Microseismic monitoring in highly stressed granite: relation between shaft-wall cracking and in Situ stress

Abstract

The excavation of the shaft at Atomic Energy of Canada Ltd's Underground Research Laboratory has been associated with wall failures and formation of overbreaks. Whole-waveform microseismic signals due to crack propagations induced by shaft sinking between 324 and 443 m of depth were recorded using a microcomputer-based acquisition system. An increased level of microseismic activity was observed for a period of about 2 hr after the blasts, the activity level decreased then to a rate of a few events an hour. A velocity survey was carried out mainly for source location calibration using seismic blasting caps detonated in 32 shallow horizontal boreholes drilled following a spiral pattern along the shaft. Both P- and S-wave velocities showed a gradual increase with depth elevation of the shots. A similar increase was observed in the dynamic Young's modulus. The location of the shots could be determined to an accuracy of less than 1 m using P- and S-wave arrival times of signals recorded after the shots and a least-squares source location algorithm. Processing of the excavation-induced data showed that cracking occurred around the bottom and the walls of the shaft near the newly-created faces. The damage area inside the wall is estimated at 1.5 m. Some correlation was observed between the horizontal distance of source locations from the shaft centre and the amount of overbreak observed along the shaft. Statistical methods were used to quantitatively analyse the distribution of source locations of blast-induced microseismic events in the horizontal plane. A clear clustering was observed in the NE-SW quadrant. This is compatible with the orientations of the minimum horizontal stress component and the overbreaks observed along the shaft.