the caisson as
soon as a foundation could be secured on sound rock. The latter was
found at a depth of 38 ft. below mean high water. With the cutting edge
seated at that depth, the top of the caisson was only 2 ft. above mean
high water, and as this was insufficient protection against high tides,
a 10-ft. extension was ordered for the top. Work, however, went on
without delay on the remainder of the excavation. The junction between
the cutting edge and the rock was sealed with concrete and grout. The
caisson was lowered at an average rate of 0.53 ft. per day. The size of
the shaft below the cutting edge was 62 ft. 7 in. by 32 ft. The average
rate of excavation during the sinking in soft material was 84 cu. yd.
per day. The average rate of rock excavation below the final position of
the cutting edge was 125 cu. yd. per day. There were night and day
shifts, each working 10 hours. Excavation in earth cost $3.96 per cu.
yd., of which $1.45 was for labor and $2.51 for top charges, etc. The
excavation of rock cost $8.93 per cu. yd., $2.83 being for labor and
$6.10 for top charges.

The final elevations of the four corners of the cutting edge, together
with their displacement from the desired positions, are shown in Table
1.

RIVER TUNNELS.

The four river tunnels, between the Manhattan and Long Island City
shafts, a distance of about 3,900 ft., were constructed by the shield
method. Eight shields were erected, one on each line in each shaft, the
four from Manhattan working eastward to a junction near the middle of
the river with the four working westward from Long Island City. Toward
the end of the work it was evident that the shields in Tunnels _B_, _C_,
and _D_ would meet in the soft material a short distance east of the
Blackwell's Island Reef if work were continued in all headings. In order
that the junction might be made in firm material, work from Manhattan in
those three tunnels was suspended when the shields reached the edge
of the ledge. The shields in Tunnel _A_ met at a corresponding point
without the suspension of work in either. An average of 1,760 ft. of
tunnel was driven from Manhattan and 2,142 ft. from Long Island City.

[Illustration: PLATE LXV, FIG. 1.--SHIELD FITTED WITH SECTIONAL SLIDING
HOODS AND SLIDING EXTENSIONS TO THE FLOORS.]

[Illustration: PLATE LXV, FIG. 2.--SHIELD FITTED WITH FIXED HOODS AND
FIXED EXTENSIONS TO THE FLOORS.]

TUNNELS DRIVEN EASTWARD FROM MANHATTAN.

_Materials and In