Tunnel _C_, being higher, was more in soft ground, and at
first it was the intention to delay its excavation until it had been
well drained by the bottom headings in the tunnels on each side. A
little later it was decided to use a shield without compressed air. This
shield had been used in excavating the stations of the Great Northern
and City Tunnel in London. It was rebuilt, its diameter being changed
from 24 ft. 8-1/2 in. to 23 ft. 5-1/4 in. It proved too weak, and after
it had flattened about 4 in. and had been jacked up three times, the
scheme was abandoned, the shield was removed, and work was continued by
the methods which were being used in the other tunnels. The shield was
rather light, but probably it would have been strong enough had it been
used with compressed air, or had the material passed through been all
earth. Here, there was a narrow concrete cradle in the bottom, with rock
up to about the middle of the tunnel, which was excavated to clear the
shield, and gave no support on its sides. The shield was a cylinder
crushed between forces applied along the top and bottom.

With the exception of this trial of a shield in Tunnel _C_, and a novel
method in Tunnel _B_, where compressed air, but no shield, was used, the
description of the work in one tunnel will do for all.

From the bottom headings break-ups were started at several places in
each tunnel where there was ample cover of rock above. Where the roof
was in soft ground, top headings were driven from the points of break-up
and timbered. As soon as the full-sized excavation was completed, the
iron lining was built, usually in short lengths.

It will be noticed on Plate XIII that there is a depression in the rock
between Station 65 and the river shafts, leaving all the tunnels in soft
ground. As this was directly under the Long Island Railroad passenger
station, it was thought best to use a shield and compressed air. This
was done in Tunnels _A_, _C_, and _D_, one shield being used
successively for all three. It was first erected in Tunnel _D_ at
Station 64 + 47. From there it was driven westward to the river shaft.
It was then taken apart and re-erected in Tunnel _C_ at Station 63 + 63
and driven westward to the shaft. It was then found that there would not
be time for one shield to do all four lines. The experience in Tunnels
_C_ and _D_ had proven the ground to be much better than had been
expected. There was considerable clay in the sand, and, with