tunnel, as
originally planned, with these pile supports.

After the shields had met and the iron lining was joined up, various
experiments and tests were made in the tunnel; screw-piles, and 16-in.
pipes, previously referred to, were inserted through the bore segments
in the bottom of the tunnel, thorough tests with these were made, levels
were observed in the tunnels during the construction and placing of the
concrete lining, an examination was conducted of the tunnels of the
Hudson and Manhattan Railroad Company under traffic, and the result of
these examinations was the decision not to install the screw-piles. The
tunnels, however, were reinforced longitudinally by twisted steel rods
in the invert and roof, and by transverse rods where there was a
superincumbent load on the tunnels; it might also be noted that on the
New York side, where the tunnels emerge from the rock and pass into the
soft material, the metal shell is of cast steel instead of cast iron.
Fig. 12 is a typical cross-section of the river tunnels as actually
constructed.

[Illustration: FIG. 11.--(Full page image)

CROSS-SECTION OF TUNNEL SHOWING TRACK SYSTEM AND SCREW-PILE.]

[Illustration: FIG. 12.--SUBAQUEOUS TUNNELS CROSS-SECTIONS]

During the investigations in the tunnels, borings were made to determine
exactly the character of the underlying material, and it was then found
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.