Finite element modelling of the Lady Loretta underground mine to inform closure strategies

Authors: Narendranathan, S; Butler, J; Mafu, M; Penney, A Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2515_18

Cite As:
Narendranathan, S, Butler, J, Mafu, M & Penney, A 2025, 'Finite element modelling of the Lady Loretta underground mine to inform closure strategies', in S Knutsson, AB Fourie & M Tibbett (eds), Mine Closure 2025: Proceedings of the 18th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-12, https://doi.org/10.36487/ACG_repo/2515_18



Abstract:
The Glencore Australia-owned Lady Loretta Mine (LLM) is an underground zinc, lead and silver mine located in the Mt Isa region of Queensland, Australia. Mining operations at LLM are scheduled for completion in late 2025, followed by works undertaken to rehabilitate the site prior to planned relinquishment in 2042. Mine workings have been excavated to 550 m below the natural surface and include more than 30 km of development drives. Mining is performed using sublevel open stoping with cemented paste fill. Additionally, four concrete-lined shafts provide ventilation to the underground workings: two fresh air intake shafts and two return air shafts. Closure of the underground workings must satisfy criteria set out in the LLM Progressive Rehabilitation and Closure Plan (PRCP) in order to be considered “safe, stable, and non-polluting” in accordance with Queensland State legislative requirements (Purtill et al. 2022). Specifically, the LLM PRCP stipulates that surface subsidence due to mining activities must not exceed 500 mm relative to pre-mining conditions. Accordingly, the authors performed 3D finite element modelling to predict post-closure surface subsidence magnitudes for two potential underground mine closure options; leaving the workings “as is” or backfilling the workings as required. The aim of the modelling was to inform closure planning and provide recommendations for targeted treatment of underground voids where appropriate. Modelled results indicated that the criteria was achieved for both closure options, with no additional treatment required beyond the paste filling of mined out stopes as scheduled during operations. As such, the decision was made to safely backfill a portion of the mine with demolition waste generated from surface decommissioning activities.

Keywords: underground, finite element modelling, geotechnical, subsidence, sublevel open stope

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