: We compare compressive failure in crystalline rocks with that in a newly tested hornfels from the Long Valley Exploratory Well, California. Strength characteristics, deformation behavior, and micromechanics of failure observed in the nearly isotropic, very-fine-grained hornfels are drastically different from those encountered in previously tested crystalline rocks. The variation of the major principal stress with the volumetric strain is linear up to the point of failure, suggesting a complete lack of dilatancy. This was corroborated by SEM analysis showing a complete lack of microcracking prior to failure, contrary to observations in crystalline rocks. Consequently, no significant effect of the intermediate principal stress on the hornfels true triaxial strength was detected. This too is surprisingly different from previously tested rocks.
Conventional triaxial tests, performed under a special case of stress conditions in which the intermediate and least principal stresses are equal ( s2= s3), have revealed that compressive failure in hard rocks takes the form of a through-going shear fracture preceded by the growth, localization and coalescence of a multitude of extensile microcracks, the long axes of which are subparallel to the maximum principal stress ( s1) direction (Wawersik & Fairhurst 1970, Tapponnier & Brace 1976, Wong 1982). In an effort to better understand rock failure behavior under the most general stress conditions, we have conducted true triaxial compression experiments in which all three principal stress magnitudes are unequal. The main objectives of these experiments were to assess rock mechanical properties such as strength, deformability and micromechanics of failure under this more realistic stress conditions and to examine the validity of Mohr-based criteria that assume no effect of the intermediate principal stress (Jaeger & Cook 1979, p. 99).