The most serious difficulties of the work came near the start. In Tunnel
_D_ blows and falls of sand from the face were frequent after soft
ground was met in the top. About six weeks after entering the full sand
face, and before the shutters had been installed, the shield showed a
decided tendency to settle, carrying the tunnel lining down with it and
resulting in a number of badly broken plates in the bottom of the rings.
Notwithstanding the use of extremely high vertical leads,[D] the sand
was so soft that the settlement of the shield continued for about
fifteen rings, the maximum being nearly 9 in. below grade. The
hydrostatic head at mid-height of the tunnel was 32-1/2 lb., and the
raising of the air pressure to 37 lb., as was done at this time, was
attended with grave danger of serious blows, on account of the recent
disturbance of the natural cover by the pulling and re-driving of piles
in the reconstruction of the Long Island ferry slips directly above. It
dried the face materially, however, and the shield began to rise again,
and had practically regained the grade when the anticipated blow-outs
occurred, culminating with the entrance of rip-rap from the river bed
into the shield and the flooding of the tunnel with 4 ft. of sand and
water at the forward end. The escape of air was very great, and, as a
pressure of more than 28 lb. could not be maintained, the face was
bulkheaded and the tunnel was shut down for three weeks in order to
permit the river bed to consolidate.
This was the most serious difficulty encountered on any part of the
work, and, coming at the very start, was exceedingly discouraging.
During the shut-down the broken plates were reinforced temporarily with
steel ribs and reinforced concrete (Fig. 1, Plate LXXIII) which, on
completion of the work, were replaced by cast-steel segments, as
described elsewhere. Practically, no further movement of iron took
place, and the loss of grade caused by the settlement of the shield,
which was by far the largest that ever occurred in this work, was not
sufficient to require a change in the designed grade or alignment of the
track. Work was resumed with the shutters in use at the face as an aid
to excavation. The features of extreme seriousness did not recur, but
for two months the escape of air continued to be extremely large, an
average of 15,000 cu. ft. per min. being required on many days during
this period.
[Illustration: PLATE LXXIII, FIG. 1.--TEMPORARY REINFORCEMENT OF BROKEN
PLATES AND REMOVAL OF A PLATE IN SECTIONS.]
[Illustration: PLATE LXXIII, FIG. 2.--HEAVY CAST-STEEL PATCH ATTACHED TO
BENT SEGMENT OF CUTTING EDGE.]
[Illustration: PLATE LXXIII, FIG. 3.--INFLOW OF SOFT CLAY THROUGH
SHIELD.]
[Illustration: PLATE LXXIII, FIG. 4.--REINFORCEMENT OF BROKEN PLATE WITH
LONG POLT AND TWISTED STEEL RODS.]
In Tunnel _B_, after passing out from under the bulkhead line, in April,
1906, the loss of air became very great, and blow-outs were of almost
daily occurrence until the end of June. At the time of the blows the
pressure in the tunnel would drop from 2 to 8 lb., and it generally took
some hours to raise the pressure to what it was before the blow. During
that time regular operations were interrupted. In the latter part of
June a permit was obtained allowing the clay blanket to be increased in
thickness up to a depth of water of 27 ft. at mean low tide. The
additional blanket was deposited during the latter part of June and
early in July, and almost entirely stopped the blows.
By the end of the month the natural clay, previously described, formed
the greater portion of the face, and, from that time forward, played an
important part in reducing the quantity of air required. During April
and the early part of May the work was under the ferry racks of the Long
Island Railroad. The blanket had to be placed by dumping the clay from
wheel-barrows through holes in the decking.
In Tunnel _A_ a bottom heading had been driven 23 ft. in advance of the
face at the time work was stopped at the end of 1905. During the ten
months of inactivity the seams in the rock above opened. The rock
surface was only from 2 to 4 ft. below the top of the cutting edge for a
distance of about 60 ft. Over the rock there were large boulders
embedded in sharp sand. It was an exceedingly difficult operation to
remove the boulders and place the polings without starting a run. The
open seams over the bottom heading also frequently caused trouble, as
there were numerous slides of rock from the face which broke up the
breasting and allowed the soft material from above to run into the
shield. There were two runs of from 50 to 75 cu. yd. and many smaller
ones.
[Footnote D: The lead of the shield is the angular divergence of its
axis from the axis of the tunnel and, in this tunnel, was measured as
the offset in 23 ft. It was called + when the shield was pointed upward
from grade, and - when pointed downward.]