Except as previously noted, the voids outside of the tunnel lining were
filled with grout ejected through the grout holes in each segment. The
possibility was always present that Portland cement, if used for grout
in the shield-driven tunnels, would flow forward around the shield and
set hard, "freezing" the shield to the rock or the iron lining, or at
least forming excrescences upon it, which would render its control
difficult. With this in mind, the contractors proposed to substitute an
English Blue Lias lime as a grouting material. Grout of fresh English
lime containing a moderate quantity of water set very rapidly in air to
the consistency of chalk. Its hydraulic properties, however, were
feeble, and in the presence of an excess of water it remained at the
consistency of soft mud. It was not suitable, therefore, as a supporting
material for the tunnel.
An American lime, made in imitation of the Lias lime, but having greater
hydraulic properties, was tried, but proved unsatisfactory. Two brands
of natural cement were also tried and rejected, but a modified
quick-setting natural cement, manufactured especially for this work, was
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
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.
The total quantity of grout used on the work was equivalent in set
volume to 249,647 bbl. of 1 to 1 Portland cement grout, of which 233,647
bbl. were ejected through the iron lining, an average of 14.93 bbl. per
lin. ft. The cost of grout ejected outside of the river tunnels was 93
cents per bbl. for labor and $2.77 for "top charges." East of the Long
Island shaft the corresponding costs were $0.68 and $1.63, the
difference being partly due to the large percentages of work done in the
normal air at the latter place.