Open Cut » Environment
As open-cut coal mines in the Bowen Basin mature, closure planning for final voids will become of paramount importance. Coal mining companies across Queensland currently plan a range of approaches in dealing with final highwalls ranging from a minimalist fencing through to comprehensive blasting and rehabilitation strategies. Most have already adopted a fencing and bunding strategy, with some allowance for local stabilization work. However, a substantial focus on closure and treatments for highwall structures and the final void has yet to be undertaken by most mining operations, principally as closure is perceived to be a somewhat distant event.
The regulatory requirement is not clear. Queensland mines operate under environmental authorities that provide for minimal treatment of the highwall, for example bunding and fencing. However at the same time the Regulator has an implicit requirement that the wall must be stable and safe. Inspections of many highwalls in Central Queensland carried out as part of this investigation confirm that for the most part, highwalls comprising Tertiary material are particularly unstable due to the dispersive, reactive and poorly consolidated nature of the Tertiary strata. Furthermore, some of the Permian highwall strata, although generally far more competent than Tertiary strata material, shows definite signs of existing instability with prospects for substantial longer term instability if left as is after mine closure.
Currently there are several hundred kilometres of active highwall in Central Queensland. More mines are developing, more are planned, and within 30 years to 40 years, the cumulative length of highwall could approach 1000 km or more. There is a considerable challenge for both the industry and the Regulator in determining the most appropriate closure treatments.
This ACARP investigation into highwall stabilisation has been undertaken to provide an insight into stability issues associated with open cut highwalls and to consider a range of alternative treatments that may have application in stabilising and rehabilitating those structures. The investigation has included inspections of representative highwalls in several Queensland mines and a desktop literature review of overseas practices. The report discusses various highwall treatment strategies that may have some application to the industry in support of its closure planning programs. Overall the investigation has attempted to:
- Identify the main mechanisms involved in highwall instability
- Discover the range of management strategies planned for highwalls at mine closure, and any trials to develop these
- Use the resultant understanding of stability in active and medium-term highwalls as a basis for proposing possible treatments for final highwalls
- Review practices and developments in other Australian states and internationally that can provide cues for potential highwall treatments in Queensland
- Develop cost comparisons of various treatments to support mining operators in initial closure cost estimations using various treatment strategies.