Authors: Schafer, KW; Bakken, KM; Ergun, S; Potter, J; Ross, BJ; Telfer, JA; Williams, CP

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Schafer, KW, Bakken, KM, Ergun, S, Potter, J, Ross, BJ, Telfer, JA & Williams, CP 2023, 'Rockfalls as precursory strain indicators leading to a large slope failure (‘Leo Failure’) at Bingham Canyon Mine, USA', in PM Dight (ed.), SSIM 2023: Third International Slope Stability in Mining Conference, Australian Centre for Geomechanics, Perth, pp. 721-732,

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Rockfalls and rockslides are ubiquitous hazards at the Bingham Canyon Mine, where a large (19 × 106 tonnes) slope failure (‘Leo Failure’) occurred on 31 May 2021. The deforming slope was monitored with continuous thermal video recordings. This study documents (based upon thermal video review) both the frequency and spatial distribution of rockfalls leading to the slope failure. Thermal video monitoring began approximately four days prior to collapse, during the final acceleration phase. The frequency of rockfalls increases exponentially leading to failure, with approximately 45% of the total documented rockfalls (158) occurring within six hours of the failure. Of the documented rockfalls 88% are spatially coincident (within 15 m) with a left, the right, and crestal failure boundaries, but not the break-out at the toe. The majority of rockfalls (60%) repeatedly originate from nearly the same positions within three different areas (clusters): Cluster 1 positioned along the right-release, a bedding-parallel fault with clay-gouge; and both clusters 2 and 3 positioned along a multi-staged left-release, a complex intra-intrusive, fractured igneous contact. Approximately twice the number of rockfalls originate from each of both Cluster 2 (N = 44) and Cluster 3 (N=33) as compared to Cluster 1 (N=19). The cumulative rockfall versus time curve for Cluster 1, along the contiguous faulted-bedding right-release, generally exhibits a progressive acceleration up to the time of failure. In contrast, a cumulative rockfall curve for Cluster 2, along a geometrically complex leftrelease, exhibits a series of three abrupt periods of rapid rockfall accelerations (e.g. 1–1.5 per day to 70–75 per day). The cyclical accelerations and greater number of rockfalls are attributed to brittle strain localisation at geometric asperities associated with an evolving ‘step-path’ left-release, as opposed to creepstrain along an inherited, clay-rich sedimentary contact reactivated as a fault.

Keywords: rockfall, slope failure, slope stability in open pit mines

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