Environmental Engineering Reference
In-Depth Information
Figure 10.12.
Effect of grouted cutoffs on position of line of seepage: (a) flownet, k g
0.2 k f , (b)
flownet, k g
0.1 k f , (c) position of line of seepage for various values of k g / k f (Cedergren,
1972).
In most cases the economic depth is likely to be less than 10 metres. Beyond this depth
slurry trench and other wall cutoffs are likely to be more economic.
If dewatering of the permeable soil is necessary, it is likely that the dewatering has to
continue while the trench is being backfilled with earthfill. Figure 10.14 shows a possible
dewatering arrangement. However provision would have to be made for careful backfill-
ing of the collector pipes with grout, and making the backfill around the pipes filter com-
patible with the Zone 1 and alluvium, or erosion may occur into the pipe or backfill.
As shown in Figure 10.8 , a filter zone may have to be incorporated into the down-
stream side of the cutoff trench to prevent erosion of the earthfill into the foundation.
If a cutoff trench can be constructed it does provide a very good quality cutoff, with a low
permeability. For compacted clayey earthfill it would not be unreasonable to expect a per-
meability of 10 8 to 10 9 m/sec. The contact with the foundation can be of high quality and,
for rock foundation, grouting can be carried out if necessary from the base of the trench.
 
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