Faulting in an Anisotropic, Schistose Rock Under General Triaxial Compression

ABSTRACT: The main objective of this paper is to discuss the mode of failure and the pattern of faults developed in samples of an anisotropic rock in a general triaxial stress field. Rectangular prismatic samples of a metamorphic, green crystalline schist were tested under conditions of a true triaxial homogeneous state of compressive stresses (s₁ > s₂ > s₃). The samples were loaded in such a way that the maximum principal stress (s₁) was oriented at angle ß=90° to the foliation planes (which is the direction of the highest compressive strength), or at angle ß=30° (direction of the lowest compressive strength). In the latter case the samples were oriented so that the angle (?) between the direction of the intermediate principal stress (s₂) and the strike of the plane of foliation was 0°, 45° or 90°. Results of the study throw a new light on the phenomenon of faulting under true three-dimensional stress conditions. They show that in stratified geologic media it might not necessarily be true that the plane of fracture (faulting) is parallel to the direction of the intermediate stress (as normally is the case in massive or isotropic rock masses), for these are the weakness planes which may control the geometry of rock deformation and faulting. It has also been revealed that seismic or aseismic faulting may take place in stratified rock masses depending on the major and the intermediate principal stress orientation relative to the planes of weakness.