Environmental Engineering Reference
In-Depth Information
(a)
(b)
Figure 10.28.
Two stage panel excavation with percussive tools and clamshell grab: (a) excavation of
pilot holes with percussive tools; (b) panel excavation with clamshell bucket (Xanthakos,
1979, reproduced with permission of McGraw-Hill).
- “Colloid protectors” (called peptizers, or dispersing agents by Xanthakos, 1979) may
be used to prevent flocculation.
Xanthakos (1979) indicates that even if trenching into soil with groundwater of high
salts content, even of seawater quality, the slurry only takes up ionic salt and the floccu-
lation can be controlled. If only saline water is available, attapulgite may be substituted
for bentonite.
More information on muds can be obtained from Xanthakos and from Chilingarian
and Vorabutr (1983).
10.4.7
Permeability of cutoff walls
ICOLD (1985) indicate that the permeabilities which can be achieved for cutoff
walls are:
-10
7
to 10
8
m/sec - for grout walls;
-10
8
to 10
9
m/sec - for plastic concrete walls;
-10
9
to 10
10
m/sec - for concrete;
- compared to 10
6
m/sec for a grout curtain.
Millet et al. (1992) give similar values.
Powell and Morgenstern (1985) reviewed published case histories and concluded that
the range of permeabilities achieved was as shown in
Figure 10.29
.
The ICOLD (1985) figures are more likely to reflect the properties of the materials in
the cutoff wall, while the Powell and Morgenstern (1985) values reflect the equivalent
permeability of the constructed wall. The permeability of well constructed well con-
structed walls should approach that given by ICOLD (1985).
