ructions from the floor sections, the only
portion then accessible.

After water-proofing the sand-walls and laying the low-tension ducts, a
second pair of carriages, with panels on one side only, for 60 ft. of
side-wall and skewback to the 15 deg. line, were set and braced against the
core-wall. These forms are shown in connection with the carriage on
Plate LXI. They were concreted to the base of the high-tension duct
bank, and, after the concrete had hardened and the bank of ducts had
been laid, the concreting was completed in a second operation.

In places where the roof was supported temporarily by posts and heavy
timbering, such as at Fifth Avenue, the form carriages could not be
used, and special methods were devised to suit the local conditions.
Usually, the panels were stripped from the carriages and moved from
section to section by hand, and, when in position, were braced to the
timbering.

The arch centers were built up of two 5 by 3 by 3/8-in. steel angles,
and, when set, were blocked up on the sidewalks opposite each other in
the two tunnels. A temporary platform was laid on the bottom chord
angles of the ribs, on which the concrete was dumped, the same as on the
form carriages. The lagging used was 3 by 3-in. dressed pine or spruce
16 ft. long, and was placed as the concreting of the arch proceeded
above the 15 deg. line on the side-wall and above the sidewalk on the
core-wall. After the arch had reached such a height that the concrete
could not be passed over the lagging directly from the main platform, it
was cast on a small platform on the upper horizontal bracing of the
centers, shown in Fig. 3, Plate LIX, and was thence shoveled into the
work. In the upper part of the arch the face of the concrete was kept on
a radial plane, and, when only 3 ft. remained to be placed, it was keyed
in from one end, the key lagging being set in about 5-ft. lengths. The
arches were concreted usually in 60-ft. lengths.

Where brick arches were used, the core-wall skewback was concreted
behind special forms set up on the sidewalks, or the arch ribs and
lagging were used for forms, and the brick arch was not started until
after the concrete had set. In laying the brick in the arch, the five
courses of the ring were carried up as high as the void between the
extrados and the rock would permit and still leave a working space in
which to place the water-proofing. This was usually not more than 3 ft.,
except on the core-wall