The Mt Bischoff mine: designing a robust closure landform

Authors: Freidman, B; Ebrahimi, M; Johnson, B; O'Kane, M; Dalton, B Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2615_54

Cite As:
Freidman, B, Ebrahimi, M, Johnson, B, O'Kane, M & Dalton, B 2026, 'The Mt Bischoff mine: designing a robust closure landform', 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. 1-14, https://doi.org/10.36487/ACG_repo/2615_54



Abstract:
Waste rock dumps containing high proportions of potentially acid forming (PAF) material present a significant long-term closure challenge, often resulting in ongoing acid and metalliferous drainage (AMD) and perpetual water treatment requirements. Conventional surface-based controls are frequently insufficient to manage oxygen ingress and water-rock interaction in these systems. The Mt Bischoff mine is a historic tin mining operation located in northwestern Tasmania, with a complex legacy of underground and open pit mining spanning more than a century. Recent open pit operations (2008–2010) resulted in the construction of several surface waste rock landforms, most notably the Happy Valley waste rock dump (HVWRD). The HVWRD presents an ongoing risk to the receiving environment, as acid neutralising capacity within the landform is predicted to deplete in the short term. To address long-term closure and water quality objectives, a revised closure strategy was developed that represents a fundamental shift from surface-based AMD controls to construction of an integrated closure landform. Central to this strategy is the progressive deconstruction of the HVWRD and relocation of waste rock into the Main Pit, transforming the pit from a hydraulic control and water quality risk into a stable, backfilled landform. This approach prioritises the management of water movement during and after construction, with the objective of maximising non-contact water and minimising contact water generation. The closure strategy is to be implemented through sequenced construction phases, including (1) dewatering of the Main Pit to enable construction activities, (2) blasting of the upper North Wall, (3) deconstruction of the HVWRD, (4) progressive backfilling and active water management, (5) North Wall backfilling and cover system construction, (6) installation of a ‘hidden’ berm and monitoring infrastructure, and (7) repurposing of the Happy Valley settling pond footprint. Sequencing of these activities is critical to controlling contact water generation, maintaining constructability and progressively reducing AMD risk throughout the works. This paper presents the closure design and construction staging for the Mt Bischoff mine. It outlines how staged earthworks, controlled material placement and integrated water management are combined to deliver a geotechnically stable, geomorphically resilient and environmentally robust landform capable of meeting long-term closure objectives. The Mt Bischoff mine is Tasmania’s first non-abandoned mine to reach the engineered closure phase, setting a benchmark for community expectations and the state’s regulatory framework around relinquishment of the mining leases.

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