Influence of water content and anisotropy on the strength and deformability of low porosity meta-sedimentary rocks under triaxial compression

Abstract

A reduction of uniaxial compressive strength (UCS) by the presence of water has been ascertained in many sedimentary rocks and even some metamorphic rocks experimentally. This paper reports and discusses the influence of water content and anisotropy on the strength and deformability of two meta-sedimentary rocks by triaxial compressive tests. Before conducting the triaxial tests, the physical properties of meta-siltstone and meta-sandstone specimens have been carefully examined. The water contents of both tested rocks are very low, for instance, 0.17% for meta-siltstone and 0.10% for meta-sandstone. The porosities of the tested rocks are analyzed by the technique of mercury intrusion porosimetry (MIP). It shows that the meta-sedimentary rocks have very low porosity, 0.18% for meta-siltstone and 0.53% for meta-sandstone on average. The thin section analysis reveals that the meta-siltstone contains more hydrophilic substances such as clay minerals than the meta-sandstone, and both two meta-sedimentary rocks are characterized by distinct beddings and laminae. Even though the water contents of two tested rocks are very low, they significantly influence the triaxial compressive strength and deformability. The reduction of strength by water content is found to be related to a reduction of friction angle in the Mohr–Coulomb failure criterion, while a reduction of mi value in the Hoek–Brown failure criterion on the other hand. The influence of water on deformability of tested rocks is reflected as a reduction of Young's modulus and increase of Poisson's ratio, which indicates that the wet meta-sedimentary rocks will deform more than that of the dry ones under the same stress condition. The experimental studies show that the anisotropy associated with bedding in rock specimens plays a weakening effect on the triaxial compressive strength for both tested rocks, appearing more severely for meta-siltstone. In general, the triaxial compression strength of rock specimens with transverse bedding planes is higher than the ones with longitudinal bedding planes no matter for meta-siltstone or meta-sandstone (both dry and wet conditions) under the same confining stress. With regard to the stiffness, both the tangent and secant Young's moduli of the two tested rocks with transverse bedding are less than the ones with longitudinal bedding, appearing more conspicuously for the meta-siltstone. The paper also discusses the axial strain calibration by two different measurement techniques, one by the linear variant difference transducer (LVDT) technique and the other one by axial strain gages. It shows that the axial strain ratio coefficient k (εLVDT/εstrain) decays with the deviator stress. In our present study, the secant modulus Estrain is usually about 1.4–1.8 times of ELVDT at the peak strength point. The correlation between mechanical properties and physical properties of the two tested rocks are also discussed in the paper.