Fracture process of a simplified laboratory model for a cross-fault tunnel with rigid reinforcement rings: Experimental and numerical insights

Tunnels often unavoidably traverse unfavorable geological structures like faults and joints . These surrounding rock structures can typically be simplified as rock mass structures containing combined defects of holes and fissures. In order to study the whole fracture process of rocks with a hole and collinear hole edge fissures after hole reinforcement, indoor biaxial compression tests were conducted, including high-speed camera photography. Utilizing RFPA3D (Realistic Failure Process Analysis) numerical simulation and rock mechanics theory, the strain and stress field throughout the entire fracture process were analyzed to study the initiation and growth of cracks in the samples. The research shows that the reinforcement ring prevented the inner wall of the hole in the specimen from peeling and fracturing, contributing to the integrity of the hole. However, a shear fracture region can still be seen at the intersection of the hole and fissures. The failure of unreinforced specimens under biaxial compression is mainly due to the expansion of wing cracks and anti-wing cracks, while that of reinforced specimens is mainly due to the expansion of secondary shear cracks. Regardless of the type of fracture mode, crack initiation usually occurs at the outer end of the collinear fissure, specifically as a tensile crack. Sometimes, the process is also accompanied by the generation of multiple secondary shear cracks and tensile cracks at the tips, resulting in other final fracture mode. Strain concentration typically occurs at the collinear fissure’s tips and intersections of circular hole and fissures.