Managing Excessive Pit Wall Deformation of Weak Rock Mass
Authors: Salina Yong, Daniel Yang, Justine Ealy, Christina Foster
Conference: International Slope Stability 2022 Symposium
Date: October 17-21, 2022
ABSTRACT
Rapid acceleration of a large-scale creeping instability occurred in an open pit operation in 2016. A 150-foot-tall back scarp formed, tension cracks appeared near the pit rim, bulging formed in the lower slope, and debris reached the pit bottom. The instability displaced material over a hundred feet and buried the main access road. Monitoring of the initially slow-moving mass had commenced in 2013 with terrestrial radar, which was integral to managing the risk along with strategic mine planning. As a result, the instability was successfully managed and did not lead to safety incidents or equipment losses. Re-establishing access necessitated routing through the slide debris, which posed a risk to the road development due to the potential of heightening the back scarp. Dewatering and removal of the slide toe also posed risks to the road development and mine economics.
Analyses were completed to evaluate the stability of the back scarp and the proposed mining sequence of the slide toe with UDEC, RocFall, and Slide3. The analyses assisted rehabilitation by identifying influential factors, estimating the rockfall impact, and identifying areas of concern. The pit was successfully rehabilitated, not only with the insights provided by the analyses but also the vigilant guidance provided by the mine geotechnical team through real-time slope monitoring and inspections, continuous maintenance, and a flexible mindset for impromptu adjustments when needed. Mining of the pit was completed in 2020 and backfilling is ongoing. Recent monitoring data indicates the instability has stabilized at a displacement rate of nearly zero. This paper summarizes a case history where excessive deformation of a pit wall exposed in weak rock mass was successfully managed.
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