Authors: Svartsjaern, M; Rentzelos, T; Shekhar, G; Swedberg, E; Boskovic, M; Hebert, Y

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Svartsjaern, M, Rentzelos, T, Shekhar, G, Swedberg, E, Boskovic, M & Hebert, Y 2022, 'Controlled reopening of the Kiirunavaara production block 22 after a 4.2 magnitude event', in Y Potvin (ed.), Caving 2022: Proceedings of the Fifth International Conference on Block and Sublevel Caving, Australian Centre for Geomechanics, Perth, pp. 685-698,

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Large magnitude seismic events are a concern for most hard rock underground mines extracting minerals at depth. At the Kiirunavaara Mine, a long-term chain of events was initiated with a 3.0 magnitude event in 2008 in production block 19 on mining level 907 m. In the aftermath of the event, this production block started to trail behind the adjacent mining blocks creating an upward pointing wedge over three blocks, with block 19 at the top. In 2020 another large magnitude event (4.2 Mw) occurred in the same block, now designated block 22, while opening level 1022 m. The event caused significant damage to the mine infrastructure over a large volume affecting overall mine output for months. Because of the significant damage to the infrastructure, it was not possible to resume mining on the topmost levels in block 22. Mining resumed instead from the lower levels, creating a remnant pillar between the existing sublevel and the resumed cave. The behaviour of this pillar has been analysed in detail using a coupled FLAC3D-CAVESIM numerical model. The modelling used a global-local approach where the mine scale stress field from sublevel cave mining until 2020 was superimposed on a local model where the resumption of mining activities in block 22 was explicitly simulated for several alternative layouts and sequences. To alleviate the seismic hazard from an uncontrolled pillar collapse after resumed mining, the best option sequence was sought to facilitate controlled pillar caving. Variation studies were performed to find high impact parameters controlling the pillar behaviour. Parameters studied included looking at rock mass quality, horizontal sequencing and alterations in the footprint of the resumed cave. From the modelling effort, it was concluded that the confinement of the remnant pillar was the major factor controlling the pillar caving and seismic potential. With support from underground damage assessments and the numerical models, it was decided to attempt to resume the mining from level 1079 m (abandoning the damaged levels 1022 and 1051). A new set of footwall drives were excavated inside the orebody to replace the damaged drives at the footwall contact. From the new footwall drives, the existing crosscuts were rehabilitated towards the old footwall drive as far as possible to reduce the confinement of the remnant pillar above. Resuming production at 1079 will attempt to reset the global mining sequence, with the production block no longer lagging compared to adjacent blocks.

Keywords: numerical modelling, SLC, cave initiation, mine seismicity

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