An innovative test to study cracking behavior of fractured rock-like material under stepped excavation unloading

Fractures are commonly found throughout rock masses. The propagation and coalescence of these fractures can significantly weaken the surrounding rock strength during underground excavations. In order to investigate the cracking behavior of pre-existing fractures affected by excavation unloading, this study proposed an innovative laboratory excavation test to simulate excavation unloading in deep rock masses. Corresponding numerical stepped excavation tests were also conducted using RFPA (Realistic Failure Process Analysis). The excavation area in specimen was pre-filled with polylactic acid (PLA), and stepped excavation under constant normal stress was performed using an electric cutter. The results reveal that the stress-strain curves of the specimens during stepped excavation can be categorized into four stages based on deformation behavior and the crack process. There are four different patterns of crack coalescence, ultimately leading to specimen failure. Numerical results indicate that the inhomogeneous deformation caused by stepped excavation is responsible for crack initiation, propagation, and eventual specimen failure. The rock bridges between fissures and excavation areas break in three modes: shear sliding, segregation due to tensile cracks, and the formation of ubiquitous fractures. Furthermore, the inclination of the fissure and the initial normal load applied to the specimens have a significant impact on the crack coalescence modes and initiation. These findings can contribute to a better understanding of crack evolution behaviors, including initiation, propagation, coalescence, and transfixion, during excavation unloading in underground engineering projects.