Experimental study and discrete element method modeling of compression and permeability behaviors of weakly anisotropic sandstones

Abstract

Full understanding of the compression and permeability behaviors of weakly anisotropic sandstones is a major challenge. The present work aims to address this challenging task through a series of triaxial compression tests equipped with permeability measurement on sandstone specimens to examine their mechanical properties and permeability evolution during the failure of weakly anisotropic rock. The influences of bedding angle and osmotic pressure are examined. It is revealed that when the bedding angle is lower than 30°, the strength decreases with an increasing angle and sliding fracture along the weak bedding occurs. When the bedding angle increases to 45°, the strength increases simultaneously accompanied by a mixed failure of cracks crossing the bedding and sliding along the bedding. When the bedding angle increases to 90°, the strength decreases as the angle increases, and cracks crossing the bedding dominates the failure. These phenomena indicate that under a certain osmotic pressure, the lower the bedding angle, the greater the influence of bedding on the strength. After loading, the degree of anisotropy for permeability is larger than that of strength as permeability is more sensitive to the distribution of cracks (i.e., the seepage channels), which is essentially controlled by the bedding orientations. Numerical simulations are also performed with the use of two-dimensional discrete element method, in which the numerical model is calibrated to match the stress-strain and permeability-strain responses of the experimental results. Simulation results reveal that concentrations of contact force chains appear near the shear band of the specimen, and the zones of sparse force chains are found in the surrounding areas. For the specimen with a larger bedding angle, the permeability when it fails increases faster than other orientations.