side. The felt and pitch water-proofing was then
laid for that height, joined to the previous water-proofing on the
side-walls, and was followed by the brick armor course over the
water-proofing and by the rock packing, after which another lift of
brick was laid and the operations were repeated. The large void (Fig. 1,
Plate LXII) above the core-wall gave convenient access for working on
top of the adjacent sides of the roof, and the keying of the arches and
the water-proofing and rock packing above the core-wall were usually
carried on from that point, the work progressing from one end.

The concrete for all work above the floor was dumped on the platform of
the carriages, to which it was transported in the early part of the work
in cars running on a high-level track laid on long ties, resting on the
finished sidewalks. This arrangement, although requiring a large amount
of timber for the track, permitted the muck to be carried out on the
low-level track without interference. Later, when the advance of the
heading had ceased and the heavy mucking was over, all concrete was
transported on the floor level, and the cars were lifted to the carriage
platforms by elevators and were hauled by hoisting engines up a movable
incline. The latter method is shown by Fig. 3, Plate LIX.

_Water-Proofing._--The water-proofing referred to above was in all cases
felt and pitch laid with six thicknesses of felt and seven of pitch. The
sub-contractor for the work was the Sicilian Asphalt Paving Company. All
joints were lapped at least 1 ft., and, where work was suspended for a
time and a bevel lap could not be made, the edges of the felt were left
unpitched for 1 ft. and the newer work was interlaced with the old. This
method was not always successful, however, on account of the softening
of the unpitched felt on long-continued exposure to the water. The felt
used was mainly "Tunaloid," together with some "Hydrex." It weighed
about 12 lb. per 100 sq. ft. when saturated and coated on one side only,
and contained about 25% of wool. The coal-tar pitch used had a melting
point of 100 deg. Fahr.

After the completion of the tunnel, the concrete arch showed some
leakage and in places unsightly lime deposits. It was determined to
attempt to stop these leaks by the application of a water-proof cement
coating on the intrados of the arch. Extended experimental application
of two varieties of materials used for this purpose--"Hydrolithic"
cement