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