In the Three-Track Tunnels, a heavy brick arch was used for those
portions constructed in tunnel, while, in the open-cut sections, the
roof was of concrete. Both were completely water-proofed on the roof and
sides, and in the tunnel sections the space above the brick roof was
filled with rock packing. On account of the unstable nature of the rock
encountered throughout, the voids in the packing were afterward filled
with grout.
By reference to the cross-sections, Plate XII, it will be seen that the
haunches of the arch were tied together by steel I-beams anchored in the
concrete, with the object of making the structure self-supporting in the
event of the removal of the adjacent rock for deep cellar excavations.
This construction materially influenced the contractor's method of
placing the masonry lining.
After depositing the floor concrete, by the same method that was used in
the Twin Tunnels, a timber trestle (Fig. 2, Plate LXII) was erected to
the height of the underside of the I-beam ties, the posts being footed
in holes, about 3 in. deep, left in the concrete floor to prevent
slipping. In the open-cut sections the sand-wall forms were of undressed
plank tacked to the studding and braced from the trestle; in the tunnel
section they were spiked to the face of the posts supporting the
timbering.
The side-wall forms were made up in panels about 3 by 10 ft., and were
clamped to studs by U-shaped irons passing around the stud and bolted to
the cleats on the back of the panels, the studs being braced from the
trestle. The side-wall concrete was deposited in three sections. The
first was brought up just above the sidewalk and formed the bench for
the high-tension ducts; the second carried the wall up to the springing
line. Before placing the third section the I-beam ties were set in
position (Fig. 3, Plate LXII) on top of the trestle, and the reinforcing
rods in the haunch of the arch were hung from them. The concrete was
carried up to a skewback for the arch, as shown in the brick-roof
cross-section (Plate XII) and embedded the ends of the ties.
The centers for the arches stood on the I-beam ties, and the tops of the
hangers, for the permanent support of the ties near their center, were
inserted through the lagging. The brick arch, water-proofing, and rock
packing were laid up in lifts, in the same manner as in the Twin Tunnel,
with grout pipes built in at intervals of about 8 ft. The concrete arch
was placed in sections, from 25 to 50 ft. in length, with a rather wet
mixture and a back form on the steep slope of the extrados.
The concrete for the sand-walls and lower part of side-walls was handled
on tracks and platforms laid on cantilever beams at mid-height of the
trestle, as shown by Fig. 3, Plate LXII. For the walls above the
springing line, the tracks were laid on top of the I-beam ties, and some
of the arch concrete, also, was delivered from the mixer at that level
and hauled up an incline to the level of the top of the arch. By far the
greater part, however, was turned out from mixers set on the completed
arch, and was transported on tracks hung in part from the street
timbering.
_Completion._--Except in the heavily-timbered portions, such as at Fifth
Avenue, where the load had to be transferred from posts to the completed
masonry section by section, the lining of the tunnels presented no
special difficulty. The large number of small forms to be set, and the
mutual interference of the concreting and duct-laying operations proved
to be the most troublesome features of the work.
The restoration of the streets, public utilities, etc., at the open-cut
sections was a slow and tedious operation, but the tunnels themselves
were completed in March, 1909, 3 years and 10 months after the inception
of the work. The finished tunnels are shown by the photograph, Fig. 4,
Plate LXII, taken at the junction of the twin and three-track types.