Pull-out characteristics of rock bolts under the influence of "natural" coupled fractures

This study investigates the pull-out characteristics of rock bolts with "natural" fractures induced by coupled dynamic-static load, which is imperative for assessing the stability of engineering structures and averting engineering disasters. "Natural" fractures under dynamic-static coupled load were simulated using a biaxial compression model and a SHPB model utilizing the PFC2D software. These coupling fractures were then incorporated into the bolt pull-out model after extraction and amplification. The impacts of these fractures on bolt's anti pull-out properties and the related micro-mechanical mechanism of bolt-rock interaction were analyzed. The results are as following: (1) The axial bolt load peak value, new crack numbers, and AE event peak values generally decrease as the pull-out specimen's damage degree increases. There's a critical threshold for impact damage affecting the bolt's pull-out characteristics, indicating that significant structural failure of the pull-out specimen must occur for notable changes in bolt pull-out characteristics. (2) The bolt's peak axial load declines with increasing fracture numbers and impact time and is weakly related to the peak displacement. Bolt tensile stiffness decreases with the rise in fracture number, biaxial compression confining pressure, and impact time. (3) The contact force accumulation within the rock mass evolves from a layered to a networked form as confining pressure increases. The distribution number of dynamic load-induced fractures directly impacts the rock mass's loading area and contact force transfer path. (4) The magnitude of normal and tangential contact forces is positively correlated with the load level. Rapid changes in particle's indirect contact force lead to a certain deviation in the distribution direction of normal and tangential contact forces during the post-peak phase.