Study on the Evolution of Rock Fracture under True Triaxial Intermediate Principal Stress

An investigation of rock fracture characteristics subjected to the true triaxial intermediate principal stress is crucial for understanding the failure mechanism of surrounding rock in an actual underground engineering project and guiding the project's implementation. In this study, a series of different true triaxial stress path tests were conducted to investigate the distribution of macroscopic rock fractures and internal cracks under various stress paths using a true triaxial servo test system and combining it with CT scanning technology. The results indicated that, due to the intermediate principal stress with constant direction, the true triaxial stress path has obvious directionality compared to the regular triaxial rock macroscopic and internal fracture distribution law and the fracture planes are aligned along the sigma 2 direction. Once the direction of the intermediate principal stress changes, the corresponding crack propagation direction immediately reverses and expands along the new intermediate principal stress direction. The direction of rock fracture is strongly dependent on the state (direction) of the intermediate principal stress, i.e., the direction of sigma 2 plays a decisive role in the macroscopic rock fracture characteristics. Accordingly, the "effect of intermediate principal stress-induced rock fracture" was proposed in this study. Based on the results of the CT scans in which the fracture dip angle within the rock is approximately parallel to the sigma 2 direction under different true triaxial paths, the intermediate principal stress plays a vital role in rock rupture. Combined with experimental and theoretical analyses, the rationality of this effect was verified. In addition, a numerical calculation method for the true triaxial rupture evolution of rocks with random fractures was proposed using ABAQUS software (version 2020), and a numerical calculation model for typical roadway excavation was developed. The rupture growth of the roadway surrounding rock was characterized by stratified evolution along the sigma 2 direction corresponding to the actual engineering. In addition, the numerical analysis of a typical roadway excavation with two different intermediate principal stress orientations indicated that the fracture of the surrounding rock in both kinds of roadways is roughly along the sigma 2 direction, which further supports the rationality of the "effect of intermediate principal stress-induced rock fracture."