Identifying crack initiation and propagation thresholds in brittle rock

Recent work at the Underground Research Laboratory of Atomic Energy of Canada Limited in Pinawa, Manitoba, has shown that high compressive stresses near the tunnel face significantly contribute to the loss of strength, and eventual failure of the rock, through stress-induced brittle fracturing. A program of laboratory testing has been undertaken to investigate the effects of brittle fracture on the progressive degradation of rock mass strength. The work carried out in this study involves a detailed analysis of the crack initiation and propagation thresholds, two key components in the brittle-fracture process. This paper describes new techniques developed to enhance existing strain gauge and acoustic emission methodologies with respect to the detection of these thresholds and their effects on the degradation of material strength.