river were very slightly modified. Plate VII is a plan and profile of
the tunnels as constructed.

[Illustration: Plate VII.--Plan, Profile, and Triangulation, North River
Tunnels]

The Board of Engineers early in 1902 took up the question of supports
for the tunnels under the North River, and various plans and schemes
were considered. It was finally decided to support the tracks on
screw-piles carried through the lining of the tunnels, as originally
proposed by the writer.

In order to know something of the capacity of screw-piles in the actual
material to be passed through, it was resolved to test them. A caisson
was sunk at the end of one of the Erie Railroad piers on the New Jersey
side near the line of the tunnels, and, to obtain parallel conditions as
much as possible, the excavation was carried down to the proposed grade
of the tunnel. Various types of screw-piles were sunk therein and tests
were made, not only of the dead load carrying capacity, but also with
the addition of impact, when it was found that screw-piles could be sunk
to hard ground and carry the required load. The final part of the test
was the loading. The screw-pile, having a shaft 30 in. in diameter and a
blade 5 ft. in diameter, was loaded with 600,000 lb., with the result
that, for a month--the duration of this loaded test--there was no
subsidence.

Again, and after the iron tunnel lining had been constructed across the
river, tests were made of two types of supports: One a screw-pile 29-1/2
in. in diameter with a blade 4 ft. 8 in. in diameter and the other a
wrought-iron pipe 16 in. in external diameter. Tests were made, not only