from 9 to 12 in. above the level of the top of the masonry,
by October 1st, they encroached 9 in. within the line of masonry. It was
then decided to remove the rock for a distance of 48 ft. west of the
shaft, and build this portion of the tunnel in open cut. The posts
supporting the deck forming the street surface were replaced by an
A-frame structure similar to that developed for the 32d Street open cut,
without interruption of the street traffic.

After making the open cut to the westward of the shaft, there was a slip
in the rock north of and adjoining the shaft. Fortunately, the timbers
did not give way entirely, and no damage was done. The open cut was
extended eastward for a distance of 46 ft., making the total length of
tunnel built in open cut on this street 94 ft.

East of the shaft, for a distance of about 125 ft., the rock was broken
and could not be excavated to full size without timbering the roof, but
between this section of poor rock and those already mentioned in
connection with the work at Fifth Avenue, there was a stretch of 600 ft.
of good rock where all the spoil was handled with a steam shovel.

TWIN-TUNNEL LINING.

The masonry lining for the tunnels was not started until the late fall
of 1906, after excavation had been in progress for a year and a half. At
that time concreting was started in the single tunnels westward from the
First Avenue Shafts, and by spring was in full swing in the Twin
Tunnels.

The plans contemplated the use of a complete concrete lining except
where large quantities of water were encountered; in which case the
arches, beginning at a point 15 deg. above the springing line, were to be
built of vitrified paving brick. By reference to Plate XII it will be
seen that the water-proofing, which in the concrete-roof tunnels
extended the full height of the sides to the 15 deg. line, was carried in
the brick-roof tunnels completely around the extrados of the arch. The
cross-sections also show the location of the electric conduits which
were buried in the mass of the side and core-walls and which limited the
height to which the concrete could be carried in one operation.

The same general scheme of operations was used wherever possible
throughout the Twin-Tunnel work, but was subject to minor modifications
as circumstances dictated. Concrete was first deposited in the bottom,
to the grade of the flow line of the drains; after it had set,
collapsible box forms, of 2-in. plank w