17TH-CANADIAN-GEOTECHNICAL-COLLOQUIUM - THE EFFECT OF COHESION LOSS AND STRESS PATH ON BRITTLE ROCK STRENGTH

Stress-strain curves for brittle rocks show three characteristic stres s levels: crack initiation, long-term strength, and peak strength. Dam age-controlled testing at low confining stresses has shown that the lo ng-term and peak strengths are sensitive to the amount of induced dama ge, i.e., the greater the amount of damage, the lower the long-term an d peak strengths. These tests also showed that the brittle-failure pro cess is characterized by a loss of cohesion as friction is mobilized. Excavation of a circular test tunnel in massive brittle rock resulted in failure around the tunnel. The back-calculated strength for the fai led rock around the tunnel is approximately one-half of that measured in laboratory tests. Crack-induced damage of Lac du Bonnet granite, bo th in the laboratory and in situ, begins when the load exceeds approxi mately one-third of the unconfined compressive strength. However, the stress level associated with failure is a function of loading path. In the laboratory, where the loading path monotonically increases, the u ltimate strength of an unconfined sample is 225 MPa. Numerical studies suggest that in situ the loading path around the tunnel is more compl ex, involving stress increase and decrease and stress rotation. For th is loading path, failure initiates at a stress between 100 and 120 MPa . Conventional frictional failure criteria did not adequately predict the extent of brittle failure measured around the circular tunnel. The results from the damage-controlled laboratory tests and the microseis mic monitoring carried out during tunnel construction indicate a const ant-deviatoric-stress criterion is a reliable indicator for predicting the onset of damage. This criterion was also found to give a reasonab le prediction for the maximum depth of failure around the test tunnel. The fundamental assumption in the constant-deviatoric-stress criterio n is that at low confining stresses, such as those which occur around underground openings, the brittle-failure process is dominated by cohe sion loss.