tant
questions connected with the design of these tunnels was their probable
stability under the long-continued action of a heavy and rapid railroad
traffic. The tunnels are lighter than the materials which they displace
even when the weight of the heavy live load is included. In the East
River the character of the material seemed to justify the conclusion
that the tunnels would not be displaced even under the action of the
live load. In the North River, however, the tunnels are enveloped by a
soft silt and it was at first apprehended that some system of supports
would be advisable to carry the heavy traffic and insure the tunnels
against displacement under its action. To meet this contingency, which
was then believed to be a very serious one, it was proposed to sink
cast-iron screw-piles through the bottom of each tunnel into and through
the underlying silt until satisfactory bearing material was reached. The
pile supports were worked out in sufficient detail to be embraced in the
contract for the construction of these tunnels, with provision, however,
for omitting them should it be determined subsequently that their use
was undesirable. The contract plans contained provisions for sliding
joints where the piles pass through the tunnel floor, so that the live
load might be carried directly to the pile heads by a system of girders,
and also for attaching the piles directly to the tunnel, the two plans
being alternatives.

Investigations, made during the progress of the work to determine the
physical character of the silt and its action on the tunnels, suggested
the possibility that the use of pile supports might be inadvisable. This
view was confirmed by actual experience in the operation of the tunnels
of the Hudson Companies between Hoboken, N. J., and Morton Street,
Manhattan, which were opened to traffic in February, 1908. The stability
of these tunnels under traffic gave further assurance that supports were
unnecessary under the North River tunnels of the Pennsylvania Railroad
Company, and they were therefore dispensed with.

_Cross-Passages Between the Tunnels._--The Bergen Hill tunnels, the land
portions of the North River tunnels and the tunnels under Manhattan are
connected by cross-passages at intervals varying from 50 to 300 ft. As
it was the desire of the Management to provide every arrangement
possible to insure the safety of its passengers and employees and also
to provide for the convenience of inspection