g 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