Triaxial mechanical behavior and damage fracture mechanism of thick-walled hollow cylindrical sandstone

To investigate the influence of excavation on the stability of underground engineering, firstly, the influence of confining pressure and inner diameter on the strength and deformation characteristics of thick-walled hollow cylindrical sandstone specimens was studied by using a thick-walled hollow cylinder triaxial compression tests; secondly, the damage and fracture characteristics of the thick-walled hollow cylindrical sandstone were explored by real-time acoustic emission(AE) monitoring and CT scanning; and lastly, discrete element simulation was used to reveal the microscopic damage fracture mechanisms of the thick-walled hollow cylindrical sandstone specimens under the triaxial compression condition. Mechanical test results showed that the elastic modulus and tangent modulus of the thick-walled hollow cylinder sandstone specimens only showed strong positive correlation with the confining pressure, and there was no significant relationship with the inner diameter of the thick-walled hollow cylinder sandstone; the confining pressure had a significant reinforcing effect on the peak strength of the thick-walled hollow cylinder sandstone, and the nonlinear Hawk-Brown criterion could better characterize the relationship between them. AE test results indicated that the peak AE ringing counts of the specimens were decreased due to the increase of the confining pressure; at the lower confining pressure, with the increase of the hole diameter, the peak AE ringing counts tended to decrease, while at the higher confining pressure, the peak AE ringing counts had no significant correlation with the hole diameter. The CT results showed that with the increase of the confining pressure and hole diameter led to the decrease of the macroscopic fracture angle of the thick-walled hollow cylinder specimens and the increase of the fracture degree of the inner wall; under the same confining pressure, the increase of the hole diameter significantly increased the three-dimensional cracking volume percentage and fractal dimension; for the same hole diameter sandstone specimen, the uniaxial compression of the specimen had the highest three-dimensional cracking volume percentage and fractal dimension, and under the triaxial compression condition, the cracking volume percentage of the specimen's internal cracks decreases with increasing confining pressure, but its fractal dimension is significantly positively correlated with the confining pressure. Numerical simulation analysis revealed that with the increase of confining pressure, shear cracks inside the thick-walled hollow cylindrical sandstone gradually dominated and the total number of cracks showed an increasing trend; with the increase of hole diameter, the total number of cracks inside the specimen gradually decreased; and compared with the confining pressure, the number of cracks had a higher sensitivity to the hole diameter.