Environmental Engineering Reference
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properties. It is not sufficient to rely on a specification and assume those who built the
dam followed the specification. Too often it has been found that materials with a higher
fines content or coarser than specified have been used, particularly near the crest of the
dam, as borrow areas became worked out. There seemed to be a lack of awareness in the
1950s and 1960s of the need for good filters near the crest of the dam, so the quality con-
trol was sometimes relaxed. Some dams even had the filters omitted near the crest (even
though they were shown on drawings). Such “sins and omissions” were not restricted to
small organisations - it happened in large organisations with good reputations as dam
engineers.
The authors prefer to have available the following:
- Particle size distributions on samples taken from the dam during construction (“record
testing”);
- As-constructed drawings;
- Photographs of construction (illustrate whether, for example, filters were placed ahead
of or after rockfill, or were placed in high lifts with subsequent risk of segregation);
-Test pits into the dam, to log and sample the relevant zones, with associated particle
size distributions.
9.3.2
Continuing and excessive erosion criteria
Mark Foster (Foster, 1999; Foster and Fell, 1999a, 2001) used the test data from Sherard
(1985a, b) and additional tests using no-erosion test equipment similar to that shown in
Figure 9.3 to develop the concept of no erosion, some erosion, excessive erosion and contin-
uing erosion shown in Figure 9.20.
The terms are defined as:
(i) No erosion : filter seals with practically no erosion of the base material;
(ii) Some erosion : filter seals after “some” erosion of the base material;
(iii) Excessive erosion : filter seals, but after “excessive” erosion of the base material;
(iv) Continuing erosion : the filter is too coarse to allow the eroded base materials to seal
the filter.
The continuing erosion boundary was determined by carrying out additional filter tests,
using a modified version of the NEF test, called Continuing Erosion Filter (CEF) tests, to
Continuing Erosion
Boundary
CONTINUING
EROSION
DF15
EXCESSIVE
EROSION
Excessive Erosion
Boundary
SOME
EROSION
No Erosion
Boundary
NO EROSION
Other factor e.g. DB85
Figure 9.20.
Filter erosion boundaries (Foster, 1999; Foster and Fell, 1999a, 2001).
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