To take care of the drainage of the tunnels, a sump with a pump chamber
above it was provided for each pair of tunnels. The sumps were really
short tunnels underneath the main ones and extending approximately
between the center lines of the latter. They were 10 ft. 9-1/2 in. in
outside diameter and 44 ft. long. The water drops directly from the
drains in the center lines of the tunnels into the sumps. Above the
sumps and between the tunnels, a pump chamber 19 ft. 5 in. long was
built. Above the end of the latter, opposite the sump, a cross-passage
was constructed between the bench walls of the two tunnels. This passage
gives access from either tunnel through an opening in the floor to the
pump chamber and through the latter to the sump.
From the preliminary borings it was thought that the sumps were located
so that the entire construction would be in rock. This proved to be the
case on Tunnels _C_ and _D_, but not on Tunnels _A_ and _B_. The
position of the rock surface in the latter is shown by Fig. 3. After the
excavation was completed in Tunnel _B_, January 1st, 1908, the plates
were removed from the side of the tunnel at the cross-passage, and a
drift was driven through the earth above the rock surface across to the
lining of Tunnel _A_. The heading was timbered as shown by Fig. 3. There
was practically no loss of air from the drift, but the clay blanket had
been removed from over this locality and the situation caused some
anxiety. In order to make the heading as secure as possible, the 24-in.
I-beams, shown on Fig. 3, were attached to the lining of the two
tunnels. The beams formed a support for the permanent concrete roof arch
of the passage, which was placed at once. At the same time plates were
removed from the bottom in Tunnel _B_ over the site of the sump, and a
heading was started on the line of the sump toward Tunnel _A_. As soon
as the heading had been driven beyond the center line of the pump
chamber, a bottom heading was driven from a break-up westward in the
pump chamber and a connection was made with the cross-passage. The iron
lining of the pump chamber was next placed, from the cross-passage
eastward. The soft ground was excavated directly in advance of the
lining, and the ground was supported by polings in much the same manner
as described for shield work. On account of bad ground and seams of sand
encountered in the rock below the level of the cross-beams, the entire
west wall of the pump chamber was placed before enlarging the sump to
full size. This was also judicious, in order to support as far as
possible the iron lining of the tunnels. The sump was then excavated to
full size. The iron lining of the sump and the east wall of the pump
chamber were placed as soon as possible. The voids outside the iron
lining of the sump and the pump chamber were filled as completely as
possible with concrete, and then thoroughly grouted. Finally, the
concrete lining was put in place inside of the iron.
As shown by Fig. 3, the excavation of these chambers left a considerable
portion of the iron lining of the tunnels temporarily unsupported on the
lower inner quarter. To guard against distortion, a system of diagonals
and struts was placed as shown.
The floor of the pump chamber was water-proofed with felt and pitch in a
manner similar to that described for the caissons at Long Island City.
It was not possible to make the felt stick to the vertical walls with
soft pitch, which was the only kind that could be used in compressed
air, and, therefore, the surfaces were water-proofed by a wall of
asphalt brick laid in pitch melting at 60 deg. Fahr. Forms were erected on
the neat line, and the space to the rock was filled with concrete making
a so-called sand-wall similar to that commonly used for water-proofing
with felt and pitch. The bricks were then laid to a height of four or
five courses. The joints were filled with pitch instead of mortar.
Sheets of tin were then placed against the face of the wall and braced
from the concrete forms. As much pitch as possible was then slushed
between the brick and the sand-wall, after which the concrete in the
main wall was filled up to the top of the water-proofing course. The tin
was then withdrawn and the operation repeated. This method was slow and
expensive, but gave good results. Ordinary pitch could not be used on
account of the fumes, which are particularly objectionable in
compressed air. The 60 deg. pitch was slightly heated in the open air before
using.
[Illustration: FIG. 3.]
The sump and pump chamber on Tunnels _C_ and _D_ differed from the one
described only in minor details; but, being wholly constructed in rock,
presented fewer difficulties and permitted a complete envelope of
water-proofing to be placed in the top.