Size effect and anisotropy in transversely isotropic rocks

Abstract

Transversely isotropic rocks, making up around 75% of the earth's surface, are widely encountered in civil, mining, petroleum, geothermal and radioactive-waste disposal engineering. In Hong Kong, the rock cavern development plan has commenced in recent years to increase the land supply and improve the environment. Some strategic cavern areas are also inevitably located in anisotropic formations. Anisotropy, as one of the most distinct features possessed by these kind of rocks, generally originates from the mineral foliation in metamorphic rocks, the stratification in sedimentary rocks, and the discontinuities in rock masses due to stress and geological history. Size effect is another important characteristic owned by brittle and semi-brittle rocks. Numerous investigations into size effect in isotropic rocks have been conducted, and many size-effect models have been proposed for isotropic rocks. However, considering the influence of anisotropy, size effect in transversely isotropic rocks is very different from that in isotropic rocks. To date, there are few studies in relation to size effect in transversely isotropic rocks. In this study, a transversely isotropic slate rock from a quarry was obtained as the test material. The first principal objective is to investigate the anisotropy and size effect in slate under indirect tensile conditions. A series of Brazilian tests were performed on slate samples with different diameters at different anisotropic angles. Size effects on elastic properties, indirect tensile strength and fracture pattern were analysed. Size effect on indirect tensile strength was found to be correlated with the anisotropy. A unified size-effect relation was proposed and validated against the experimental data to capture the ascending and descending size-effect trends and the relationship among indirect tensile strength, sample size and anisotropic angle. Furthermore, the influence of three-dimensional anisotropy on the tensile behaviour of transversely isotropic rock was investigated using a particle-based discrete element approach. Considering various foliation orientations relative to loading direction and sample axis, the tensile strength, fracture mechanism and micro-cracking were systematically studied. The second principal objective is to investigate the anisotropy and size effect in slate under compressive conditions. A series of uniaxial and triaxial compression tests were conducted on slate samples. In response to the test results, a size-effect model developed from coal was extended to the transversely isotropic rock. Both uniaxial and triaxial compressive strengths were found to follow a cosine relation. It was also found that the size-effect behaviours in uniaxial and triaxial compressive strengths were similar. Two size-dependent failure criteria were proposed by incorporating the size-effect model for uniaxial compressive strength into the modified Hoek-Brown and Saeidi failure criteria, respectively, and were verified against the experimental data. For the first time, the relationship among compressive strength, specimen size, anisotropic angle and confining pressure was comprehensively captured for transversely isotropic rock. Lastly, without evident size effect, the anisotropic triaxial residual strength was captured well by an improved cohesion loss model. Two equations delineating the range for ratio of residual to peak strength were proposed for transversely isotropic rocks. Overall, these findings in slate may be applicable to other transversely isotropic rocks.