The relationship between mining activities and mining-induced seismicity is vital. Having a better understanding of it allows mine operators to anticipate and plan for necessary ground support and to reduce those risks which might compromise the safety and productivity aspects of a mine. Drs. Hani Mitri and Atsushi Sainoki with the Department of Mining and Materials Engineering of McGill University, working with Vale Canada Limited’s Copper Cliff Mine, undertook a comprehensive investigation and evaluation of the fault-slip potential in an underground mine. The project, supported by CEMI and NSERC through a Collaborative Research & Development grant examined and analyzed mining induced fault-slip, microseismicity, and violent shear rupture data using conventional continuum numerical modeling techniques and a new, advanced discontinuum modelling approach. Commercially available software, FLAC3D and 3DEC, was also used.
SIGNIFICANT FINDINGS OF VALUE
The outcome of the investigation led to two major findings:
- Development of a methodology to distinguish aseismic fault-slip from seismic fault-slip by using numerical modelling techniques and microseismic data to investigate whether the shear movement along the targeted fault is seismic or not (Figure 1). Currently, positive fault-slip potential in mines is perceived as a strong indicator of the likelihood of a future seismic event thereby requiring implementation of potentially unnecessary and costly preventive measures, such as the installation of dynamic support. Avoiding this would represent real value to mines. The developed methodology achieves this by enabling the ground control specialist to make informed decisions on the need for dynamic support element deployment, based on the type of anticipated future fault-slip potential, (i.e. whether seismic or aseismic).
- Development of a new perspective on the cause of mining-induced seismicity that takes place away from mine openings. It is common belief that mining-induced stress change causes seismicity in a massive, highly stressed rockmass. While this is true, the stress state numerically analyzed or measured away from mine openings, is often less than the critical threshold needed to create seismic activity. The McGill study has shed light on the influence of fracture networks in the rockmass and on the associated occurrence of mining-induced seismicity. A discontinuum numerical analysis successfully demonstrated that complex interactions of fractures away from mine openings can accumulate sufficient elastic strain energy to cause localized violent shear rupture (rockbursting). The result is an index of fracture-network- induced strain energy (Figure 2). A guideline or index such as this makes it possible to assess the severity of potential mining-induced seismicity, allow for optimization of mine planning, support system selection and mine design. Theoretically, this could enable reliable productivity improvements and potentially, lower costs.