Highwall slope instability related to large pit-ward dipping thrust fault occurred in a large strip coal mine in the Bowen Basin, Queensland. The inability to identify the location of thrusts and predict potential instability proved difficult. A series of highwall instability events developed into a 700m-long tension crack located 80m from the crest, resulting in a large block of the highwall creeping towards the active open pit workings. The risk of further highwall instability was high with the potential for premature closure of the pit and loss of coal reserves at a time of high global demand for quality coking coal.
The geotechnical setting, the failure mechanism identified by detailed geotechnical investigation and the geotechnical engineering approach to managing risk associated with this issue while continuing mining operations are described.
The softwall method consists of a highwall slope design where the rock mass is blasted beyond the pit limit to disrupt rock defects. The softwall design was successfully adopted as the preferred geotechnical slope design to manage potential highwall instability in the final mining strip for the pit.
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