Environmental Engineering Reference
In-Depth Information
Figure 10.8.
Control of erosion of earthfill into permeable foundation using a filter.
When checking the design of the horizontal drain the following should be noted:
-Water from the abutments will flow towards the lowest part of the drain before
flowing out the toe of the dam. Hence the required flow capacity per unit width in
this area will be greater than the average flow per unit width under the dam;
- If a 3 layer filter drain is used, Zone 2B will dominate the discharge capacity;
- Conservative estimates of foundation permeability should be used for the design
of the horizontal drain, since failure to provide adequate capacity can lead to
failure of the dam, or the requirement for a stabilizing berm if the problem is
recognised;
- Similarly, conservative estimates of the horizontal drain permeability should be used,
and care taken to ensure the drain is not contaminated by soil from the foundation
during construction. Generally it is good practice to provide a 3 layer drain, so there
is a large discharge capacity;
- The downstream toe should be designed to ensure the drain outlet is not blocked by
erosion of soil off the embankment or the abutment. It is wise to provide a berm at
the toe of the dam above the drain to collect any material eroded off the downstream
face of the dam. (as shown in
Figure 10.1
)
;
- Place the filter drain in contact with the higher permeability strata in the foundation.
So, as shown in Figure 10.8, the lower permeability sandy clay-silty sand has been
removed. Specify that the foundation not be rolled or trafficked by construction
equipment which would reduce the permeability under the filter drain.
10.3.2
Treatment of the sides of the cutoff trench
Erosion of the earthfill core into the sides of the cutoff trench may occur where:
- There are open jointed rocks, with a joint opening large enough to allow the soil to
erode into the joint e.g. in columnar basalt and other volcanics, pyroclastic rocks, open-
jointed sandstone or other rock;
- Coarse gravels in alluvial or colluvial soils are present;
- There are fissures or holes in lateritic soils or weathered rock.
Figure 10.8 shows one way of controlling erosion into a soil foundation. The filter
would have to satisfy critical filter criteria (see Chapter 9) in regard to the Zone 1 earth-
fill, and the “openwork” layer of alluvium. In open jointed rock it is normal to cover the
open joints with shotcrete or other concrete, before placing the earthfill. This may also be
done in cutoffs into soil and weathered rock.
