Environmental Engineering Reference
In-Depth Information
greatest during operation of the storage, particularly early in the operation when there is
little blanketing effect from tailings. It will be exacerbated if water is allowed to pond over
the tailings and adjacent to the natural ground. Seepage will continue after shutdown,
even when the tailings are covered, because infiltration of rainfall will occur. Granted this
is the case, it will often be more practic-able to limit expenditure on seepage “control” (or
“prevention”) and to concentrate on design measures to collect the seepage. This may
then be pumped back to the storage or the process plant, or diluted with surface runoff
before release at acceptable concentrations. Measures which may be taken include:
19.6.5.1
Toe drains
A drain may be provided at the downstream toe of the embankment, to collect seepage
which emerges at that location.
These drains can be reasonably successful in intercepting seepage, but only if the seep-
age naturally emerges in this location. In many cases, the flow rates will be such that the
phreatic surface stays below the level of the drain. Even when the seepage is sufficient to
raise the phreatic surface to flow to the drain, much may still bypass by flowing beneath
the drain. Ideally the drain has to penetrate to a low permeable stratum, but this is often
not practicable.
Such drains may also intercept surface runoff from the downstream face of the dam and
groundwater from downstream (if the water table is high) and, if the seepage is to be returned
to the dam or process plant, may exacerbate water management problems if a “no release”
system is being operated.
19.6.5.2
Pump wells
Seepage can be collected by constructing water wells into pervious strata downstream of the
tailings embankment and pumping from these back into the storage or to the process plant.
Such a well system can be reasonably successful in intercepting seepage but there are
some disadvantages:
- The pumps lower the piezometric pressures downstream of the storage, so gradients
and seepage rates from the storage may be increased;
- The wells also attract water from downstream and so may also add to water manage-
ment problems in no release operations;
- The wells have to be pumped continuously to be effective, with all the associated costs;
- The well screens and pumps are susceptible to corrosion and blockage and require
maintenance and periodic replacement;
- It is unlikely that it is practicable to operate the wells after shutdown of the storage, so
another method may be needed to handle long term seepage.
19.6.5.3
Seepage collection and dilution dams
In many cases, a practical way of collecting seepage from tailings storages will be to con-
struct a seepage collector dam or dams.
Figure 19.32
shows such a system.
The seepage collector dams may be located sufficiently close to the storage to collect the
bulk of seepage, but not too far away so as to limit the external catchment, e.g. Dam A on
Figure 19.32. In this case water would normally be pumped back into the dam or to the
process system.
Alternatively, one may deliberately locate the collector dam sufficiently far downstream
to ensure that the runoff from the catchment to the dam is sufficient to dilute the seepage
to acceptable water quality, e.g. Dam B in Figure 19.32.
Whether such an approach is acceptable will depend on the particular circumstances
for the tailings storage. For example, it may be unacceptable to have substandard water
