This depth was about 10 feet above the sub-grade of the tunnel. Three
rows of piles were next driven on each side of the trench from the
west bank to the middle of the river and on them working platforms
were built, forming two wharves 38 feet apart in the clear. Piles were
then driven over the area to be covered by the subway, 6 feet 4 inches
apart laterally and 8 feet longitudinally. They were cut off about 11
feet above the center line of each tube and capped with timbers 12
inches square. A thoroughly-trussed framework was then floated over
the piles and sunk on them. The trusses were spaced so as to come
between each transverse row of piles and were connected by eight
longitudinal sticks or stringers, two at the top and two at the bottom
on each side. The four at each side were just far enough apart to
allow a special tongue and grooved 12-inch sheet piling to be driven
between them. This sheathing was driven to a depth of 10 to 15 feet
below the bottom of the finished tunnel.

A well-calked roof of three courses of 12-inch timbers, separated by
2-inch plank, was then floated over the piles and sunk. It had three
timber shafts 7 x 17 feet in plan, and when it was in place and
covered with earth it formed the top of a caisson with the sheet
piling on the sides and ends, the latter being driven after the roof
was in place. The excavation below this caisson was made under air
pressure, part of the material being blown out by water jets and the
remainder removed through the airlocks in the shafts. When the
excavation was completed, the piles were temporarily braced and the
concrete and cast-iron lining put in place, the piles being cut off as
the concrete bed was laid up to them.

The second or eastern section of this crossing was carried on by a
modification of the plan just mentioned. Instead of using a temporary