Environmental Engineering Reference
In-Depth Information
In many cases the presence of a water pond does not create difficulties, but if the water
is contaminated it may be unacceptable. In this event it will be necessary to construct a
spillway as shown or to fill the depression with waste rock, more tailings etc. However
this filling will in itself induce additional settlement. Mounding of the tailings and cover
prior to shutdown can help alleviate the problem. If tailings are placed subaerially and
well desiccated, they will have dried to a high strength, low compressibility landfill and,
provided the surface is adequately contoured, water ponding will not be a problem.
19.6.6.2
Erosion control
This can be a major issue. Wind erosion, as well as water erosion, has to be considered.
The embankment side slopes are best covered by rock if this is available, otherwise flat
(flatter than say 3H to 1V) slopes with good vegetation are required. This is often impos-
sible to achieve on a year round basis.
The tailings surface may be covered with soil, waste rock or a combination of the two
and vegetated to control erosion and limit infiltration. Some tailings can be successfully
revegetated without the need for cover. Cement stabilization has also been used.
Figure 19.47
shows some examples.
It is often necessary to carry out trials during operation to determine what measures
will be successful.
Blight (1988) and Blight and Caldwell (1984) describe measures taken to alleviate ero-
sion from abandoned gold tailings dams. Ritcey (1989) also discusses the design of cover.
Forrest et al. (1990) and Corless and Glenister (1990) also discuss erosion control measures.
19.6.6.3
Seepage control
In many cases the deposited tailings will have contaminants trapped in the accompanying
water when operations cease. These contaminants will continue to seep from the tailings
as they consolidate, and the water will infiltrate through the cover leaching contaminants
as it passes through the tailings.
As was the case during operation, this may or may not be a problem, depending on the
concentration of contaminants that reaches groundwater wells or streams in the vicinity. If
the contaminants are likely to be a problem, measures will have to be taken to limit infil-
tration. This can be achieved by redirecting external catchment flows (see
Figure 19.33
)
,
contouring the surface of the tailings to encourage runoff, avoiding ponding of water
as shown in
Figure 19.45
,
and encouraging transpiration by planting vegetation on the
tailings.
It is unrealistic to consider that a “clay cover” can be provided which will “seal” the
tailings. In the first place, the clay will have a finite permeability even if well compacted
and, in any case, the clay may be difficult to compact, because the tailings do not form a
strong base. The permeability of the cover will be increased by cracking due to desiccation
and settlement and penetration by roots and animals.
It is more realistic to design the total system on the assumption that there will be long
term seepage, and if the resultant contaminant concentrations are too high, to design
seepage collector dams to allow dilution to acceptable concentrations prior to release.
Figure 19.46
shows the components involved in assessing the infiltration into tailings.
In this diagram P
precipitation (rainfall); E
evapotranspiration; R
runoff;
I
(SW).
Figure 19.47 shows some alternative “covers” which may be used. These are in increas-
ing degree of sophistication and increasing cost from (a) to (d). It should be emphasized
immediately that it is seldom practicable to use such complicated and costly multiple layer
systems as shown in (d) for tailings (or waste rock). Such systems may be used in cover-
ing hazardous waste landfills.
infiltration; SW
soil water stored in the “cover” and I
P-E-R-
