Size Effect and Anisotropy in a Transversely Isotropic Rock Under Compressive Conditions

A series of uniaxial and triaxial compression tests were performed on slate samples with different diameters at different foliation orientations with respect to the direction of the major principal stress. The size effect and anisotropy in slate, as a transversely isotropic rock, were investigated, and the research focused on aspects of elastic properties, uniaxial compressive strength (UCS), triaxial compressive strength (TCS), and triaxial residual strength (TRS). In the five elastic constants for slate, only the Young’s modulus parallel to the isotropic plane is size dependent. The UCS follows a descending size-effect model developed from coal. The size-effect behaviors of the UCS and TCS are similar. Two size-dependent failure criteria are proposed by incorporating the size-effect model for UCS into the modified Hoek–Brown and Saeidi failure criteria and are verified against experimental data. This is the first time that the relationship among the compressive strength, specimen size, foliation orientation and confining pressure has been comprehensively captured for transversely isotropic rock. Without an evident size effect, the anisotropic TRS has also been effectively captured by a modified cohesion loss model, and two bound equations for the brittleness index are finally proposed for transversely isotropic rock. This work promises to provide an upscaling method for determining the mechanical parameters of transversely isotropic rocks in practical engineering.