG ISLAND CITY. |
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Shaft. | North. | South. |
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Corner. | High. | East. | North. | High. | East. | North. |
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Northeast|0.21 ft.|0.08 ft.|0.05 ft.|0.32 ft.|0.15 ft.|0.28 ft.|
Northwest|0.22 " |0.08 " |0.02 " |0.00 " |0.15 " |0.12 " |
Southwest|0.27 " |0.14 " |0.02 " |0.18 " |0.45 " |0.12 " |
Southeast|0.23 " |0.14 " |0.05 " |0.39 " |0.45 " |0.28 " |
================================================================
=============================================================================
LOCATION.| MANHATTAN. |
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Shaft. | North. | South. |
-----------------------------------------------------------------------------
Corner. | High. | East. | South. | High. | East or West.|North or South.|
-----------------------------------------------------------------------------
Northeast|0.23 ft.|0.74 ft.|0.38 ft.|0.00 ft.|0.06 ft. east.|0.04 ft. south.|
Northwest|0.00 " |0.74 " |0.22 " |0.08 " |0.06 " " |0.13 " north.|
Southwest|0.11 " |0.31 " |0.22 " |0.21 " |0.45 " west.|0.13 " " |
Southeast|0.46 " |0.31 " |0.38 " |0.04 " |0.45 " " |0.04 " south.|
=============================================================================
The cost of labor in compressed air chargeable to concreting was $3.40
per cu. yd.
After the roof of each working chamber had been removed, the shield was
erected on a timber cradle in the bottom of the shaft, in position to be
shoved out of the opening in the west side of the caisson. Temporary
rings of iron lining were erected across the shaft in order to furnish
something for the shield jacks to shove against.
The roof of the working chamber was then re-erected about 35 ft. above
its original position and about 8 ft. above the tunnel openings. This
time, instead of the two small shafts which were in use during the
sinking of the caisson, a large steel shaft with a T-head lock was
built. This is illustrated in Fig. 2, Plate LXIV. The shaft was 8 ft. in
diameter. Inside there was a ladder and an elevator cage for lowering
and hoisting men and the standard 1-yd. tunnel cars. At the top, forming
the head of the T, there were two standard tunnel locks.