Environmental Engineering Reference
In-Depth Information
used for soaking. The sodium sulphate penetrates fine cracks in the aggregates and
expands on crystalization in the drying phase, breaking apart the aggregate particles.
The test is particularly severe for most dam applications where high salt contents are
not present. This is reflected in the dam specifications in
Table 9.10
, where the require-
ments are less severe than for coarse concrete aggregates (similar for fine aggregates).
If materials are testing as marginal for this test, it is important to inspect the prod-
uct of breakdown. In some rocks the product will be a fine silt and sand, which is
likely to affect filter permeability markedly. In others it is simply the breakup of coarse
particles into two or three smaller particles, which does not affect permeability greatly.
It is quite common to have different performance in these tests depending on the
size fraction being tested. This is particularly the case for naturally occurring aggre-
gates from a river which drains a mixed geological environment where the mineralogy
of the individual particles may vary substantially.
The authors' opinion is that for important tailings dams which are storing water with
a high acidity or high salts content, it may be necessary to impose more severe require-
ments on sulphate soundness than normally accepted for water dams. It may also be nec-
essary to test with the more severe magnesium sulphate if the water in the tailings dam has
a high magnesium sulphate content.
9.5.2.2
Possible effects if carbonate rocks are used as filter materials
Rocks containing significant proportions of carbonate minerals (e.g. limestone, dolomite
or marble) may perform satisfactorily in the tests in 9.5.2.1, but could be unsuitable for
use in filter zones, because any of the following effects may be possible, during the lifetime
of the dam.
(a) Change of grading, due to dissolution;
(b) Partial dissolution and recemention;
(c) Interlocking of grains due to pressure-solution.
Effect (b) or (c) could cause a filter zone to become cohesive and be capable of sustain-
ing an open crack or erosion tunnel. All 3 possible effects are discussed in more detail in
3.7.7, 3.7.7.1, 3.7.7.2 and 2.9.4.
The authors consider that carbonate materials should not be adopted for use in a filter
zone, until the risk of malfunction due to any of the above, during the lifetime of the dam,
has been assessed by experienced geochemical specialist or team.
9.5.2.3
Effects if rocks containing sulphide minerals are used as filter materials
Rocks containing small amounts (1% or less) of sulphide minerals may also perform sat-
isfactorily in the tests in 9.5.2.1, but in service, the sulphides will oxidise and produce sul-
phuric acid and sulphate minerals. Effects can include clogging or cementation of the
filter, by sulphate minerals (or iron hydroxides?) and acid drainage problems (see 2.9.4
and 3.7.8). The effects may also include weakening of the filter rock and acid attack on
other embankment materials.
The authors recommend that rocks containing sulphide minerals should be avoided,
whenever possible, when selecting filter zone materials.
9.5.2.4
Other investigations for filter materials
In the light of all of the above, the authors recommend that as well as the laboratory tests
discussed in 9.5.2.1, the following should always be undertaken:
(i) If the proposed material is bedrock, quarried and crushed, or alluvial sand/gravel, the
mineral composition of the particles should be determined.
