Research On The Instability Failure Mechanism Of Subway Tunnel Excavation Under The Influence Of Water-rich Sand Layer And The Thickness Of Safe Water Barrier

In recent years,with the rapid development of China's national economy,the scale and quality of rail transit are getting bigger and bigger.As an important part of urban rail transit,subway has the advantages of large volume of traffic,fast speed,accurate running time,less land occupation and no influence from external factors such as weather,which can effectively alleviate problems such as population density,housing shortage and traffic congestion in the process of urbanization.However,due to the influence of construction conditions,design parameters and stratum conditions,subway tunnel inevitably passes through the complex engineering geological conditions such as soft stratum,water-rich stratum and flowing sand stratum,which brings significant adverse effects on construction,construction and operation and maintenance.Among them,it is common for subway tunnel to pass through water-rich stratum,and engineering risks such as collapse,water inrush and mud gushing and surface settlement are easy to occur in the excavation process.The stability of tunnel surrounding rock is very serious.Reasonable determination of the thickness of the safe water-proof layer between the main structure of the tunnel and the water-rich sand layer is an important premise to ensure the safe construction of this kind of tunnel.The excavation of subway tunnel with water-rich sand layer in Pi-Miao section of Qingdao Metro Line 2 is taken as the research object,and the FLAC3 D finite difference software is used to study the influence of six kinds of factors on the stability of surrounding rock,including different thickness of water-rich sand layer,thickness of water-rich sand layer,underground water level and mechanical parameters of water-rich soil layer.According to the principle of single variable,a total of 25 groups of schemes were designed to simulate the instability failure law of subway tunnel excavation in water-rich sand stratum under the influence of different factors.According to the results of numerical simulation,the sensitivity of different factors on the stability of tunnel excavation is analyzed based on the theory of grey correlation analysis,and the correlation degree of the factors affecting the stability of tunnel surrounding rock in water-rich sand layer is ranked.At the same time,based on the limit equilibrium method,the analytical solution of the safe thickness of the waterproof layer is derived,and the method of predicting the safe thickness of the waterproof layer under the influence of the water-rich sand layer is proposed.The main work includes the following three aspects:(1)Based on the finite difference software FLAC3 D,30 groups of comparison schemes were designed considering six influencing factors,including different thickness of water-rich sand layer,thickness of water-rich sand layer,underground water level and mechanical parameters of water-rich sand layer,to conduct a comparative study on the instability failure mechanism of subway tunnel excavation under the influence of water-rich sand layer.The results show that the tunnel vault is the weak part of surrounding rock deformation and failure in water-rich sand layer.With the increase of the thickness,cohesion and friction Angle of the waterproof layer,the deformation of surrounding rock and the area of plastic zone at the top decrease continuously.With the increase of the thickness of water-rich sand layer,the weight of water-resistant layer or the rise of groundwater level,the deformation of surrounding rock at the top and the area of plastic zone increase continuously.The thickness and cohesion of waterproof layer are the key factors affecting the stability of surrounding rock of tunnel with water-rich sand layer.(2)According to the deformation and failure characteristics of surrounding rock of metro tunnel under the influence of water-rich sand layer,based on the theory of grey correlation analysis and taking the settlement of tunnel vault as the reference matrix,the grey correlation difference of influencing factor matrix is analyzed.By calculating the correlation degree,the order of factors affecting the stability of surrounding rock of tunnel with water-rich sand layer is obtained: cohesion of water-rich sand layer,thickness of water-rich sand layer,underground water level,friction Angle of water-rich sand layer,weight of water-rich sand layer and thickness of water-rich sand layer.The results show that the cohesive force and thickness of waterproof layer are the key factors affecting the stability of surrounding rock.(3)based on the limit analysis theory,constructs the rich water sand layer under the influence of the tunnel vault water-resisting layer within the scope of rupture mechanism of soil mechanics model,combining with the principle of virtual power tunnel safety thickness of water-resisting layer was deduced theory formula,this paper proposes a forecasting method of safe thickness of water-resisting layer and analysis for the influence of different factors on the safety thickness of water-resisting layer are studied.Among them,with the increase of soil cohesion,soil weight and friction Angle of the waterproof layer,the thickness of the tunnel waterproof layer decreases gradually.With the increasing of the supporting force,the thickness of the waterproof layer decreases.