Chapman, P, Salfate, E & Hatton, C 2024, 'How long is ‘long-term’? Carrying seismic risk through the post-operational period', in AB Fourie, M Tibbett & G Boggs (eds), Mine Closure 2024: Proceedings of the 17th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1177-1184, https://doi.org/10.36487/ACG_repo/2415_84
(https://papers.acg.uwa.edu.au/p/2415_84_Chapman/)
Abstract: Recent trends in industry have led to triggers for static liquefaction being assumed in the absence of evidence to the contrary for tailings prone to liquefaction. However, for seismic liquefaction, the risk is often accepted provided that a target Factor of Safety (FoS) is achieved for the earthquake design event stipulated in standards and codes. In the post-operational period, the triggers associated with static liquefaction can be modelled and, in many cases, ruled out. However, for the assessment of ‘long-term’ conditions following the end of operations (i.e. post-closure), the earthquake design event under consideration is often greater. Rather than installing additional risk mitigation measures, the ‘long-term’ performance discussion is often focused on the time period during which sufficient drain down will occur such that the post-seismic FoS is acceptable. This raises the question: ‘How long is ‘long-term’?’ Or, ‘How long will the owner carry the post-seismic risk after operations cease?’
In many cases, mitigation measures such as buttresses are put in place to maintain compliance under static conditions during the operational period. As the tailings storage facility (TSF) drains down, the potential for failure under static conditions is often reduced. This provides an opportunity to reconsider the extent of mitigation measures required to maintain geotechnical stability over time. For sites that are material constrained there is the potential to consider reusing material that was previously in place to manage the static stability risk without adversely affecting the stability of the facility.
This paper presents two case studies related to the drain down of the phreatic surface in the post-operational period for two TSFs located in semi-arid climates but with contrasting tailings behaviour — a hard rock sandlike tailings versus a finer, clay-like residue — with the difference in hydraulic conductivity relative to the foundation playing a key role in the drain down period. The paper explores the drain down periods in the context of the time frame over which the post-seismic FoS would become compliant under an elevated design earthquake event. The potential to reuse the buttress material for other TSFs is also explored, noting that removal of the buttress material can have a positive influence on the stress state of the foundation, mimicking a pre-loading arrangement.

Keywords: tailings, failure mode, seismicity, stability