Geotechnical challenges for final landform: a case study at Werris Creek

Authors: Guy, G; Forrest, B Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2615_134

Cite As:
Guy, G & Forrest, B 2026, 'Geotechnical challenges for final landform: a case study at Werris Creek', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-16, https://doi.org/10.36487/ACG_repo/2615_134



Abstract:
Final landforms for open pit mine closure must ultimately demonstrate that they are safe, stable and nonpolluting by meeting long-term performance expectations. These landforms are typically designed and shaped using waste rock material at low-angle geometries. Consequently, closure landforms are typically developed to meet conservative design acceptance criteria (DAC), incorporating elevated factors of safety to address long-term uncertainties associated with hydrogeological rebound, progressive material degradation and evolving geomechanical conditions. As open pit operations approach completion, progressive reshaping of in-pit and ex-pit waste rock dumps in accordance with the final closure geometry operate under the assumption that the remaining resource extraction will not compromise the final landform. Significant unplanned instability late in the mine life may compromise the DAC through undermining the confidence in the geotechnical model, consequently raising questions about stability thresholds and whether closure requirements can still be achieved or a substantial redesign is required.  This paper presents a case study of an open pit coal mine that experienced a major endwall failure within the final 6 months of resource extraction. This occurred during advanced stages of landform development, posing significant challenges to both operational continuity and closure compliance. The study details the integrated geotechnical assessment undertaken to characterise the failure, and risk mitigation strategies implemented to enable continued resource recovery alongside adaptive slope redesign and waste placement methodologies. The case study outlines the modifications to the final landform design to achieve long-term stability and closure performance objectives which included re-establishing the confidence in the geotechnical model. The  outcomes demonstrate the importance of adaptive closure planning, robust monitoring and contingencybased design frameworks in managing late-stage geotechnical instability and ensuring sustainable post-mining landforms.

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