cement grout ejected through similar holes, reinforced the rolled-steel
ring against external water pressure. In two of the tunnels the concrete
lining was carried completely through the junction, and covered the
whole construction, while in the remaining two tunnels it was omitted at
the rolled-steel ring, leaving the latter exposed and set back about 3
in. from the face of the concrete.

[Illustration: FIG. 2.]


GROUTING.

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