Environmental Engineering Reference
In-Depth Information
Ice Walls
Ice walls provide a temporary expedient for controlling seepage during the construction
of open excavations, tunnels, and shafts. They are most useful in thick deposits of “run-
ning sands” and saturated silts, or where grout materials may contaminate the water sup-
ply. Ice walls have been used for shaft construction to depths of over 1000 ft in the mining
industry for many years. The procedure for deep shaft construction is as follows
(Lancaster-Jones, 1969):
1.
A freezing plant is installed with adequate capacity to ensure that the wall will
be sufficiently thick and continuous.
2.
One or two rings of cased boreholes are drilled outside the shaft perimeter to
depths sufficient to penetrate fully the saturated, potentially troublesome zone,
and brine pipes are installed.
3.
The ice cylinder is formed by pumping brine solution of calcium chloride
injected at
20
°
C, or even liquid propane at
44
°
C.
4.
The cooling process usually takes from 2 to 4 months, and when complete, the
shaft sinking begins. Care is required not to damage the brine pipes, which must
continue to function during the excavation.
5.
Concrete is normally used for lining but the temperature changes during hydra-
tion; when the ice thaws it causes substantial cracking, requiring repairs with
cement or epoxy grouts.
The method is time consuming and costly, generally causing delays of about 6 months
or more. Careful installation is required to ensure a thick and continuous wall, since even
a small flow through an opening can be disastrous during excavation. Substantial ground
heave may occur as a result of freezing operations near the surface, followed by ground
collapse when thawing occurs. The impact on nearby structures must be evaluated care-
fully to guard against distress. The amounts of heave and subsequent collapse depend on
the types of materials near the surface.
Grouting
Applications
Grout injection into pervious soils and rock formations is a common, permanent solution
to contain flows, but often provides an imperfect wall. Grouts are also used to strengthen
soil and rock formations. During construction, grouting is used beneath dam foundations,
in tunneling, and in excavations. In the last case, grout is injected behind pervious sheet-
ing to provide flow control. Postconstruction installations usually serve as corrective
measures to control flows through or beneath embankments.
Types
Grouts can consist of soil-cement mixtures, cement, or chemicals. Common cement and
chemical grouts, their composition, and application in terms of the characteristics of the
materials to be treated are summarized in
Table 8.7
;
some are also shown in
Figure 8.35.
Selection
of the grout type depends on the porosity of the geologic formation to be treated,
the rate of groundwater flow, and the desired compressive strength of the grouted formation.
In general, sand-cement grouts are used to seal large cavities and fractures, and clay and
Portland cement grouts are used to seal relatively small fractures and coarse-grained soils.
Microfine cement is used to grout openings that cannot be penetrated by Portland cement
(Weaver, 1991). Bentonite gels are relatively low-cost and seal sands, but do not bring about
any strength increase. Chemical grouts are used in fine-grained soils with effective particle
