that the hard material noted in the preliminary wash-borings was a layer
of gravel and boulders overlying the rock. When the borings in the
tunnels reached this material it was found to be water-bearing and the
head was about equivalent to that of the river. Rock cores were taken
from these borings, and the deepest rock was found at about the center
of the river at an elevation of 302.6 ft. below mean high water. Rods
were then inserted in each bore hole and thereby attached to the rock
and used as bench-marks in the tunnels. From these bench-marks, using
specially designed instruments, very accurate observations of the
behavior of the tunnels could be made, and from these the very
interesting phenomenon of the rise and fall of the tunnels with the tide
was verified, the tunnels being low at high tide and the average
variations being about 0.008 ft. in the average tide of about 4.38 ft.:
the tidal oscillations are entirely independent of the weight of the
tunnels, since observations show them to have been the same both before
and after the concrete lining was in position. There was considerable
subsidence in the tunnels during construction and lining, amounting to
an average of 0.34 ft. between the bulkhead lines. This settlement has
been constantly decreasing since construction, and appears to have been
due almost entirely to the disturbances of the surrounding materials
during construction. The silt weighs about 100 lb. per cu. ft. (this is
the average of a number of samples taken through the shield door, and
varied from 93 to 109 lb. per cu. ft.), and contains about 38% of water.
It was found that whenever this material was disturbed outside the
tunnels a displacement of the tunnels followed. The tunnels as above
noted have been lined with concrete reinforced with steel rods, and
prior to the placing of the concrete the joints were caulked, the bolts
grummeted, and the tunnels rendered practically water-tight; the present
quantity of water to be disposed of does not exceed 300 gal. per 24
hours in each tunnel 6,100 ft. long.