Environmental Engineering Reference
In-Depth Information
(ii) If material is found to contain even small amounts (e.g. 1% to 5%) of carbonates,
gypsum, or much less than 1% of sulphide minerals, then it should be subjected to
further laboratory and field studies to assess its likely performance in the proposed
filter zone.
In the field the studies are primarily of the observational type. For all types of
materials a search should be made for existing (preferably very old) deposits of the
materials e.g. old fills, pavements or natural screes. In the case of alluvial materials
being considered for use, the actual deposit in question is also studied.
The deposits are carefully exposed by trenching or pitting. Evidence of any cohe-
sive behaviour is looked for during excavation and the exposed faces and spoil are
examined for any particles which have become cemented together. Evidence of
weathering or solution of particles is also looked for.
When judging the suitability or otherwise of a material, from the results of such
observational tests, differences between the environments (particularly moisture) in
the old deposit and the proposed filter zone must be taken into account.
(iii) Volcanic ash materials should also be checked carefully for long term chemical stability
and for any tendency to develop cohesion when compacted. As well as the laboratory
and field tests described in (ii) above, field compaction trials are advisable for sand-sized
volcanic ash materials.
9.6
DIMENSIONS, PLACEMENT AND COMPACTION OF FILTERS
9.6.1
Dimensions and method of placement of filters
9.6.1.1
Some general principles
The following factors should be taken into account when selecting the dimensions of zones
of filters.
(a)
For filters upstream and downstream of the dam core;
- The theoretical width to achieve filtering action (see below);
- The discharge capacity required along the filter zone e.g. for chimney drains (see
Section 10.3);
- The particle size distribution of the filter and the potential for segregation. Wider
filter zones should be used if segregation is likely and narrower zones may be used
for more uniformly graded materials;
- Potential displacement under major earthquake - filters need to be wide enough to
remain effective even if displacements occur in an earthquake;
- The size of the dam - there is a tendency to use minimum width of 2.5 m or 3 m for
very large (
100 m to 150 m high) dams, to allow for differential movement during
construction and simply for conservatism;
- The method of placement, as detailed below.
(b)
For horizontal, or near horizontal filters e.g. in horizontal drains, on the surface of
erodible foundations;
- The theoretical thickness to achieve filtering action (see below);
- The discharge capacity required along the filter zone e.g. for a horizontal drain (see
Section 10.3);
- The particle size distribution of the filter, the potential for segregation and maximum
particle size. As a guide, the horizontal filter layer thickness should be not less than
20 times the maximum particle size, to ensure that, given good segregation control,
there are very unlikely to be continuous coarse zones through the filter layer;
- The method of placement, as detailed below.
