eventually made satisfactory, and by far the largest part of the
river-tunnel grouting was done with this material mixed 1 to 1 by
volume. East of the Long Island shafts the work which was built without
shields was grouted principally with Portland cement and sand mixed 1 to
1 by volume.

In the river tunnels large quantities of the English lime were used neat
as grout over the top of the tunnel in attempts to stop losses of air
through the soft ground. It was not of great efficiency, however, in
this respect until the voids outside of the lining had been filled above
the crown. Its properties of swelling 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