Reimagining closure at Mt Bischoff

Authors: Johnson, B; Dalton, B; O'Kane, M; Freidman, B; Taylor, I; Ebrahimi, M; Meek, I; Sawyer, R Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2615_79

Cite As:
Johnson, B, Dalton, B, O'Kane, M, Freidman, B, Taylor, I, Ebrahimi, M, Meek, I & Sawyer, R 2026, 'Reimagining closure at Mt Bischoff', 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. 1-9, https://doi.org/10.36487/ACG_repo/2615_79



Abstract:
Mt Bischoff mine is a historic tin mine located in the northwestern region of Tasmania, with a complex legacy of underground mining activity spanning 125 years of operations. More recently, open pit mining led to construction of ex-pit mine rock stockpiles and waste rock dumps. One of these, the Happy valley waste rock dump (HVWRD), contains non-acid-forming (NAF) and potentially acid-forming (PAF) materials and has contributed to generation and mobilisation of acidity, metal(loid)s and sulphate. Advancing closure execution of the open pit operation was presented with the challenge of an existing approved closure plan compliant with permitting requirements, albeit having identified long-term performance risks and uncertainties relative to long-term water quality objectives. This paper presents improved understanding of system behaviour that identified water transport as the dominant control mechanism. By integrating stakeholder feedback and updated technical conceptualisations, the project developed a defensible pathway towards reimagining site closure.  A central technical shift in the site’s approach to closure was an improved understanding of the dominant residual risk. While earlier rehabilitation efforts for the HVWRD focused on limiting oxygen ingress, further assessment revealed that substantial acidity had already been generated and stored within the HVWRD landform and was being transported by water movement to the base/toe of the landform. Consequently, the closure strategy was reimagined to focus on management of water transport as the key driver rather than oxygen ingress alone. The resulting current closure design adopts an optimised and integrated landform plan that features relocation of the HVWRD into the main pit as a result of the substantive challenges in managing water flow through the landform, which is situated in the steep and narrow Happy Valley. This approach also reduces water management risks associated with the main pit, specifically regarding storage and water quality. Execution of the strategy involves dewatering the pit, which was deemed as being required for subsequent backfilling with mine rock sourced from the HVWRD (both NAF and PAF materials), and installing a barrier- type cover system that would result in very low net percolation. This case study demonstrates that reimagining closure through a performance-led lens can enable a successful relinquishment trajectory that meets both regulatory and corporate requirements.

Keywords: mine closure, water management, cover system, landform design

References: