Authors: Clayton, C; Jackson, A; Price, J; Bidwell, A; Elmo, D

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Clayton, C, Jackson, A, Price, J, Bidwell, A & Elmo, D 2020, 'Case study: analysis of a highwall toppling failure and development of a successful mine re-entry plan using RS2, RocFall and Dan-W at a coal mine in Canada', in PM Dight (ed.), Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 383-398,

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A highwall failure at an open pit coal mine in western Alberta, Canada occurred on 9 July 2017. A larger failure occurred on 5 August 2017. Radar monitoring successfully identified this second failure prior to the event. The initial assessment of the failure mechanism suggested toppling of steep bedding. The stratigraphy of the highwall consists of thinly to thickly bedded sedimentary strata dipping south at angles of about 65 to 70° into the wall. Neither failure resulted in injury or damage to vehicles or infrastructure, however, mining of the pit was suspended pending an assessment of the wall stability and the development of an appropriate reentry mining plan. Following the failure, two principal hazards remained. The eastern portion of the highwall slope was characterised by potential rockfall hazards beneath the failed slope area. The western portion of the slope was characterised by a larger rock mass which had not failed and presented a toppling hazard and potential debris runout hazard. Prior to re-entry into the pit, a re-entry strategy was developed to allow mining of the remaining coal to be completed safely. The overall stability of the slope geometry was evaluated using RS2™ finite element modelling methods, and re-entry strategies involving crest unloading to reduce the remaining failure hazard were assessed. A rockfall analysis for the eastern portion of the slope was completed using RocFall™, calibrated against observed conditions. A runout analysis of the western portion of the slope was completed using Dan-W™, calibrated against observed conditions. The results of the rockfall and runout analyses were used to design appropriately sized berms located at the toe of the slope. The re-entry plan incorporated the rockfall berm design, a rockfall runout setback distance for the berm, and crest unloading on two sequential single benches. The mitigation strategy was supplemented by radar monitoring. This paper summarises the analyses that were completed to develop a safe, fit-for-purpose re-entry strategy that has since been successfully implemented.

Keywords: slope failure, finite element analysis, rockfall analysis, runout analysis, DAN-W, RocFall, RS2

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