Open Cut » Environment
The aim of this project was to evaluate the performance of current rehabilitation techniques on dispersive Tertiary spoil. Studies included:
- Examination of the fundamental mechanisms of dispersion-related erosion;
- Sampling and testing of corresponding spoiled and unmined Tertiary overburden;
- Visual inspection of rehabilitated Tertiary spoil; and
- Computer modelling to investigate mechanical stresses and moisture flow.
Results from these revealed that the most significant change brought about by mining is simply that dispersive material is placed where free water can come into contact with it. Dispersive soil profiles that have not been mined, but that have water contacting the subsoil due to other disturbance, show erosion at least as severe as observed on Tertiary spoil. Further, because dispersion can initiate in still water, standard measures to control erosion by limiting runoff velocity are much less effective on spoils containing dispersive clay.
Consistent with this, the most successful rehabilitation strategies appear to be those that separate dispersive spoil from rainfall runoff, such as by burying or covering it with other spoil. Where this is not practical, continuous grass cover may provide a level of erosion protection, although there is risk regarding its long-term integrity. Tree and shrub cover without established grass and elevated ponds without soil or other spoil cover are not recommended for dispersive Tertiary spoil.
Even without mining, dispersive soil lands do not have high capability for grazing use, because of their susceptibility to severe erosion upon minor disturbance. Thus, it is suggested that alternative land values, for example nature conservation, be applied to rehabilitated spoil where dispersive behaviour cannot be prevented by burial.