S)]

[Sidenote: _Concrete-lined
Tunnel_]

Between 33d Street and 42d Street under Park Avenue, between 116th
Street and 120th Street under Broadway, between 157th Street and Fort
George under Broadway and Eleventh Avenue (the second longest
double-track rock tunnel in the United States, the Hoosac tunnel being
the only one of greater length), and between 104th Street and Broadway
under Central Park to Lenox Avenue, the road is in rock tunnel lined
with concrete. From 116th Street to 120th Street the tunnel is 37-1/2
feet wide, one of the widest concrete arches in the world. On the
section from Broadway and 103d Street to Lenox Avenue and 110th Street
under Central Park, a two-track subway was driven through micaceous
rock by taking out top headings and then two full-width benches. The
work was done from two shafts and one portal. All drilling for the
headings was done by an eight-hour night shift, using percussion
drills. The blasting was done early in the morning and the day gang
removed the spoil, which was hauled to the shafts and the portal in
cars drawn by mules. A large part of the rock was crushed for
concrete. The concrete floor was the first part of the lining to be
put in place. Rails were laid on it for a traveler having moulds
attached to its sides, against which the walls were built. A similar
traveler followed with the centering for the arch roof, a length of
about 50 feet being completed at one operation.

[Illustration: FOUR COLUMN (TOWER) VIADUCT CONSTRUCTION]

[Illustration: MANHATTAN VALLEY VIADUCT, LOOKING NORTH]

[Illustration: ERECTION OF ARCH, MANHATTAN VALLEY VIADUCT]

On the Park Avenue section from 34th Street to 41st Street two
separate double-track tunnels were driven below a double-track
electric railway tunnel, one on each side. The work was done from four
shafts, one at each end of each tunnel. At first, top headings were
employed at the north ends of both tunnels and at the south end of the
west tunnel; at the south end of the east tunnel a bottom heading was
used. Later, a bottom heading was also used at the south end of the
west tunnel. The rock was very irregular and treacherous in character,
and the strata inclined so as to make the danger of slips a serious
one. The two headings of the west tunnel met in February and those of
the east tunnel in March, 1902, and the widening of the tunnels to the
full section was immediately begun. Despite the adoption of every
precaution sugg