Optimization analysis of initial support of water-rich fault zone tunnel based on fluid-solid interaction

The design of tunnel supporting structure often relies on engineering analogy method and experience method, and there is no reasonable design method. Moreover, in order to ensure the stability of tunnel in water-rich fault zone, the designers often increase the supporting parameters, this makes the tunnel factor of safety too large, resulting in construction difficulties and unnecessary economic waste. Based on the fluid-structure coupling theory, the whole process of tunnel excavation and support across F8 water-rich fault zone is simulated by FLAC^3D in the background of longnan tunnel of Ganzhou-Shenzhen railway passenger dedicated line combined with the engineering and hydrogeological conditions of the fault zone and the construction characteristics of the six CD methods, the center axis, left vault area and right vault area of the middle part of the fault zone are selected to establish the deviation square sum function (DEVSQ) . The simulation results show that: (1) The reasonable space of steel arch frame is 1.0 m, the reasonable thickness of shotcrete is 26−30 cm, and the optimal thickness of shotcrete is 28 cm by cubic curve fitting. (2) When the space between steel arch frames is 1.0 m, the volume of the plastic zone decreases with the increase of the thickness of the initial branch of shotcrete. When the thickness of the initial branch of shotcrete is greater than 28 cm, the volume of the plastic zone decreases very little, it is not economical to increase the thickness of shotcrete in order to improve the safety of the initial-support structure, and the rationality of the optimization of the spacing of the initial-support steel arch frame and the thickness of shotcrete is verified. (3) Under the optimal initial support condition, the obvious location of seepage is arch foot > side wall > arch bottom > Arch crown. (4) Comparing the simulation results with the monitoring results, although there are some differences between the two results, the variation regularity is similar and the order of magnitude is the same, the simulation results can reflect the actual situation. The research results can provide important reference value and theoretical basis for other tunnel construction and support under similar stratum conditions.