Predicting ash settlement magnitudes and timing at the Hazelwood mine in-pit ash retention facility to support mine closure

Authors: Butler, J; Xue, YS; Missen, J; Truden, P; Lowe, J Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2615_45

Cite As:
Butler, J, Xue, YS, Missen, J, Truden, P & Lowe, J 2026, 'Predicting ash settlement magnitudes and timing at the Hazelwood mine in-pit ash retention facility to support mine closure', 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. 1-12, https://doi.org/10.36487/ACG_repo/2615_45



Abstract:
The Engie owned-and-operated Hazelwood mine, located in the Latrobe Valley in Victoria, Australia, ceased brown coal winning operations in 2017 following the decision to close the nearby Hazelwood Power Station. Mine closure execution works remain ongoing, with active rehabilitation completion not expected for a number of years. As part of mine closure, the rehabilitation of an in-pit storage facility, the Hazelwood Ash Retention Area (HARA), is required to control the environmental risk associated with potential ash migration. The closure concept for the facility consists of ash surface reshaping followed by placement of a compacted clay cap and topsoil layer, encapsulating the underlying ash material. The authors were approached to investigate the potential implications of ash settlement on the efficacy of the cap, where it was understood that differential settlement of the ash may impact the drainage performance of the landform or compromise the integrity of the cap, if substantial. Accordingly, the authors employed Asaoka’s (1978) observational method combined with Terzaghi’s (1943) 1D consolidation theory to estimate the magnitude of remaining settlement due to cap loading. Outcomes of the assessment demonstrated that for a proposed 0.5 m capping layer, an additional allowance of 50 mm would accommodate the predicted differential settlement subsequent to initial cap placement. Accordingly, the cap was designed to be placed above the final design level to accommodate the predicted subsidence. The closure design for the HARA subsequently received approval from relevant regulatory authorities, with works to cap the HARA anticipated to be completed by late-2026. This paper presents findings from the HARA case study, incorporating both empirical observations and theoretical analyses, to assess ash settlement behaviour in support of the closure design.

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