The Influence of Pre-Existing Deformation and Alteration Textures on Rock Strength, Failure Modes and Shear Strength Parameters

This study uses the uniaxial compressive strength (UCS), the indirect tensile strength (ITS) and the point load tests (PLT) to determine the strength and deformation behavior of previously deformed and altered tonalite and anorthosite. In general, veined samples show higher strength because the vein material has both cohesive and adhesive properties while fractures have no cohesion, only frictional resistance. This implies that each rock category has to be treated independently and absolute strength predictions are inaccurate. Thus, the conversion factor k is a sample specific parameter and does not have a universal value. The ratio of UCS/ITS appears to be related to the rock strength and can be used to classify rocks based on their strength. The shear strength parameters, the friction angle and the cohesion, cannot be calculated for rocks with pre-existing planes of weakness. Reactivation is favoured only for planes oriented less than 20° to the maximum stress. For planes oriented between 20° and 50° to the maximum stress, failure occurs by a combination of reactivation and newly formed fractures, while for orientations above 50°, new shear fractures are favoured. This suggest that the Byerlee’s law of reactivation operates exclusively for planes oriented ≤10° to the maximum stress