In many places where the rock surface was
penetrated, a fine sand was found that was probably quicksand. The
material above the rock in the open-cut sections was mostly sand.
[Illustration: FIG. 2. METHOD OF EXCAVATING WITH FULL-WIDTH HEADING
CROSS-TOWN TUNNELS, MANHATTAN
SIDE ELEVATION
FRONT ELEVATION
PLAN SHOWING POSITION OF COLUMNS FOR DRILLING FACE]
The concurrence of the watercourse, shown on General Viele's map of
Manhattan Island (Plate IX[D]), with the points where difficulties in
the construction of the tunnels were encountered has been noted in a
previous paper.
In all cases where the course of this ancient stream was crossed (except
at its final intersection of 33d Street), the rock was found to be very
soft and disintegrated, a large quantity of water was encountered, and
heavy timbering was required. The construction at these localities will
be taken up later. In addition, disintegrated rock, but of a less
troublesome character, was invariably met under the depressions in the
rock surface developed by the borings from the streets and test holes
from the tunnels. Many of these places required timbering, and no
timbering was elsewhere necessary except at the portals. These
coincident conditions were especially marked in 32d Street, which for a
long distance closely adjoins the course of the former creek.
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