The approach on either side is a double-track concrete
arched structure. The total length of the section is 1,500 feet.
The methods of construction employed were novel in subaqueous
tunneling and are partly shown on photographs on pages 62 and 63.
The bed of the Harlem River at the point of tunneling consists of mud,
silt, and sand, much of which was so nearly in a fluid condition that
it was removed by means of a jet. The maximum depth of excavation was
about 50 feet. Instead of employing the usual method of a shield and
compressed air at high pressure, a much speedier device was contrived.
The river crossing has been built in two sections. The west section
was first built, the War Department having forbidden the closing of
more than half the river at one time. A trench was dredged over the
line of the tunnel about 50 feet wide and 39 feet below low water.
This depth was about 10 feet above the sub-grade of the tunnel. Three
rows of piles were next driven on each side of the trench from the
west bank to the middle of the river and on them working platforms
were built, forming two wharves 38 feet apart in the clear. Piles were
then driven over the area to be covered by the subway, 6 feet 4 inches
apart laterally and 8 feet longitudinally. They were cut off about 11
feet above the center line of each tube and capped with timbers 12
inches square. A thoroughly-trussed framework was then floated over
the piles and sunk on them. The trusses were spaced so as to come
between each transverse row of piles and were connected by eight
longitudinal sticks or stringers, two at the top and two at the bottom
on each side.
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