ken 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.

IRON LINING.

The standard cast-iron tunnel lining was of the usual tube type, 23 ft.
in outside diameter. The rings were 30 in. wide, and were composed of
eleven segments and a key. The webs of the segments were 1-1/2 in. thick
in the central portion, increasing to 2-3/8 in. at the roots of the
flanges, which were 11 in. deep, 2-1/4 in. thick at the root, and 1-1/2
in. at the edge, and were machined on all contact faces. Recesses were
cast in the edge of the flanges, forming a groove, when the lining was
in place, 1-1/2 in. deep and about 3/8 in. wide, to receive the
caulking. The bolt holes were cored in the flanges, and the bosses
facing the holes were not machined. The customary grout hole was tapped
in the center of each plate for a standard 1-1/4-in. pipe. In this work,
experience indicated that the standard pipe thread was too fine, and
that the taper was objectionable. Each segment weighed, approximately,
2,020 lb., and the key weighed 520 lb., the