ling and quick setting in the dry sand
at that point then became of value. The use of dry lime in the face,
where the escaping air would carry it into the voids of the sand and
choke them, was much more promptly efficacious in checking the loss.

With the exception of the English lime, all grout was mixed 1 to 1 with
sand in a Cockburn continuous-stirring machine operated by a 3-cylinder
air engine. The grout machine was placed on the lower floor of the
trailing platform shown on Plate LXXII, while the materials were placed
on the upper platform, and, together with the water, were fed into the
machine through a hole in the upper floor. The sand was bagged in the
yard, and the cars on which the materials were sent into the tunnels
were lifted by an elevator to the level of the upper floor of the
trailing platform before unloading.

Great difficulty was experienced in preventing the waste of the fluid
grout ahead of the shield and into the tail through the space between it
and the iron lining. In a full soft ground section, the first condition
did not usually arise. In the full-rock sections the most efficient
method of checking the waste was found to be the construction of dams or
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"