中文摘要：

巷道开挖过程中卸荷速率对岩体破裂特性有显著影响,且破裂特性表现出明显的时间效应。针对岩体在卸荷条件下的受力特征,利用颗粒流程序,对脆性大理岩进行围压卸载数值模拟,研究不同卸荷速率下卸荷结束瞬间和卸荷后持续点的岩石试样破裂特性和机理。结果表明：围压卸载过程中,卸荷变形率△ε_i随卸荷速率的增大而减小,且侧向变形比轴向变形更敏感;在卸荷结束瞬间,微裂纹主要集中在试样上下端部并形成剪切破裂带,其中张拉裂纹数目是剪切裂纹的3~6倍,试样的破裂程度S_1随卸荷速率的增大呈指数递减规律变化;在卸荷后持续点,卸荷速率较慢的情况下岩石试样破坏形式为宏观剪切破裂面,卸荷速率较快的情况下岩石试样破坏形式为块体剥落及上下端部颗粒（块体）弹射,试样的破裂程度S_2随卸荷速率的增大呈指数递增规律发展;卸荷速率越快,达到卸荷后持续点时,试样累计释放的颗粒动能越大,岩爆程度越大。

英文摘要：

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 3-45 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.