Study On The Stabilityof Excavation Foundation Pit In Subway Area And Optimization Of Support Scheme

With the continuous advance of urbanization,the city will be more and more foundation pit engineering,the construction environment will be more complex,which makes engineering researchers pay more attention to the stability of the foundation pit.In the current foundation pit engineering design and construction,people have long been troubled,it is difficult to strike a balance between the economy and security.To solve this contradiction,it is necessary to optimize the support scheme of foundation pit.Therefore,the stability of the foundation pit and the optimization of the support is an urgent problem to study and research now.This paper intends to analyze the stability of the excavation pit in Chongqing Metro Line 4,and optimize the support scheme.The main work and research contents are as follows:(1)To collect the hydrogeological conditions of the project,the distribution of nearby ground buildings,underground pipeline conditions,support system design,excavation design,construction organization design and other relevant information.It is concluded that the geological and hydrological conditions of the project are good and the construction conditions are convenient.The influence of the sliding surface and the precipitation on the foundation pit is mainly considered,and the influence of the construction process on the deformation of the surrounding buildings should been controlled.(2)Using limit equilibrium method and finite element method to analyze the stability of the foundation pit of the original scheme,comparing the difference between displacement and internal force of foundation pile and ground surface settlement under two methods,And compared with the monitoring measurement data.Through the analysis,it is concluded that the foundation pit engineering is stable at all stages of the construction.the calculation results of limit equilibrium method is larger than the finite element method,the two results of the monitoring and the finite element method are not the same,the calculation results of finite element method is closer to the actual situation,the original design is conservative,it is necessary to optimize.(3)Using the finite element method to analyze the influence factors of the stability of the foundation pit.The main influencing factors are the number of steel support layers,the vertical spacing and horizontal spacing of steel support,the stiffness,prestress of the steel support,pile diameter and pile spacing.studying the influence of each factor on the displacement and internal force of the pile,summarizing the influence of each factor on the stability of the foundation pit:the increase of the number of supporting layers can reduce the deformation of the foundation soil and the retaining pile and reduce the bending moment of the pile.the prestressing can reduce the deformation of the enclosure structure,and when the prestress reaches a certain value,the effect of controlling the deformation becomes insignificant,and when the prestressing force is too large,the enclosure structure will be deformed to the outside of the foundation pit;the influence of the change of the stiffness,the horizontal spacing and the pile spacing on the deformation and internal force of the retaining pile is very small.(4)Using the above analysis results to optimize the retaining structure of the foundation pit,through the comparison of the proposed scheme and the original scheme,the optimized scheme is as follows:a layer of steel support is set up,the horizontal distance of steel support is 4m,the steel support dimension is "Φ609mm",pipe wall thickness t=12mm,pile spacing adjusted to 2m.Through the work and research,the design of the foundation pit is optimized,and the investment of the project can be reduced under the stability of the foundation pit.The research results of this paper provide reference for the design and construction of similar projects in the future.