Under such conditions the normal concrete
methods, to be described later, could not be used, and special forms
were substituted.
[Illustration: FIG. 3. CONSTRUCTION OF TWIN TUNNELS, THROUGH EXCAVATION
STARTED FOR THREE-TRACK TUNNEL IN 33D STREET NEAR 5TH AVENUE]
In this section of the work the most serious difficulties were
encountered near Fourth Avenue a short distance east of the Intermediate
Shaft, and beneath the site of the old pond shown on General Viele's
map. The rock cover was known from the boring to be very thin, and the
presence of the subway overhead caused some anxiety. The excavation was
at first taken out to practically full width and timbered, but the rock
became so treacherous that the heading was narrowed to a width
sufficient for one tunnel only. With this span the rock in the roof held
without timbering. As the masonry lining approached, sufficient trimming
was done to permit the placing of the core-wall and one arch. Above the
completed core-wall and brick arch the voids were filled solid with
rubble masonry to give an unyielding support to the roof. The excavation
of the remaining width of tunnel was then undertaken. Near the west side
of Fourth Avenue, the excavation broke out of rock at the top, and fine
sand and gravel with a large quantity of water were encountered.
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