When they had been
driven just beyond the line of the fourth track, their ends were
connected by a tunnel parallel with the axis of the subway. The rock
in the bottom of all these tunnels was then excavated to its final
depth. In the small tunnel parallel with the subway axis, a bed of
concrete was placed and the third row of steel columns was erected
ready to carry the steel and concrete roof. When this work was
completed, the earth between the traverse tunnels was excavated, the
material above being supported on poling boards and struts. The roof
of the subway was then extended sidewise over the rock below from the
second to the third row of columns, and it was not until the roof was
finished that the rock beneath was excavated. In this way the subway
was finished for a width of four tracks. For the fifth track the earth
was removed by tunneling to the limits of the subway, and then the
rock below was blasted out.
[Illustration: MOVING WEST SIDE WALL TO WIDEN SUBWAY FOR THIRD
TRACK--135TH STREET AND BROADWAY]
[Illustration: SUBWAY THROUGH NEW "TIMES" BUILDING, SHOWING
INDEPENDENT CONSTRUCTION--THE WORKMEN STAND ON FLOOR GIRDERS OF
SUBWAY]
[Illustration: COLUMNS OF HOTEL BELMONT, PASSING THROUGH SUBWAY AT 42D
STREET AND PARK AVENUE]
In a number of places it was necessary to underpin the columns of the
elevated railways, and a variety of methods were adopted for the work.
A typical example of the difficulties involved was afforded at the
Manhattan Railway Elevated Station at Sixth Avenue and 42d Street.
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