Environmental Engineering Reference
In-Depth Information
The following should be noted:
- It is difficult to predict pore pressures near the toe of a dam, because they are greatly
affected by local variations in permeability (see for example Figures 11.14 and 11.15).
Hence it is necessary to be conservative and to provide piezometers to check the
assumptions;
- The berm will be most effective if it is free draining;
- The pressure relief wells will provide greatest benefit at each well and least benefit midway
between wells. For conservatism the design should be based on the pore pressures midway
between wells. Again, piezometers are required to check the effectiveness of the wells.
10.4
CONTROL OF FOUNDATION SEEPAGE BY CUTOFFS
10.4.1
General effectiveness of cutoffs
As shown in
Figure 10.1
, seepage through a permeable foundation can be reduced by con-
structing a low permeability cutoff through the permeable material. This may consist of:
- cutoff trench filled with earthfill;
-slurry trench;
- concrete diaphragm wall;
- contiguous or intersecting bored piles;
-sheet pile wall;
-grout curtain.
Such cutoffs will have varying degrees of effectiveness depending on their permeability
and the depth to which they are taken. This is illustrated in
Figures 10.11
and
10.12
which are reproduced from Cedergren (1972).
Figure 10.11 shows the effect on the line of seepage in the downstream shell of an
embankment and the effect on exit gradients of cutoffs constructed to varying propor-
tions of the total depth of a permeable foundation (note the example assumes the foun-
dation and downstream fill zone have the same permeability).
It can be seen that there is not a significant improvement even with a cutoff which is
90% penetrating. Only where the cutoff is fully penetrating and properly connected into
the low permeability zone will the seepage pressures be controlled. This is often difficult
to achieve as will be discussed below.
Figure 10.12 shows the effect of fully penetrating grouted cutoffs on the line of seepage
in the downstream zone. Unless the grouted zone (or cutoff constructed by another
method) has a permeability much less than the foundation, there is little reduction in
downstream pore pressures. This applies to soil and rock foundations.
The effectiveness of partially penetrating cutoffs will depend on layering of lower and
higher permeability soils in the foundation. If there are continuous low permeability lay-
ers present, partially penetrating cutoffs can be effective. However it must be expected
that, unless a cutoff is fully penetrating and of low permeability (say 10 to 100 times less
than the permeable foundation), it will have little benefit in the reduction of leakage and
exit gradients.
Figure 10.13
shows some important implications of constructing effective slurry trench or
similar cutoffs. As shown in Figures 10.13(a) and (b), the seepage through the dam may
flow towards the foundation downstream of the cutoff, and a filter may be required under
the earthfill to prevent erosion. Figure 10.13(c) shows the problem of how far a cutoff
should penetrate into the abutments, to prevent seepage flowing around the ends.
