Controlled reopening of the Kiirunavaara production block 22 after a 4.2 magnitude event

Abstract

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.

Managing geotechnical uncertainty and risk in mining

“…In May 2020, Kiirunavaara mine (Kiruna) in Sweden experienced a major seismic event, which caused extensive damage over more than 1000 m in tunnels on several levels (Boskovic 2022;Dineva et al, 2022;Svartsjaern et al 2022;Mawson et al, 2022). The Swedish National Seismic Network (SNSN) recorded it as a moment magnitude MW 4.2 seismic event, which is considerably larger than any seismic event previously experienced at Kiruna and was felt several thousand kilometres away.…”

“…Importantly, no-one was exposed in the area at the time of the MW 4.2 seismic event and re-entry was only permitted after the seismicity returned to normal levels. Details of the MW 4.2 seismic event, aftershocks, detailed analyses and damage, and rehabilitation strategies have been published at RASIM10 (Boskovic 2022;Dineva et al, 2022) and CAVING2022 (Svartsjaern et al, 2022;Mawson et al, 2022), and a workshop on the incident was held at CAVING2022. All the information in this section has been drawn from these publications.…”