This part of the route includes the former site of the ancient Collect
Pond, familiar in the early history of New York, and the excavation
was through made ground, the pond having been filled in for building
purposes after it was abandoned for supplying water to the city. The
excavations through Canal Street, adjacent, were also through made
ground, that street having been at one time, as its name implies, a
canal.
From the City Hall to 9th Street was sand, presenting no particular
difficulties except through the territory just described.
At Union Square rock was encountered on the west side of Fourth Avenue
from the surface down. On the east side of the street, however, at the
surface was sand, which extended 15 feet down to a sloping rock
surface. The tendency of the sand to a slide off into the rock
excavation required great care. The work was done, however, without
interference with the street traffic, which is particularly heavy at
that point.
[Illustration: DUCTS IN SIDE WALLS--EIGHT ONLY OF THE SIXTEEN LAYERS
ARE SHOWN]
[Illustration: REINFORCED CONCRETE CONSTRUCTION]
[Illustration: ROOF SHOWING CONCRETE-STEEL CONSTRUCTION--LENOX AVENUE
AND 140TH-141ST STREETS]
[Illustration: SECTION OF SUBWAY AT PEARL STREET
This construction was made necessary by encountering a layer of Peat
resting on Clay]
[Illustration: SURFACE RAILWAY TRACKS SUPPORTED OVER EXCAVATION ON
UPPER BROADWAY]
[Illustration: SUBDIVISION OF 36" AND 30" GAS MAINS OVER ROOF OF
SUBWAY--66TH STREET AND BROADWAY]
The natural difficulties of the route were increased by the network of
sewers, water and gas mains, steam pipes, pneumatic tubes, electric
conduits and their accessories, which filled the streets; and by the
surface railways and their conduits.
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