Clayton, C, Barnett, R & Slater, M 2020, 'Case study: back-analysis of a historical open pit highwall failure at a coal mine in Canada', in PM Dight (ed.), Slope Stability 2020: Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 873-888, https://doi.org/10.36487/ACG_repo/2025_57
(https://papers.acg.uwa.edu.au/p/2025_57_Clayton/)
Abstract: An open pit highwall up to 400 m in height is being planned at a coal mine in southeastern British Columbia, Canada. A geotechnical drilling program completed in 2016 identified that a thick (20‒40 m), weak, soil-like coal seam will be exposed along the base of the proposed highwall slope. A historical highwall failure at an adjacent open pit occurred in 2011 where successful monitoring resulted in no personnel injury or equipment damage. Following the failure, a review identified that a thick seam of sheared coal at the base of the slope might have contributed significantly to the failure; however, no back-analysis was performed.
The discovery of a similar thick, friable, soil-like coal seam which will be exposed at the toe of the proposed highwall emphasised the need for a back-analysis of the historical slope failure to assist with understanding the previous failure relative to the design of the future wall. A back-analysis was then completed using limit equilibrium and finite element numerical modelling methods and included a review of monitoring, climate, and observational data. The finite element method best captured the site observations and data associated with the failure.
This paper presents the key findings from the back-analysis including the results of the finite element modelling, as well as conclusions which were relevant to the design of the proposed highwall slope.

Keywords: back-analysis, slope failure, slope deformation, weak coal, radar monitoring