Numerical study on path dependence in transient unloading of rock masses under non-hydrostatic pressure

Abstract

Understanding dynamic mechanical behavior during rock mass transient excavation is crucial for gaining insights into the safe construction of underground engineering projects such as tunnels, mining, and hydraulic stations. However, descriptions of the dynamic response and failure evolution on surrounding rock under an inhomogeneous stress field and arbitrary unloading path are still ambiguous. In this study, we established a rapid calculation framework for dynamic response induced by transient unloading under non-hydrostatic pressure based on modal decomposition and Duhamel integral. A piecewise fitting expression for the peak circumferential stress increment was proposed to predict the dynamic overshoot. Moreover, we achieved precise control over arbitrary transient unloading paths under non-hydrostatic pressure conditions for arbitrary shaped tunnel by introducing the equivalent nodal force release technique within the explicit dynamics code LS-DYNA. Our results show that the dynamic stress redistribution and progressive damage of the surrounding rock exhibit significant path dependence. The faster the unloading, the larger the dynamic increment. We found that the shear damage and fracture zone around the excavation face results from the combined effect of transient stress redistribution and the time-dependent evolution of wave propagation. The geometric corner regions of non-circular tunnel further enhanced the dynamic amplification effect induced by transient unloading. These results emphasize the important role of unloading path in controlling the overall dynamic behavior and the local stress concentration.