To meet the development trend of large diameters and high water pressures in shield tunnels, the double-gasket waterproofing system has become the mainstream design scheme for segmental joints in large underwater shield tunnels. The existing double sealing gasket waterproof system can be divided into two types: double sealing gaskets arranged on both sides of the joint (inner and outer double) and double sealing gaskets arranged on the outer side of the joint (outer double), but the influence of these two kinds of waterproof gaskets arrangement on the mechanical properties of segment structures has not been revealed. Based on Jiangyin-Jingjiang Yangtze River Tunnel, the influence of the arrangement of double waterproof sealing gaskets on the mechanical properties of the through joint assembly and staggered joint assembly segment structures is compared by a three-dimensional refined numerical calculation model for the whole segmental ring structure. The results show that: When the arrangement of double waterproof sealing gaskets changes from inner and outer double to outer double, the deformation of the segment structures slightly increases, the axial force of the segment structures changes less, the maximum positive bending moment decreases, and the maximum negative bending moment increases. The safety factor of the segment structures with the outer double sealing gasket arrangement is slightly lower than that with the inner and outer double channel sealing gasket arrangement, but both have higher safety factors and a larger safety margin. Overall, the arrangement of double sealing gaskets on the outer side significantly improves the waterproof ability of the segmental joint, but does not cause excessive deformation of the segment structures. The overall structural system still maintains a considerable safety margin and is currently the most effective option for resisting high water pressure and enhancing the durability of the shield tunnel waterproofing system.

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.