As previously mentioned, clay was dumped over the tunnels in varying
depths at different times. A material was required which would pack
into a compact mass and would not readily erode under the influence of
the tidal currents of the river and the escape of the great volumes of
air which often kept the water in the vicinity of the shields in violent
motion. Suitable clay could not be found in the immediate vicinity of
the work. Materials from Shooter's Island and from Haverstraw were tried
for the purpose. The Government authorities did not approve of the
former, and the greater portion of that used came from the latter point.
Although a number of different permits governing the work were granted,
there were three important ones. The first permit allowed a blanket
which roughly followed the profile of the tunnels, with an average
thickness of 10 ft. on the Manhattan side and somewhat less on the Long
Island City side. The second general permit allowed the blanket to be
built up to a plane 27 ft. below low water. This proved effective in
checking the tendency to blow, but allowed considerable loss of air.
Finally, dumping was allowed over limited and marked areas up to a plane
of 20 ft. below low water. Wherever advantage was taken of this last
authority, the excessive loss of air was almost entirely stopped. After
all the shields had been well advanced out into the river, the blanket
behind them was dredged up, and the clay used over again in advance of
the shield.
Soundings were taken daily over the shields, and, if marked erosion was
found, clay was dumped into the hole. Whenever a serious blow occurred,
a scowload of clay was dumped over it as soon as possible and without
waiting to make soundings. For the latter purposes a considerable
quantity of clay was placed in storage in the Pidgeon Street slip at
Long Island City, and one or two bottom-dump scows were kept filled
ready for emergencies. Mr. Robert Chalmers, who had charge of the
soundings for the contractor, states that "the depressions in the
blanket caused by erosion due to the escape of air were, as a rule,
roughly circular in plan and of a curved section somewhat flat in the
center." Satisfactory soundings were never obtained in the center of a
violent blow, but the following instance illustrates in a measure what
occurred. Over Tunnel _B_, at Station 102+80, there was normally 36 ft.
of water, 7 ft. of clay blanket, and 20 ft. of natural cover. Air was
escaping at the rate of about 10,000 cu. ft. per min., and small blows
were occurring once or twice daily. On June 22d, soundings showed 54 ft.
of water. A depth of 18 ft. of the river bottom had been eroded in about
two days. On the next day there were taken out of the shield boulders
which had almost certainly been deposited on the natural river bed. Clay
from the blanket also came into the shields on a number of occasions
during or after blows. The most notable occasion was in September, 1907,
when the top of the shield in Tunnel _D_ was emerging from the east side
of Blackwell's Island Reef. The sand in the top was very coarse and
loose, and allowed the air to escape very freely. The fall of a piece of
loose rock from under the breast precipitated a run of sand which was
followed by clay from the blanket, which, in this locality, was largely
the softer redredged material. Mucking out the shield was in progress
when the soft clay started flowing again and forced its way back into
the tunnel for a distance of 20 ft., as shown in Fig. 3, Plate LXXIII.
Ten days of careful and arduous work were required to regain control of
the face and complete the shove, on account of the heavy pressure of the
plastic clay.
The clay blanket was of the utmost importance to the work throughout,
and it is difficult to see how the tunnels could have been driven
through the soft material on the Manhattan side without it.
The new material used in the blanket amounted to 283,412 cu. yd., of
which 117,846 cu. yd. were removed from over the completed tunnels and
redeposited in the blanket in advance of the shields. A total of 88,059
cu. yd. of clay was dumped over blows. The total cost of placing and
removing the blanket was $304,056.