Net distance optimization of super-large section tunnel using new elastoplastic-damage model combining tensile and shear yield modes

The minimum reasonable value of the net distance is a key problem to be solved in design and construction of small net distance tunnels, especially in super-large section tunnels. In this study, the elastoplastic-damage model (EDM) is improved by introducing a hyperbolic yield criterion where tensile/compressive and shear failure modes are combined together. The new model is solved by explicit finite difference method and implemented into Fast Lagrangian Analysis of Continua (FLAC3D) commercial software. Taking the Ganggou Tunnel as an engineering prototype, a series of numerical simulations of tunnel excavation process with different net distances are conducted through improved EDM. Based on comprehensive criteria for determining reasonable net distance, numerical results of distribution of plastic failure zone, displacement deformation and stress of surrounding rock are analysed to determine reasonable values of net distance for tunnels. Finally, the reasonable net distance values in different condition are given, e.g. 1B (B refers to the excavation span of tunnel) for V-grade rock tunnel, 0.75B for IV-grade rock tunnel and 0.5B for III-grade rock tunnel. These results can provide valuable references for design and construction of similar engineering projects.