Field monitoring of an undercut paste backfill and implications for analysis and design

Authors: Jafari, M; Grabinsky, M; Chacaltana, P; Wei, W

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DOI https://doi.org/10.36487/ACG_repo/2655_26

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Jafari, M, Grabinsky, M, Chacaltana, P & Wei, W 2026, 'Field monitoring of an undercut paste backfill and implications for analysis and design', in AB Fourie, M Horta, M Oliveira & S Wilson (eds), Paste 2026: Proceedings of the 28th International Conference on Paste, Thickened and Filtered Tailings, Australian Centre for Geomechanics, Perth, pp. 1-12, https://doi.org/10.36487/ACG_repo/2655_26

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Abstract:
Mining underneath previously placed backfill can offer important operational efficiencies for underground mines. However, determining the strength of backfill that can be safely undercut has posed a long-standing design challenge. Mitchell (1991) proposed an analysis method involving 4 independently evaluated failure mechanisms, the most commonly cited involving assumed flexural bending. Curiously, an earlier publication also included a fifth failure mechanism involving stope wall closure, although this was later omitted for failure analysis but included as a mitigating term in the flexural analysis. Subsequently, Grabinsky et al. (2024) presented a method to assess rock mass closure–backfill interaction which broadly delineates conditions where closure will be dominant, negligible, or intermediate. Field verification of actual conditions is therefore important, with two existing field studies showing the extreme conditions postulated by Grabinsky et al. (2024). The proposed paper features a heavily instrumented underhand cut-and-fill stope at Macassa mine. The experimental design, instrumentation layout and installation details, undercutting details, and field results will be presented. This case study demonstrates the intermediate condition postulated by Grabinsky et al. (2024) where the closure effect dominates the Mitchell flexural effect, yet is not so extensive as to crush the backfill. The paper concludes with recommendations for analysis and design of future undercut backfills.

Keywords: cemented paste backfill, instrumentation, underhand cut-and-fill

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