Environmental Engineering Reference
In-Depth Information
Figure 9.24.
Particle size distribution for core and filters for continuing erosion tests, Eucumbene Dam
(Bell, et al., 2001).
It is important to recognize the limitation of the laboratory test apparatus used to assess
the performance of a filter. If the filter is a non-erosion filter or perhaps a filter with a
slightly coarser grading, the ultimate seal may only affect a very small area of the soil/filter
interface in the test. On the other hand, for a very coarse filter, such as the in-place filter for
Eucumbene dam, one must be cautious when deciding if this test indicates “ultimate” seal-
ing or whether it shows significant localized weakening of the soil with the high likelihood
of the leak moving sideways. Care must also be taken in the laboratory test to prevent ero-
sion up the sides of the sample.
9.3.3
Excessive fines content in filters or transition zones
It is unfortunately not uncommon to be presented with information showing that the fil-
ters or transition zone in an existing dam were constructed with fines content (% passing
0.075 mm) greater than the 5% normally accepted as an upper limit and/or that the fines
are, contrary to accepted practice, plastic.
The question which arises is whether the filter or transition will “hold a crack”, and not
perform its filter function. Vaughan (1982) and ICOLD (1994) describe a simple test
attributed to Vaughan for assessing this.
“A simple test, suitable for use in a field laboratory, has been devised to examine filter
cohesion. It consists of forming a cylindrical or conical sample of moist compacted filter,
either in a compaction mould, or in a small bucket such as is used by a child on a beach;
