Small-scale variations in mining-induced stresses, monitored in a seismically active underground mine

Kiirunavaara mine, owned by LKAB, is the largest iron ore underground mine. It is about 4 km long and has been mined with sublevel caving since the late 1950s. The current mining levels in the main orebody are 1079 m and 1108 m. The sublevel height is 29 m and it takes about two years to mine one sublevel in the southern half of the mine. As such, the mining-induced stress changes were regarded to be relatively slow. There are 2700 MW ≥ 0 seismic events recorded on average every year, with the largest event recorded on 18 May 2020 MW 4.3. Not many rock mechanical monitoring campaigns have been undertaken during the mine’s history, except for a mine-wide seismic system which has been in operation since 2009. In 2013, 3D stress measurements with the Borre stress cell were undertaken at level 1165 m (production area Block 34), with subsequent installation of 3D CSIRO-HI cells for long-term stress change monitoring. The primary purpose of the installation was to measure the changes in the induced stress (magnitude and direction) as the mining progressed downwards. The cell was installed 143 m below the current production level and at the time of the installation, there were indications that the measured stresses were already affected by the mining-induced stresses. At the time, the local stress state within different production areas was defined only by large-scale generic numerical modelling. Since the installation, large-scale changes in the induced stresses have been recorded. The current study is focussing on the small-scale variations in the induced stresses which can be related to larger seismic events, development, and production blasting for the time period mid-July 2015 to mid-March 2017. Stress changes before, during, and after 15 seismic events with mL ≥ 1.0 were analysed. Clear pattern of stress changes for eight events was found. No stress change was observed during 25 development blasts (distance < 50 m) and 19 closest productions blasts (125–135 m). Cyclic behaviour of the stress changes was observed with increases starting at 6:00 am which is most probably related to the shift/mucking start at this time. The stress change for a sample of data showed a striking similarity to the seismic activity in the area. The outcome of this work is part of the ongoing project to find patterns/scenarios which will be used for short-term hazard assessment and closing criteria in the mine. The results can be used also for further studies on seismic event preparation processes and co-seismic stresses and deformations.