Numerical study on transient unloading fracture mechanism of deep hard rock under 3D stress path induced by drilling and blasting

Transient unloading and rapid stress adjustment of surrounding rock during deep drilling and blasting excavation can easily induce hard rock fracture. Aiming at the dynamic adjustment of the magnitude and direction of the three principal stresses in transient unloading, the proposed spherical discrete element model was used to reveal the influence mechanism of principal stress magnitude, stress rotation and strain rate on transient unloading fracture for the first time. The results show that the transient unloading fracture rate of hard rock is significantly correlated with the magnitude and change trend of principal stress and has obvious inertia. The fracture rate is positively correlated with the maximum differential stress (σ₁-σ₃) and strongly negatively correlated with the minimum principal stress (σ₃). Both transient unloading and stress rotation will induce tensile strain rates with certain inertia and directionality. The stress rotation causes the transient unloading fracture rate significantly related to the strain rate component along the σ₃ axis (effective strain rate) rather than the principal strain rate. The maximum differential stress, minimum principal stress and effective strain rate constitute the stress-strain rate conditions for rapid fracture of rock under transient unloading. Increasing σ₃ will suppress the rate effect of transient unloading fracture, and there is a threshold (8.4 MPa in this study). The mutual influence between the tensile deformation and fracture of rock under transient unloading is weak and is related to the initial maximum unloading direction and the minimum principal stress direction, respectively, which is obviously different from the static mechanical response.