Seismicity and stress changes subsequent to destress blasting at the Galena Mine and implications for stress control strategies

The US Bureau of Mines conducts research at the Galena Mine, Wallace, ID, with the aim of mitigating the effects of rock bursting. Destress blasting is commonly used as a stress control technique at the mine. A digital seismic array and an array of borehole pressure cells (BPC's) had been installed near the site of a slope undergoing mining and periodic destressing. The instrumentation was being monitored at the time of a destress blast of the 46-99 slope. No significant seismic events occurred coincident with the destress. However, the destress was followed by a 2 1/2-week period of increased seismic activity, including two damaging events on February 7, 1990, at 034500 (hour, minute, and second) and 122020 Pacific standard time. BPC measurements indicated coseismic ground pressure changes on the order of 200 to 300 kPa associated with the damaging events. Fault plane solutions and dislocation models established that stress changes induced by the event at 034500 may have been significant in promoting the occurrence of the event of 122020. Theoretical investigations suggest that applying knowledge of the existing field, an understanding of rock burst mechanics, and fracture mechanics principles can improve destress effectiveness.