_Materials and Inception of Work._--The materials encountered are shown
in the profile on Plate XIII, and were similar in all the tunnels. In
general, they were found to be about as indicated in the preliminary
borings. The materials met in Tunnel _A_ may be taken as typical of all.
From the Manhattan shaft eastward, in succession, there were 123 ft. of
all-rock section, 87 ft. of part earth and part rock, 723 ft. of all
earth, 515 ft. of part rock and part earth, 291 ft. of all rock, and 56
ft. of part rock and part earth.
The rock on the Manhattan side was Hudson schist, while that in the reef
was Fordham gneiss. Here, as elsewhere, they resembled each other
closely; the gneiss was slightly the harder, but both were badly seamed
and fissured. Wherever it was encountered in this work, the rock surface
was covered by a deposit of boulders, gravel, and sand, varying in
thickness from 4 to 10 ft. and averaging about 6 ft.
The slope of the surface of the ledge on the Manhattan side averaged
about 1 vertical to 4 horizontal. The rock near the surface was full of
disintegrated seams, and was badly broken up. It was irregularly
stratified, and dipped toward the west at an angle of about 60 degrees.
Large pieces frequently broke from the face and slid into the shield,
often exposing the sand. The rock surface was very irregular, and was
covered with boulders and detached masses of rock embedded in coarse
sand and gravel. The sand and gravel allowed the air to escape freely.
By the time the shields had entirely cleared the rock, the material in
the face had changed to a fine sand, stratified every few inches by very
thin layers of chocolate-colored clayey material. This is the material
elsewhere referred to as quicksand. As the shield advanced eastward, the
number and thickness of the layers of clay increased until the clay
formed at least 20% of the entire mass, and many of the layers were 2
in. thick.
At a distance of about 440 ft. beyond the Manhattan ledge, the material
at the bottom of the face changed suddenly to one in which the layers
of clay composed probably 98% of the whole. The sand layers were not
more than 1/16 in. thick and averaged about 2 in. apart. The surface of
the clay rose gradually for a distance of 40 ft. in Tunnels _A_ and _B_,
and 100 ft. in Tunnels _C_ and _D_, when gravel and boulders appeared at
the bottom of the shield. At that time the clay composed about one-half
of the face.
The surfaces of both the clay and gravel were irregular, but they rose
gradually. After rock was encountered, the formations of gravel and clay
were roughly parallel to the rock surface.
As the surface of the rock rose they disappeared in order and were again
encountered when the shields broke out of rock on the east side of the
Blackwell's Island Reef. East of the reef a large quantity of coarse
open sand was present in the gravel formations before the clay appeared
below the top of the cutting edge. In Tunnels _C_ and _D_ this was
especially difficult to handle. It appears to be a reasonable assumption
that the layer of clay was continuous across the reef. Wherever the clay
extended above the top of the shield it reduced the escape of air
materially. It is doubtless largely due to this circumstance that the
part-rock sections in the reef were not the most difficult portions of
the work.
While sinking the lower portions of the shafts the tunnels were
excavated eastward in the solid rock for a distance of about 60 ft.,
where the rock at the top was found to be somewhat disintegrated. This
was as far as it was considered prudent to go with the full-sized
section without air pressure. At about the same time top headings were
excavated westward from the shafts for a distance of 100 ft., and the
headings were enlarged to full size for 50 ft. The object was to avoid
damage to the shaft and interference with the river tunnel when work was
started by the contractor for the cross-town tunnel.
[Illustration: PLATE LXVI, FIG. 1.--REAR OF SHIELD SHOWING COMPLETE
FITTINGS.]
[Illustration: PLATE LXVI, FIG. 2.--SHIELD WITH LOWER PORTION OF
BULKHEAD REMOVED.]
The shields were erected on timber cradles in the shaft, and were shoved
forward to the face of the excavation. Concrete bulkheads, with the
necessary air-locks, were then built across the tunnels behind the
shields. The shields were erected before the dividing walls between the
two contracts were placed. Rings of iron tunnel lining, backed by
timbers spanning the openings on the west side, were erected temporarily
across the shafts in order to afford a bearing for the shield jacks
while shoving into the portals. The movement of the shield eastward was
continued in each tunnel for a distance of about 60 ft., and the
permanent cast-iron tunnel lining was erected as the shield advanced.
Before breaking out of rock, it was necessary to have air pressure in
the tunnels. This required the building of bulkheads with air-locks
inside the cast-iron linings just east of the portals. Before erecting
the bulkheads it was necessary to close the annular space between the
iron tunnel lining and the rock. The space at the portal was filled with
a concrete wall. After about twenty permanent rings had been erected in
each tunnel, two rings were pulled apart at the tail of the shield and a
second masonry wall or dam was built. The space between the two dams was
then filled with grout. To avoid the possibility of pushing the iron
backward after the air pressure was on, rings of segmental plates, 5/8
in. thick and 13-7/8 in. wide, were inserted in eighteen circumferential
joints in each tunnel between the rings as they were erected. The plates
contained slotted holes to match those in the segments. After the rings
left the shield, the plates were driven outward, and projected about 5
in. When the tunnel was grouted, the plates were embedded.
The bulkheads were completed, and the tunnels were put under air
pressure on the following dates:
Line _D_, on October 5th, 1905;
Line _C_, on November 6th, 1905;
Line _B_, on November 25th, 1905;
Line _A_, on December 1st, 1905.
This marked the end of the preparatory period.
In the deepest part of the river, near the pier-head line on the
Manhattan side, there was only 8 ft. of natural cover over the tops of
the tunnels. This cover consisted of the fine sand previously described,
and it was certain that the air would escape freely from the tunnels
through it. To give a greater depth of cover and to check the loss of
air, the contractor prepared to cover the lines of the tunnels with
blankets of clay, which, however, had been provided for in the
specifications. Permits, as described later, were obtained at different
times from the Secretary of War, for dumping clay in varying thicknesses
over the line of work. The dumping for the blanket allowed under the
first permit was completed in February, 1906. The thickness of this
blanket varied considerably, but averaged 10 or 12 ft. on the Manhattan
side. The original blanket was of material advantage, but the depth of
clay was insufficient to stop the loss of air.
The essential parts of the shields in the four tunnels were exactly
alike. Those in Tunnels _B_ and _D_, however, were originally fitted
with sectional sliding hoods and sliding extensions to the floors of the
working chambers, as shown by Fig. 1, Plate LXV. The shields in Tunnels
_A_ and _C_ were originally fitted with fixed hoods and fixed extensions
to the floors, as shown in Fig. 2, Plate LXV. A full description of the
shields will be found in Mr. Japp's paper.
The shields in each pair of tunnels were advanced through the solid rock
section about abreast of each other, until test holes from the faces
indicated soft ground within a few feet. As the distance between the
sides of the two tunnels was only 14 ft., it was thought best to let
Tunnels _B_ and _D_ gain a lead of about 100 ft. before Tunnels _A_ and
_C_ opened out into soft ground, in order that a blow from one tunnel
might not extend to the other. Work in Tunnel _C_ was shut down on
December 23d, 1905, after exposing sand to a depth of 3 ft. at the top,
and it remained closed for seven weeks. Work in Tunnel _A_ was suspended
on September 29th, 1905. By the time Tunnel _B_ had made the required
advance, it, together with Tunnels _C_ and _D_, was overtaxing the
capacities of the compressor plant. Only a little work was done in
Tunnel _C_ until July, 1906, and work in Tunnel _A_ was not resumed
until October 22d, 1906.