Study Of Water-rich Tunnels Stability Based On Fluid-solid Coupling Limit Analysis

China’s southwest region is mountainous and rich in groundwater,and the hydrogeological conditions faced by road tunnels during construction are complex and variable.A large number of engineering practices at home and abroad show that tunnels through water-rich geological sections are often prone to engineering disasters such as water surges,mud surges and even collapse,so it is necessary and practical to carry out research on the stability of tunnels under the action of flow-solid coupling.Taking the Daxuechengfuxian tunnel as the engineering background,this paper analyzes the difference between the tunnel surrounding rock stability with and without seepage by theoretical analysis;analyzes the tunnel stability with or without considering the fluid-solid coupling effect,different seepage control factors and grouting circle parameters by numerical simulation and evaluates its stability using the safety factor of the surrounding rock calculated by the ultimate strain method,the main research contents and results are as follows:（1）An axisymmetric plane strain problem model based on the Mohr-Coulomb yield criterion is used to compare and analyze whether to consider the elastic-plastic solution of circular tunnels under the effect of seepage by arithmetic examples.The results show that the surrounding rock stress,radial displacement and plastic zone radius increase under the effect of seepage,and the surrounding rock tangential stress is most significantly affected.Using the finite difference overload method of numerical limiting analysis method,the limiting shear strain of the reinforced surrounding rock used in this paper is calculated in FLAC3D as 9.50×10^-4,and taking the connection of the limiting shear strain zone as the overall instability criterion of the surrounding rock to solve the safety factor.（2）To address the limitations of the theoretical solution,FLAC3D software is used to respectively numerical simulate the tunnel three-step method and CRD method excavation for the fully fluid-structure coupled model and the non-fluid-structure coupled model.Analyzed the change law of seepage field,displacement field,stress field,plastic zone,force of support structure and surrounding rock safety factor based on ultimate strain.The results show that the two excavation methods show the same variation law under the effect of fluid-solid coupling compared to that without considering fluid-solid coupling,the arch top and arch bottom vertical displacement and rate is increased,the range of plastic zone and ultimate strain zone of surrounding rock is expanded,the safety factor is reduced,the initial support arch bottom is transformed from compressive stress to tensile stress,and the overall stability of the tunnel is decreased.（3）On the basis of the flow-solid coupling model,the influence law of different initial water levels and tunnel groundwater discharge on the tunnel stability is studied by numerical simulation,and the relative weak position of the tunnel is determined.The results show that:the groundwater flow rate at the foot of the arch is the fastest and the pressure of water outside the initial support is the largest,so the construction should pay attention to the reinforcement and drainage prevention treatment at the foot of the arch;with the initial water level increases tunnel stability significantly reduced,the arch bottom is the most sensitive to the height of the water level,the high water level is prone to bottom drum damage;with the increase in tunnel drainage,the rise of the arch bottom of the surrounding rock is reduced,and the force at the bottom of the initial support arch is improved,due to the effective drainage of pore water the safety factor of the surrounding rock is improved,increasing the tunnel drainage can significantly reduce the impact of the arch bottom by ground water.（4）The influence law of different grouting circle parameters on tunnel stability is studied by numerical simulation.The results show that:increasing the thickness of grouting circle and permeability ratio all help to improve the water plugging effect of anti-seepage circle,but in the case of increasing permeability ratio,the safety factor of the surrounding rock is slightly reduced due to the ultra-static pore pressure acting on the rock fractures for a long time,so the tunnel taking grouting should be paired with corresponding drainage measures to control groundwater together.The reasonable thickness of the grouting circle in the project is 5m and the permeability coefficient ratio is 50.（5）Construction simulation of a water-rich section of the relying project.The results show that:the change trend of the simulated and field measured surrounding rock deformation is basically the same,the actual project should be applied in time after excavation for the initial support and closed into a ring;when taking the upper limit of5.0 m³/m·d as the drainage volume of the mountain ridge tunnel drainage limit standard,the water level does not exceed 60m from the vault of the tunnel in the grouting thickness of 5m,the permeability ratio of 50 the safety factor of the surrounding rock are greater than 1.15.The proposed grouting design scheme can provide design and construction reference for similar projects.