bulkheads outside the lining between it and the rock surface. For this
purpose, at intervals of about 30 ft., the leading ring and the upper
half of the preceding one were disconnected and pulled forward
sufficiently to give access to the exterior. A rough dam of rubble, or
bags of mortar or clay, was then constructed outside the iron, and the
rings were shoved back and connected up. In sections containing both
rock and soft ground, grout dams were built at the cutting edge at
intervals, and were carried up as high as circumstances permitted.

The annular space at the tail of the shield was at all times supposed to
be packed tight with clay and empty bags, but the pugging was difficult
to maintain against the pressure of the grout. For a time, 1/2-in.
segmental steel plates, slipped down between the jackets and the iron,
were used to retain the pugging, but their displacement resulted in a
number of broken flanges, and their use was abandoned. In their place,
2-in. segmental plates attached to the jack heads were substituted with
more satisfactory results. Notwithstanding these devices, the waste of
grout at the tail was very great.

The soft ground material on various portions of the work acted very
differently. The clay and "bull's liver" did not cave in upon the iron
lining for several hours after the shield had passed, sometimes not for
a day or more, which permitted the space between it and the iron to be
grouted. The fine gray or beach sand and the quicksand closed in almost
at once. The quicksand has a tendency to fill in under the iron from the
sides and in places to leave a cavity at about the horizontal diameter
which was not filled from above, as the sand, being dried out by the
air, stood up fairly well and did not cave against the iron, except
where nearly horizontal at the top.