The Crack Initiation Stress, Crack Damage Stress, and Failure Characteristics of Mudstone Under Seepage Conditions in Different Principal Stress Directions

In deep underground engineering projects, the rock mass is frequently subjected to extreme environments characterized by high geostress and high permeation pressure. This makes the rock mass highly prone to disasters such as collapses, significant deformations, and water seepage. Among these factors, the direction of seepage plays a critical role. In this study, true triaxial tests were performed to investigate the characteristic stress and failure behaviors of mudstone under seepage conditions in different principal stress directions. The test results indicate that, under permeation pressure (σp), the characteristic stresses are significantly reduced. After TTS-1 shifts to TTS-2, the permeability of the mudstone decreases significantly. A volumetric dilation hysteresis effect of mudstone was discovered. Furthermore, the increase in β1 and decrease in β2 indicate that, after the transition from TTS-1 to TTS-2, the stable crack propagation stage in the mudstone is prolonged, while the unstable crack propagation stage is shortened. In the σ1–σ3 plane, after TTS-1 shifts to TTS-2, the change in the included angle between the mudstone fracture surface and the σ1 direction shows a reverse trend with the increase in σp.