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.
[Illustration: PLATE LXIV, FIG. 1.--TUNNELING IN COMPRESSED AIR WITHOUT
SHIELD.]
[Illustration: PLATE LXIV, FIG. 2.--T-HEAD AIR-LOCK.]
[Illustration: PLATE LXIV, FIG. 3.--CUTTING EDGE OF CAISSON ASSEMBLED.]
[Illustration: PLATE LXIV, FIG. 4.--CAISSON SUPPORTED ON JACKS AND
BLOCKS.]