There were only two instances of damage to the essential structural
features of the shields. The most serious was in Tunnel _D_ where the
cutting edge at the bottom of the shield was forced up a slightly
sloping ledge of rock. A bow was formed in the steel casting which was
markedly increased with the next few shoves. Work was suspended, and a
heavy cast-steel patch, filling out the bow, was attached to the bent
segments, as shown in Fig. 2, Plate LXXIII. No further trouble was
experienced with the deformed portion. The other instance was in Tunnel
_B_, from Long Island, where a somewhat similar but less serious
accident occurred and was treated in a like manner.
_Bulkheads._--At Manhattan, bulkheads had to be built near the shafts
before the tunnels could be put under pressure. After 500 ft. of tunnel
had been built on each line, the second bulkheads were constructed. The
air pressure between the first and second bulkheads was then reduced to
between 15 and 20 lb. When the shields had been advanced for 1,500 ft.,
the third set of bulkheads was built. Nearly all the broken plates which
were removed were located between the first and third bulkheads at
Manhattan. Before undertaking this operation, the doors of the locks in
the No. 3 bulkheads were reversed to take pressure from the west. By
this means it was possible to carry on the work of dismantling the
shields under comparatively low pressure simultaneously with the removal
of the broken plates.
At Long Island City the roofs of the caissons served the purpose of the
No. 1 bulkheads. Two other sets of bulkheads were erected, the first
about 500 ft. and the second about 1,500 ft. from the shafts.