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Stress-strain curves for brittle rocks show three characteristic stress levels: crack initiation, long-term strength, and peak strength. Damage-controlled testing at low confining stresses has shown that the long-term and peak strengths are sensitive to the amount of induced damage, i.e., the greater the amount of damage, the lower the long-term and peak strengths. These tests also showed that the brittle-failure process 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 failed rock around the tunnel is approximately one-half of that measured in laboratory tests. Crack-induced damage of Lac du Bonnet granite, both in the laboratory and in situ, begins when the load exceeds approximately 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 ultimate strength of an unconfined sample is 225 MPa. Numerical studies suggest that in situ the loading path around the tunnel is more complex, involving stress increase and decrease and stress rotation. For this 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 microseismic monitoring carried out during tunnel construction indicate a constant-deviatoric-stress criterion is a reliable indicator for predicting the onset of damage. This criterion was also found to give a reasonable prediction for the maximum depth of failure around the test tunnel. The fundamental assumption in the constant-deviatoric-stress criterion is that at low confining stresses, such as those which occur around underground openings, the brittle-failure process is dominated by cohesion loss.Key words: rock mass strength, brittle failure, cohesion loss, deviatoric stress, stress path.Résumé : Les courbes contrainte-déformation des roches fragiles montrent trois niveaux caractéristiques de contraintes : l'initiation de fractures, la résistance à long terme et la résistance de pic. Des essais à dommage contrôlé réalisés sous de faibles contraintes de confinement a montré que les résistances à long terme et de pic sont sensibles à la quantité de dommages induits, i.e., plus la quantité de dommages est importante, plus les résistances à long terme et de pic sont faibles. Ces essais ont aussi démontré que le processus de rupture fragile est caractérisé par une perte de cohésion à mesure que le frottement est mobilisé. L'excavation d'un tunnel d'essai circulaire dans un roc massif fragile a résulté en une rupture autour du tunnel. La résistance calculée à rebours de la roche en état de rupture autour du tunnel est environ la moitié de la valeur obtenue dans les essais de laboratoire. Le dommage ...

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