air pressure between the first and second bulkheads was then reduced to
between 15 and 20 lb. When the shields had been advanced for 1,500 ft.,
the third set of bulkheads was built. Nearly all the broken plates which
were removed were located between the first and third bulkheads at
Manhattan. Before undertaking this operation, the doors of the locks in
the No. 3 bulkheads were reversed to take pressure from the west. By
this means it was possible to carry on the work of dismantling the
shields under comparatively low pressure simultaneously with the removal
of the broken plates.

At Long Island City the roofs of the caissons served the purpose of the
No. 1 bulkheads. Two other sets of bulkheads were erected, the first
about 500 ft. and the second about 1,500 ft. from the shafts.


SETTLEMENT AT SURFACE OF GROUND.

The driving of such portions of the river tunnels, with earth top, as
were under the land section, caused a settlement at the surface varying
usually from 3 to 6 in. The three-story brick building at No. 412 East
34th Street required extensive repairs. This building stood over the
section of part earth and part rock excavation where the tunnels broke
out from the Manhattan ledge and where there were a number of runs of
sand into the shield. In fact, the voids made by those runs eventually
worked up to the surface and caused the pavement of the alley between
the buildings to drop 4 or 5 ft. over a considerable area. The tunnels
also passed directly under the ferry bridges and racks of the Long
Island Railroad at East 34th Street. Tunnels _B_ and _D_ were constantly
blowing at the time, and, where progress was slow, caused so much
settlement that one of the racks had to be rebuilt. Tunnel _A_, on the