Mitigating against induced stress from the Merensky Reef pillars on the UG2 Reef mining environment, Tumela 1 Shaft, Anglo American Platinum

Hunter, Carol
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Tumela 1 Shaft of Amandelbult Section, Anglo American Platinum, is situated south of the town of Thabazimbi in Limpopo Province of South Africa. The Platinum Group Metals (PGMs) are mined from two prominent, narrow tabular, shallow dipping reefs; the Merensky Reef and the Upper Group 2 (UG2) Reef. This mine has experienced an increase in seismic activity from the Merensky Reef stable pillar failures which has increased the risk of mining UG2 Reef close to these potentially unstable pillars. Also, the UG2 Reef areas under Merensky Reef remnants are encountering unusual induced stress effects, which is abnormal for the UG2 Reef shallow depth of mining. The Merensky Reef pillars are in a critical state of stability and stress redistribution by minor UG2 Reef mining can trigger seismic events. In several cases, Merensky Reef seismic events, remote from any UG2 Reef mining, are also triggered. This makes Merensky Reef pillar failures unpredictable. As these seismic events are unpredictable, one must cater for the possibility that seismic shake-down and the effects of induced stress on UG2 Reef can potentially injure people underground. The seismicity and the effects of the induced stress from the Merensky Reef remnants cannot be prevented or predicted - but they can be managed. To mitigate is to lessen the severity of an occurrence. In the context of mine seismicity, this will involve looking at the entire seismic event: from the source, any triggers, the event properties, and the effect on the UG2 Reef. To be proactive, there is a need to understand what is occurring on the Merensky Reef horizon before mitigation against the stress effect on the UG2 Reef can be done. There is limited information available on this area of research, so the challenge is to ensure sufficient investigations and knowledge transfer to carry forward as the mine progresses. The focus of this research is to investigate the induced stress from the Merensky Reef pillars on the UG2 Reef mining environment at Tumela 1 Shaft Mine. This is done by two approaches: firstly, underground investigations into whether the Merensky Reef dynamic pillar failures and Merensky Reef remnants affect the UG2 Reef mining as expected; and secondly, to investigate additional strategies to manage the induced stress on UG2 Reef mining whilst maximising extraction. Numerical modelling methods are used, and research suggestions have been applied. Since the seismic monitoring is in place and there are strategies to mitigate against most of the seismic events, the risk still lies with the exposure of mining personnel to potential shake-down and also to the effects of the Merensky Reef stresses on the UG2 Reef. The prediction of seismic events is not possible but a correct, proactive approach can reduce the exposure of individuals to the risks associated with seismic events. Tumela 1 Shaft multi-reef environment is unique, so further mitigation measures against the effects of the induced Merensky Reef stress on the UG2 Reef mining are suggested.
A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, 20 December 2019