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 total weight being 9,102 lb.
per lin. ft. of tunnel. Fig. 1 shows the details of the standard heavy