[Illustration: PLATE LXII, FIG. 1.--WATER-PROOFING OVER BRICK
ARCHES.]
[Illustration: PLATE LXII, FIG. 2.--TRESTLE USED IN CONCRETING IN
THREE-TRACK TUNNEL.]
[Illustration: PLATE LXII, FIG. 3.--METHOD OF STREET SUPPORT OVER
OPEN-CUT EXCAVATION.]
[Illustration: PLATE LXII, FIG. 4.--JUNCTION OF TWIN AND THREE-TRACK
TUNNELS.]
THREE-TRACK TUNNEL LINING.
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.
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