As has been said, the typical subway near the surface was used for
about one-half of the road. Since the sewers were at such a depth as
to interfere with the construction of the subway, it meant that the
sewers along that half had to be reconstructed. This indicates but
very partially the magnitude of the sewer work, however, because
nearly as many main sewers had to be reconstructed off the route of
the subway as on the route; 7.21 miles of main sewers along the route
were reconstructed and 5.13 miles of main sewers off the route. The
reason why so many main sewers on streets away from the subway had to
be rebuilt, was that, from 42d Street, south, there is a natural
ridge, and before the construction of the subway sewers drained to the
East River and to the North River from the ridge. The route of the
subway was so near to the dividing line that the only way to care for
the sewers was, in many instances, to build entirely new outfall
sewers.

[Illustration: THREE-TRACK CONCRETE ARCH--117TH STREET AND BROADWAY]

A notable example of sewer diversion was at Canal Street, where the
flow of the sewer was carried into the East River instead of into the
Hudson River, permitting the sewer to be bulkheaded on the west side
and continued in use. On the east side a new main sewer was
constructed to empty into the East River. The new east-side sewer was
built off the route of the subway for over a mile. An interesting
feature in the construction was the work at Chatham Square, where a
6-1/2-foot circular brick conduit was built. The conjunction at this
point of numerous electric surface car lines, elevated railroad
pillars, and enormous vehicular street traffic, made it imperative
that the surface of the street should not be disturbed, and the sewer
was built by tunneling. This tunneling was through very fine running