Study on fracture failure mechanism and crack propagation law of granite-shotcrete composite structure

Deep buried tunnel engineering is in the high ground stress environment, shotcrete, and surrounding rock jointly bear the huge pressure of surrounding rock mass. In order to ensure the stability of tunnel structure, it is necessary to study the mechanical characteristics of rock-shotcrete structure. In this study, two kinds of granite with different mechanical properties were selected as research materials, and the beam of granite-shotcrete composite structure was made after pouring the same shotcrete. Based on the particle flow theory, numerical simulation was carried out by using the discrete element software PFC3D. After coupling the macro and micro mechanical parameters of each material, a model fitting the actual specimen was established. By comparing the experimental and numerical simulation results of four-point bending deformation and failure of specimens, it can be found that the fracture process can be divided into three stages: elastic deformation, the specimen cracks rapidly, and the specimen was unstable. Combined with the mechanical analysis, the bending failure mechanism was obtained. The bearing capacity of the composite structure is mainly affected by the tensile strength of shotcrete; secondly, the granite with higher elastic modulus can make the composite specimen show higher bearing capacity and better deformation resistance; when the granite stiffness is constant, the granite strength has little effect on the bending failure process of the composite specimen. The research results of this paper have certain reference value to ensure the normal and safe use of deep buried tunnel lining.