Mechanical properties and failure characteristics of specimens containing a couple of flaws under unloading condition has been studied by particle-flow-code (PFC) simulation. The results show that different unloading rates (Δt) and flaw inclination angles (α) result in variations in the unloading strength, deformation and crack effect for the pre-flawed specimens. Compared with intact specimens (without pre-flaws), the unloading strength of the pre-flawed specimens decreases nearly 5.5~20%, and faster unloading rates or smaller flaw inclination angles will lead to lower strength. Furthermore, the crack effect are investigated to understand the failure mechanism. It is observed that unloading rate exerts little effect on the failure modes but can affect the number of cracks, and slower rates will generate more cracks in the specimens. In addition, the changes in flaw inclination angles will influence the failure modes and cracks number. When α = 0° and 30°, the final failure modes of the specimens are similar with each other, while α = 60°specimen is more likely to be inhibited in the generation of cracks.
Excavations in rock mass under high geostress are at great risk, because unloading process would have an important effect on the failure characteristics of rock mass. And the discontinuities such as joints, cracks, fissures, weak surfaces, faults, etc. in rocks play significant roles the mechanical behaviors of rock mass under excavation unloading (Wong and Chau 1998). The mechanical properties of intact rock during unloading under high geo-stress has been widely investigated by laboratory experiments (Bauch et al., 2004; Zhao et al., 2014; He et al., 2010; Huang et al., 2014; Du et al., 2016) and theoretical analyses (Li et al., 2013; Wang et al., 2010). However, numerical simulation of pre-flawed rock specimens, which refer to rock specimens containing pre-existing flaws, under unloading condition have rarely been investigated.