Effect of Unloading Direction on Rock Failure under True Triaxial Stress Conditions

Rock failures induced by excavation unloading in the direction of the minimum principal stress sigma(3) and the intermediate principal stress sigma(2) were numerically simulated and analyzed. The damage evolution process, the failure mode, and the failure mechanism under the two conditions were investigated. The results show that the unloading direction of the principal stress has a significant impact on the rock failure mode. Under the same triaxial stress level, when unloading sigma(3), rock failure is mainly caused by localized tensile-shear composite failure; when unloading sigma(2), rock failure is mainly caused by tensile buckling failure. When unloading sigma(3), the damage is mainly concentrated near the free surface; however, when unloading sigma(2), the damage range increases and develops toward the interior of the rock. The greater the intermediate principal stress is before unloading, the larger the number of splitting fractures and the magnitude of dilatation are, and the smaller the thickness of the splitting plate is. These results may shed light on the prevention and mitigation of rock instability in deep underground engineering.