Abstract: Excavation in the rock mass engineering is inevitably associated with the rock mass unloading. In this paper, the granite is selected as the test material, the rupture characteristics of the granite under linear unloading conditions are studied experimentally. The rapid dilation in the radial direction and the enhancement of the plasticity are found. Furthermore, the FJM (flat joint model) in the PFC (particle flow code), is applied to simulate the mechanical properties of the granite under unloading conditions. A new “stress/time step” unloading method, which can easily execute the simulation of the nonlinear unloading behavior, is used considering that the excavation of the rock mass in the practical engineering is mostly characterized as the nonlinear unloading behavior. Numerical simulation results show that the granite is seriously damaged when the axial pressure is increased and the confining pressure is unloaded. The load capacity was decreased sharply and no residual strength is found. As the initial confining pressure increases, the failure mode is transformed from the splitting to the shear failure. The faster the unloading speed, the slower the crack propagation is found. The research results can provide the reference for the study of the mechanical behavior and the practical engineering under unloading conditions.

Key words: granite, unloading rock mass, mechanical characteristic, numerical simulation