edge stiffening brackets, and the sinking was carried on by
methods similar to those used at Long Island. The jacks and blocking
supporting the caisson are shown in Fig. 4, Plate LXIV. As soon as the
cutting edge entered the rock, which was drilled about 6 in. outside of
the neat lines, the space surrounding the caisson was back-filled with
clay and muck to steady it and provide skin friction. As the friction
increased, the walls were filled with concrete, and as the caisson
slowly settled, it was checked and guided by blocking. The cutting edge
finally came to rest 31 ft. below mean high water, the sinking having
been accomplished in about seven weeks, at an average rate of 0.50 ft.
per day.

The final position of the cutting edge in relation to its designed
position is shown in Table 1.

A berm about 4 ft. wide was left at the foot of the caisson below which
the rock was somewhat fissured and required timbering. The cutting edge
of the caisson was sealed to the rock with grout on the outside and a
concrete base to the caisson walls on the inside, the latter resting on
the 4-ft. berm. Following the completion of the shaft, the permanent
sump was excavated to grade for use during construction.

_North Shaft._--The north shaft had to be sunk in a very restricted
area. The east side of the caisson cleared an adjoining building at one
point by only 1 ft., while the northwest corner was within the same
distance of the east line of First Avenue. As in the case of the Long
Island shafts, the steelwork for only the lower 40 ft. was ordered at
the start. This height was completely assembled before sinking was
begun. The caisson was lowered in about the same manner as those
previously described. The bearing brackets for the hydraulic jacks were
attached, as at the south shaft, to the inside of the cutting-edge
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