Mechanical Response and Instability of Granite with Parallel Non-Persistent Joints Under Rapid True-Triaxial Unloading: A PFC3D Numerical Study

Rapid excavation in deep rock masses imposes a non-proportional stress path that can destabilize jointed rock. A three-dimensional Particle Flow Code (PFC3D) true-triaxial model of granite containing two parallel non-persistent joints was used to examine the effects of joint length, joint spacing, joint dip angle, and minimum principal stress, followed by 25 joint length–dip angle combinations. The prescribed path rapidly released the minimum principal stress to zero after the major principal stress reached 50 MPa while maintaining the intermediate principal stress at 40 MPa, followed by continued axial loading to failure. In the single-factor tests, joint length caused the most systematic deterioration: post-unloading strength fell from 103.22 to 36.85 MPa as joint length increased from 5 to 45 mm, and specimens with 40–45 mm joints could no longer recover the load-bearing level at unloading onset. Joint dip angle produced a pronounced V-shaped anisotropy, with the lowest strength at 15–30°. More importantly, long joints altered when damage accumulated: 55.6–69.0% of the cracks formed before the post-unloading strength was reached were generated during rapid unloading, while the peak elastic energy-storage ratio decreased from 76.4% to 31.9%, indicating a transition toward dissipation-dominated behavior. In the factorial analysis, joint dip angle and joint length explained 61.67% and 24.32% of the variation in the composite strength–energy-storage response, respectively, and longer joints amplified the weakening associated with unfavorable intermediate dip angles. Across 85 numerical cases, low strength–energy-storage capacity coincided with high unloading-induced crack fractions and loss of post-unloading strength recovery. These results identify a coupled instability mechanism governed by rock-bridge degradation, earlier damage activation, and reduced elastic energy storage, providing a basis for recognizing unloading-sensitive joint configurations in deep excavation.