Influence of interface roughness and rock type on mixed mode fracture characteristics of tunnel shotcrete-rock interface

A four-point shear (FPS) test was conducted to investigate the mixed mode fracture characteristics of the shotcrete-rock interface. Real-time monitoring of the fracture process was achieved using acoustic emission (AE) and digital image correlation (DIC) techniques. The mechanical test results indicate that residual stress increases with modal Angle or roughness under the same crack mouth opening displacement (CMOD). As the modal angle increases, the crack morphology and damage mechanism of the granite-shotcrete interface change. For large modal angles, the crack expands into the granite during the expansion process, leading to a larger range of granite damage at the interface. The results obtained through the nearest neighbor method reveal that specimens with small mode angle, low roughness or weak material properties on one side do not exhibit obvious background events, whereas the cluster characteristics of density distribution are evident. The AE events with smaller nearest neighbor distances were found to be concentrated around the macroscopic cracks. Moreover, the modal angle was observed to have a significant impact on crack opening/sliding displacement, with crack mouth sliding displacement (CMSD) and crack tip sliding displacement (CTSD) both increasing significantly as the modal angle increases.