Environmental Engineering Reference
In-Depth Information
- Stringent safety precautions against gas in confined spaces;
-Downstream lagoons for long-term treatment of seepage waters.
Roadford Dam (Devon, UK)
. This is an asphaltic concrete faced rockfill embankment
45 m high. It was built in 1988-89 (Cripps et al., 1993). The rockfill materials are sand-
stone and mudstone with average pyrite content of 1.12%. A basal drainage blanket was
formed by altered dolerite, which contained 11% calcite. Some degradation of the fill was
allowed for in the selection of shear strength parameters and of freeboard.
On first filling in 1989, drainage water was found to have unacceptably high levels of
sulphate, calcium, iron and other minerals. Rapid pyrite oxidation and sulphuric acid attack
on the calcite was indicated. From the results of tests on the rockfill and blanket materials
about 30 months later, Cripps et al. (1993) conclude that the amounts of calcite removed were
so small that the effect on the geotechnical properties of the materials would be negligible.
However they note that the drainage water requires treatment, probably “for some years”.
2.9.4.2
Possible effects of sulphide oxidation in rockfill dams
The four cases discussed above indicate that if materials used in rockfill dams contain sul-
phide materials, the following can occur:
1. Seepage waters containing sulphuric acid and metallic sulphates, which may require
ongoing remedial treatment;
2. Attack on concrete by sulphuric acid and/or sulphates;
3. Lowering of strength of individual particles in rockfill, giving rise to settlement or low-
ered shear strength of the embankment.
If an embankment also contains carbonate rocks or minerals, in its rockfill, drains, filters,
or in its foundation, these could be attacked by sulphuric acid to cause any of the following:
4. Clogging or cementation of drains or filters, by sulphate minerals;
5. Loss of material and changes to gradings, in carbonate rockfill and/or filter zones, giv-
ing rise to settlement and/or impaired filter performance.
Whether or not any of the above effects occur to an extent affecting the successful oper-
ation of a project would depend on the interaction of many site-specific factors. It is pos-
sible also that some of the above effects could occur when a dam of any type is built on a
foundation (or with a storage area) containing sufficiently large amounts of sulphide min-
erals. See further discussion and recommendations in Section 3.7.7.
2.9.4.3
Sulphide oxidation - implications for site studies
In the light of all of the above it is important that site investigations for dam projects
include checking for the presence of sulphide minerals and assessment of the risk of pos-
sible problems associated with their breakdown. Such studies should include:
(a) checking for any known sulphide mineralization or acid drainage problems in the
region;
(b) checking for sulphides and for past and current oxidation effects, during geological
mapping and during logging of test excavations and drill samples;
(c) testing streams and groundwaters for pH, electrical conductivity and chemical com-
position;
(d) examining samples petrographically to assess the types, concentrations and modes of
occurrence of any sulphide minerals found during (a) or (b);
(e) accelerated weathering tests or other tests to simulate the proposed in-service
environment.
