[Illustration: FIG. 5.--(Full page image)

ARRANGEMENT OF STRUCTURES SUPPORTING NINTH AVE. DURING PROGRESS OF
EXCAVATION]

Contractor's plants were established at the Weehawken Shaft and at the
Manhattan Shaft, including at each, low-pressure air compressors of a
capacity of 13,000 cu. ft. of free air per minute and also high-pressure
air compressors for drills, hydraulic pumps, electric generators, etc.

The river tunnels passed under Pier 72, North River (old No. 62), which
was occupied by the New York Central and Hudson River Railroad Company.
The Tunnel Company leased this pier and withdrew all the piles on the
lines of the tunnels prior to the commencement of construction, and on
the remaining piles constructed a trestle for the disposal of the
excavation from the tunnels and the terminal. At the completion of the
work this pier had to be restored, and Fig. 10 shows the general
arrangements of the location of the piles and the pier structure with
reference to the tunnels.

In the tunnels which were constructed in silt farther down the river, by
the writer as Chief Engineer for the Hudson Companies, it had been
possible to shove the shield through the silt with all the doors closed,
displacing the ground and making great speed in construction owing to
the absence of all mucking. It was thought that this procedure might be
pursued in the larger tunnels of the Pennsylvania Railroad, and it was
tried, but it was almost immediately found to be impossible to maintain
the required grade without taking a certain quantity of muck into the
tunnels through the lower doors, the tendency of the shield being to
rise. By taking in about 33% of the excavation displaced by the tunnel,