Environmental Engineering Reference
In-Depth Information
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D 15F /D 85B
(a) SANDY BASE SOIL
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D 15F /D 85B
(b) FINE CLAY AND SILT, LITTLE OR NO SAND SIZED PARTICLES
Figure 9.25.
Conceptual distribution of D 15F /D 85B for (a) clean sandy base soil, (b) base soil fine clay and
silt with little or no sand sized particles, with design, no erosion and continuing erosion
criteria.
standing the sample in a shallow tray (if a bucket is used the operation is exactly as build-
ing a child's sand castle) and carefully flooding the tray with water. If the sample then col-
lapses to its true angle of repose as the water rises and destroys the capillary suctions in
the filter, then the filter is non-cohesive. Samples can be stored for varying periods to see
if cohesive bonds form with time. This test is, in effect, a compression test performed at
zero effective confining pressure and a very small shear stress, and it is a very sensitive
detector of a small degree of cohesion.”
The authors' judgement is that it is the combination of fines content, the plasticity of
the fines and degree of compaction which is important. One is more likely to accept that
a filter with excessive and/or plastic fines will collapse and not hold a crack, if it is not
well compacted. Each case should be considered on its merits, taking into account
whether zones downstream of the filter may meet continuing erosion criteria, so even if
the filter holds a crack, erosion will eventually seal.
 
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