Study on the identification method of tunnel surrounding rock failure zone based on continuous discontinuous analysis theory

Accurate analyzing the scope of tunnel excavation failure zone has important guidance and engineering significance in determining support parameters reasonably. This study focuses on the identification methods of tunnel surrounding rock failure zone, specifically the continuous medium analysis method and the continuous-discontinuous method represented by the finite element-discrete element coupling method (FDEM). Firstly, the continuous medium analysis method and FDEM identification criteria for surrounding rock failure are studied. Then the rock mass is divided into elastic rock elements and elastic-plastic interface elements. Based on the concept of equivalent continuous model, the relationship between the mechanical parameters of interface elements and rock elements and rock mass element is mathematically derived. The connection between the parameter values of these two methods is established for the first time, resolving the challenge of determining values in the continuous-discontinuous method. Finally, the ranges of surrounding rock failure zones simulated by these two methods during the excavation process of railway tunnels with different lithology and cross-sections are compared. According to the range of mechanical parameters for each class of surrounding rock mass in the specification, the range of values for the main failure parameters of surrounding rock, such as penalty parameter and fracture energy, in FDEM, is given for each class of surrounding rock. The simulation results of railway tunnel excavation with different lithology and cross sections using the continuous medium method represented by FLAC^3D and FDEM method show that the plastic zone obtained by the continuous medium method, and the failure zone obtained by the plastic limit strain, as well as the crack growth zone and failure zone obtained by the continuous-discontinuous method, are generally consistent in terms of distribution range, shape and failure mode. The method proposed in this article for determining the failure parameters of surrounding rock in FDEM is verified as reasonable and feasible.