The
girders in turn were supported on posts footed on the new underpinning
of the adjacent columns. On the completion of the tunnels, concrete
piers were built up from the roof of the tunnel to form a permanent
foundation for the center-line columns. The area to be excavated under
Sixth Avenue was enclosed by a rubble masonry retaining wall constructed
in a trench.
Open-cut excavation was started by planking over the street on stringers
resting on transverse 12 by 12-in. caps. The caps were gradually
undermined and supported on temporary posts which were then replaced by
short posts resting on 12 by 12-in. sills about 7 ft. below the cap. The
operation was then repeated and the sill was supported on another set of
short posts resting on a second sill. When the excavation had been
carried down in this manner to the level of the top of the tunnel,
diagonal 3 by 10-in. timbers were cut in between the posts and sills to
form a species of double A-frame, the legs of which rested in niches cut
in the rock and on posts carried up the face of the underpinning wall,
and the whole was stiffened with vertical tie-rods. This construction is
shown by Fig. 3, Plate LXII. The brick sewer was replaced temporarily by
one of riveted steel pipe. This pipe and the water and gas pipes and
electric conduits were suspended from the timbers as the pipes were
uncovered.
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