Time-effect properties and mechanisms of marble failure under different unloading rates

In the process of tunnel excavation, the failure properties of rock mass are obviously affected by unloading rates, and the failure properties show a clear time effect. Considering the stress characteristics of rock mass under unloading, the unloading tests on brittle marble are simulated by the particle flow code (PFC), then the failure properties and mechanisms at the unloading finished point and the continuous point are analyzed. The numerical modeling results show that the deformation rate decreases with the increasing unloading rate in the process of unloading. Moreover the lateral deformation is more sensitive than the axial deformation. At the unloading finished moment, most micro cracks formed in the samples are distributed at the top and bottom sides of the sample, and the number of tensile cracks is about 3~6 times more than that of shear cracks, then these cracks form shear fracture bands. Moreover, at this moment, the fracture degree of samples decreases exponentially with the increase of unloading rate. At the subsequent moment from the unloading finished, the failure mode of macroscopic shear fracture occurs at low unloading rates, while the failure mode turns to block spalling and particle (block) ejection at high unloading rates. Furthermore, at the subsequent moment from the unloading finished, the fracture degree increases exponentially with the increase of unloading rate. When reaching the subsequent moment from the unloading finished, the faster the unloading rates, the greater the kinetic energy release, and the rock burst is more violent.