Figure 1: Typical in-pit tailings storage
In-pit tailings storage, as the name suggests, is simply the process of backfilling abandoned open pit surface mines with tailings (figure 1). This method is very attractive to a mine operator as worked out voids can be filled at a fraction of the costs associated with designing, constructing and operating a conventional, thickened, paste or dry stack facility. Another advantage to in-pit storage is that the tailings do not require retaining walls, thus the risks associated with embankment instability are eliminated (EPA 1994).
The main disadvantage to in-pit storage of tailings is that the potential for groundwater contamination below and around the void can be very significant (DME 1999). Other disadvantages to in-pit storage are:
- Rapid rates of rise occur particularly in the early stages of deposition when the pit is at its deepest and the exposed surface area the smallest. This reduces the solar drying and desiccation potential of the tailings resulting in low strength and poor consolidation properties (DPI 2003).
- The stability of underground mines in the vicinity to an in-pit tailings facility may be jeopardised. Liquefied tailings may rush into underground voids resulting in catastrophic consequences or the increasing weight of the overlaying tailings may cause convergence in underground roadways. The Mufulira disaster of 1970 claimed the lives of 89 miners when a tailings inrush flooded the underground workings (DME 1999).
- Poor consolidation can result in long durations of surface deformation after a pit has been filled. This is mainly due to the low solids content of the tailings and the depth of the stored material. Pits will have to be continuously topped up with tailings until the consolidation rates are minimal and a rehabilitation cover can be implemented and contoured successfully (DME 1999).
- Groundwater bores will have to be installed around the pit to monitor the seepage plumes. In some cases it may be necessary to monitor these long after a pit has been filled and to even pump out the seepage by wells to prevent groundwater contamination (DME 1999).
- As a consequence where environmental laws exist the backfilling of pits is either forbidden or controlled to prevent excess groundwater contamination.
- Sterilisation of reserves as modern block cave or cut and fill underground mining methods would be too hazardous to contemplate. Remote mining operations in Australia have used in-pit storage for decades where the groundwater is saline or non-potable. Also, the aboriginal stakeholders prefer voids to be filled after mining has ceased and grant permission to backfill suitably located pits.
