| A fundamental laboratory study was conducted in the deformation and strength of Westerly granite and KTB amphibolite subjected to true triaxial compressive stress conditions (σ1 > σ2 > σ 3) with a particular attention to σ2 effect on rock failure process. It was found that σ2 strongly affects the criteria of strength for these rocks, contrary to the assumption contained in commonly accepted Mohr-type failure criteria. Under true triaxial stress conditions, crystalline rocks fail along a steeply inclined throughgoing shear fracture striking to the σ2 direction. Stress-induced microcracks also develop mainly parallel to σ2 direction, as the intermediate stress grows beyond σ3, localizing along the plane that eventually becomes the throughgoing fracture. A general strength criterion can be expressed in terms of the octahedral shear stress and the mean normal stress acting on the failure plane.; In a separate series of tests, failure of KTB amphibolite under borehole wall condition was simulated by leaving one pair of the prismatic specimens faces unjacketed and in direct contact with the confining fluid through which σ 3 is applied. These tests reveal that brittle fracture occurs at a considerably lower stress level than that in dry amphibolite, and results from the development of a swarm of densely spaced extensile fractures subparallel and adjacent to one of the unjacketed faces. It is inferred that upon dilatancy onset, confining fluid intrudes microcracks, which are predominantly subparallel to the unjacketed faces, and promotes their elongation into throughgoing fractures. A true triaxial strength criterion of the unjacketed amphibolite can be expressed in terms of the octahedral shear stress as a function of the octahedral normal stress.; The magnitudes of the maximum horizontal in situ stresses at the KTB hole, Germany, were computed based on the strength criterion of the unjacketed KTB amphibolite together with all the other known data from the same hole. It confirms previous assessments of a strike-slip stress regime, and suggests that the stress difference between the least and largest horizontal principal in situ stresses remains constant as the two increase with depth within the 3200–6000 m range. |
