POLT AND TWISTED STEEL RODS.]

In Tunnel _B_, after passing out from under the bulkhead line, in April,
1906, the loss of air became very great, and blow-outs were of almost
daily occurrence until the end of June. At the time of the blows the
pressure in the tunnel would drop from 2 to 8 lb., and it generally took
some hours to raise the pressure to what it was before the blow. During
that time regular operations were interrupted. In the latter part of
June a permit was obtained allowing the clay blanket to be increased in
thickness up to a depth of water of 27 ft. at mean low tide. The
additional blanket was deposited during the latter part of June and
early in July, and almost entirely stopped the blows.

By the end of the month the natural clay, previously described, formed
the greater portion of the face, and, from that time forward, played an
important part in reducing the quantity of air required. During April
and the early part of May the work was under the ferry racks of the Long
Island Railroad. The blanket had to be placed by dumping the clay from
wheel-barrows through holes in the decking.

In Tunnel _A_ a bottom heading had been driven 23 ft. in advance of the
face at the time work was stopped at the end of 1905. During the ten
months of inactivity the seams in the rock above opened. The rock
surface was only from 2 to 4 ft. below the top of the cutting edge for a
distance of about 60 ft. Over the rock there were large boulders
embedded in sharp sand. It was an exceedingly difficult operation to
remove the boulders and place the polings without starting a run. The
open seams over the bottom heading also frequently caused trouble, as
there were numerous slides of rock from the face which broke up the
breasting and allowed the soft material from above to run into the
shield. There were two runs of from 50 to 75 cu. yd. and many smaller
ones.

[Footnote D: The lead of the shield is the angular divergence of its
axis from the axis of the tunnel and, in this tunnel, was measured as
the offset in 23 ft. It was called + when the shield was pointed upward
from grade, and - when pointed downward.]

GUIDING THE SHIELDS.

Little difficulty was experienced at any time in driving the shield
close to the desired line, but it was much harder to keep it on grade.
In rock section, where the cradle could be set far enough in advance to
become hard before the shield was shoved over it, there was no trouble
whatever.