lining as the posts were taken out. This meant
that not more than four rings, and often only one ring, could be built
before excavation and a short length of cradle became necessary. Before
the posts under the roof bars could be built and the weight transferred
to the iron lining, a grout dam was placed at the leading end of the
iron lining, and grout was brought up to at least 45 deg. from the top. Such
workings were in progress at as many as eight places in one tunnel at
the same time. Where there was only the ordinary ground-water to contend
with, the driving of the top heading drained the ground very thoroughly,
and the enlarging was done easily and without a serious loss of ground.
Under these conditions the surface settlement was from 6 in. to 2 ft.

Under Borden Avenue, there was more water, which probably came from a
leaky sewer; it was not enough to form a stream, but just kept the
ground thoroughly saturated. There was a continued though hardly
perceptible flow of earth through every crevice in the timbering during
the six or eight weeks between the driving of the top heading and the
placing of the iron lining; and here there was a settlement of from 4 to
8 ft. at the surface.

TUNNELING IN COMPRESSED AIR WITHOUT A SHIELD.

When it became evident that there would not be time for one shield to do
the soft ground portions of all four tunnels under the Long Island
Railroad station, a plan was adopted and used in Tunnel B which, while
not as rapid, turned out to be as cheap as the work done by the shields.
Figs. 6 and 7, Plate LXIII, and Fig. 1, Plate LXIV, illustrate this work
fairly well. The operation of this scheme was about as follows: Having
the iron built up to the face of the full-sized excavation, a hole or
top heading, about 3 ft. wide and 4 or 5 ft. high, was excavated to
about 10 ft. in advance. This was done in a few hours without timbering
of any kind; but, as soon as the hole or heading was 10 ft. out, 6 by
12-in. laggings or polings were put up in the roof, with the rear ends
resting on the iron lining and the leading ends resting on vertical
breast boards. The heading was then widened out rapidly and the lagging
was placed, down to about 45 deg. from the crown. The forward ends of the
laggings were then supported by a timber rib and sill. Protected by this
roof, the full section was excavated, and three rings of the iron lining
were built and grouted, and then the whole process was repeated.