Failure characteristics of coarse and fine sandstone containing two parallel fissures subjected to true triaxial stresses

Failure characteristics of flawed rocks subjected to true triaxial stress are not studied, and the cracking mechanism is not fully understood. Especially, the damage and failure mechanisms incorporating the effect of material properties such as grain diameter are seldom revealed. In this study, true triaxial compression tests are conducted on flaws-contained specimens of coarse and fine sandstone, to explore the coupled effects of the microstructure and the pre-existing flaws on cracking characteristics and further to reveal the cracking mechanism. Acoustic emission (AE) data and the mechanical data are the main experimental output. It is found that cracking characteristics are collectively affected by the microstructure features and the geometrical features of specimen-sized pre-existing flaws. When the confining pressure is relatively large, the mechanical interaction between coarse grains inhibits the specimen's rupture and contributes to the increase of the strength of coarse sandstone. Due to the complex interaction between grain-scale microcracks, the micro-shear cracking mechanism and the micro-tensile cracking mechanism are presented in the physical process of damage progression in coarse sandstone. However, the microcrack clustering phenomenon in fine sandstone is insignificant, and the interaction between grain-scale microcracks is weak. Accordingly, the micro-shear cracking is seldom generated in fine sandstone.