Environmental Engineering Reference
In-Depth Information
Figure 19.9.
Subaerial placement of tailings (Robertson et al., 1978).
are required to contain the same quantity of tailings and long term rehabilitation is more
difficult than if subaerial deposition is used, because the surface of the tailings is wet and
low strength. If water cover is maintained on the tailings, seepage quantities from the stor-
age will be relatively high.
The method where the tailings are discharged in a “beach” above the water pond and
where the deposition is cycled so that the tailings are allowed to dry by desiccation is
known as subaerial deposition.
Desiccation induces negative pore pressures in the tailings, which results in consolidation
of the tailings to higher densities than by subaqueous methods. The permeability and com-
pressibility are also reduced, although cracking by desiccation may affect the permeability.
In the ideal situation where the climate, storage area and tailings permit, the tailings will
remain in a partially saturated condition and result in significantly reduced seepage from the
storage. Post placement consolidation may be very small and the strength such that access on
to the surface for rehabilitation can be easy. Figure 19.9 shows a conceptual design of such a
system (Robertson et al., 1978).
Blight (1988) also discusses the benefits and disadvantages of desiccation. He points
out the risks of planning to use the benefits and then not being able to achieve them. He
cites an example where loss of control of water return resulted in the achieved dry density
being about half the expected. The authors' experience is that the technique can be suc-
cessfully used, if an adequate area is provided to accommodate the rate of tailings pro-
duction, the evaporation is sufficiently high and care is taken to operate the area properly.
In such cases, even high clay content tailings have been desiccated sufficiently for use with
upstream construction.
Table 19.6
summarizes some of the advantages and disadvantages of subaerial depositions.
The authors have seen cases where increased production of tailings, poor control of
return water, low winter evaporation, and a lack of consideration of the fact that early in
a dam's life the surface area is often less than later when the tailings level has risen, have
led to virtually no desiccation being achieved.
Blight (1988) also cautions against drying the tailings to the stage where they crack, as this
may lead to a potential for piping failure. This would only be a problem in particular instances
of upstream construction using small paddocks where the desiccation cracks could penetrate
from the water to downstream, as is common in South Africa and parts of Australia.
In general, one will not aim for the “full” subaerial deposition method shown in
Figure 19.10
.
This was developed for uranium tailings, where requirements for seepage
control are very stringent. The costs of underdrainage will not be warranted in most cases.
One should, however, aim to obtain as much benefit as possible from desiccation for the
reasons outlined above. It is important to consider the “whole of life” of the facility because
many of the benefits of subaerial deposition and desiccation are in reduced rehabilitation
and long term environmental impacts.
