brackets. The east side of the caisson was in contact with the
foundations of the neighboring building, while the west side was in much
softer material. As a consequence, the west side tended to settle more
rapidly and thus throw the caisson out of level and position. To
counteract that tendency, it was necessary to load the east wall heavily
with cast-iron tunnel sections, in addition to the concrete filling in
the walls.

Soon after sinking was begun, a small test shaft was sunk to a point
below the elevation of the top of the tunnels. The rock was found to be
sound, hard, and nearly dry. It was then decided to stop 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