Study On The Stability Of Expanding Excavation Of Weak Surrounding Rock Tunnel Based On Fluid-solid Coupling

At present,most single-track railway tunnels are converted to double-line tunnels by adding second-line tunnels next to single-track tunnels.Existing structures are often used to expand and excavate tunnels with additional lines.If the construction is in rainy season or water-rich areas,the coupling between stress and seepage will become an important factor affecting the stability of the surrounding rock.This article takes the expansion of the existing parallel pilot pits of the Xinshanshutuo tunnel on the Yuhuai double line as the engineering background.Through the comparative analysis of numerical simulation and classic solution,it discusses that the stability of tunnel expansion is worse than that of excavation;it is established by FLAC3 D software The fluid-solid coupling model is compared with the ordinary model,and the analysis shows that the tunnel expansion stability considering the fluid-solid coupling is worse;the fluid-solid coupling model with different buried depths,different water levels,and different lateral pressure coefficients is established to analyze the tunnel expansion under different factors Stability and use corresponding criteria to evaluate the stability of tunnel expansion.The main research contents and results are as follows:Firstly,relevant literature is consulted and summarized,using the elastoplastic theory,according to the plane strain problem to study the distribution law of the plastic zone in the excavation and expansion of the weak surrounding rock circular cavern.The results show that the radius of the plastic zone in the direction of 45° expansion is larger than that of the excavation,and the expansion stability of the weak surrounding rock is worse.Secondly,the fluid-solid coupling and ordinary comparison model is established by FLAC3 D software,and the pore water pressure,stress,displacement,plastic zone distribution law and initial support force characteristics of the surrounding rock in the process of tunnel expansion with or without fluid-solid coupling are studied.The results show that,considering the interaction of fluid-solid coupling,the principal stress of surrounding rock,the settlement of the vault,the horizontal displacement of the arch waist,and the plastic zone all increase,and the stability of tunnel expansion in weak surrounding rock is worse.Thirdly,establish a fluid-solid coupling model with different burial depths,different water levels,and different lateral pressure coefficients to analyze the pore water pressure,stress,displacement,plastic zone distribution law and initial support force characteristics of the surrounding rock during tunnel expansion under different control factors the study.The results show that with the increase of burial depth,the rise of water level and the decrease of lateral pressure coefficient,the displacement and plastic zone of surrounding rock increase,and the change of burial depth has a significant influence on the stability of tunnel expansion in weak surrounding rock.Fourthly,parallel pilot tunnel soft surrounding rock tunnel enlarging and excavation construction simulation,and the reliability of the numerical simulation was verified by comparative analysis with the actual monitoring data.According to the relevant requirements and calculation methods of the current code,the reasonable support timing and initial support safety factor of the secondary lining were obtained Distribution.The results show that the weak surrounding rock tunnel is expanded by the two-step method,the second lining reasonable support time is 20 days,the safe step distance is 20 m,and the most unfavorable position of the initial support structure during the construction phase is the arch waist.