Environmental Engineering Reference
In-Depth Information
Figure 10.14.
Dewatering system for cutoff trench.
Figure 10.15.
Slurry trench technique (1) Permeable virgin ground, (2) Substratum, (3) Excavation,
(4) Direction of progress, (5) Trench filled with bentonite slurry, (6) Filling with aggre-
gate, (7) Finished trench (ICOLD, 1985).
Figure 10.16.
Slurry trench excavation by dragline (Xanthakos, 1979, reproduced with permission of
McGraw-Hill).
Note that it may be difficult to form a low permeability connection between the cutoff
and the underlying low permeability stratum, particularly if the surface is irregular. An air
lift or clamshell or scraper should be used as shown in Figures 10.16 and
10.17
to assist
in cleanup of loosened debris at the base of the trench.
ICOLD (1985) indicates that the backfill will normally consist of bentonite slurry, with
5-15% bentonite (by weight), a marsh funnel viscosity of greater than 40 seconds, mixed
with well-graded sand and gravel between 0.02 mm and 30 mm size. The mixture should
have a standard concrete cone slump of 100-200 mm. Xanthakos (1979) indicates that
naturally occurring clays from the site may be used, although they are unlikely to be suit-
able where their liquid limit is greater than 60% and may be difficult to mix if they remain
