Study On Mixed Support System Of Foundation Pit In Long And Narrow Metro Station

With the development of the domestic rail transportation industry,the pit engineering of the long and narrow metro station is increasing,and the mixed support system with reinforced concrete support and steel support is the most widely used.The mixed support system is an important part of the foundation pit engineering,and its parameters can significantly affect the mechanical characteristics of the foundation pit.Therefore,the reasonable mixed support system is very important for the foundation pit engineering.But its bearing characteristics are not entirely clear,this article relies on a long and narrow metro station in Foshan to carry out research.On the basis of summarizing the research results at this stage,this paper conducts the site monitoring test of the foundation pit project of a long and narrow metro station in Foshan,and studies the deformation law of the foundation pit of the mixed support system.At the same time,the MIDAS/GTS NX finite element analysis software is used to simulate and analyze.On the basis of the original model,the parameters of "two concrete and one steel" hybrid support system are analyzed.The main contents and conclusions are as follows:(1)Through site monitoring tests concluded that there is a difference mixed concrete support system with full support system.The maximum value of surface subsidence and the horizontal displacement of the wall are slightly larger than that of the whole concrete support system.The vertical displacement of the wall is independent of the support system.(2)During excavation to the third support position and the backplane is closed,the incremental displacement curve of the area of the whole concrete support system is smooth"convex" shape,and the incremental curve of the mixed support system is similar to the"triangular" shape.(3)The finite element analysis software of MIDAS/GTS NX finite element analysis software was used to analyze the foundation pit model,and the accuracy of the simulation results was confirmed by comparison with the measured values.On the basis of the original model,the comparative analysis is made by adjusting the support type of the mixed support system,and the first reinforced concrete support can be used to reduce the roof horizontal displacement of the wall is 66%.(4)Based on the model of "two concrete and one steel" mixed support system,the support stiffness of each support,the spacing of the support system and the spacing between the support systems are analyzed respectively.The convergence trend of the displacement curve in the course of the support stiffness change is obtained.The cross-sectional spacing of the mixed support system is preferably 0.3-0.4 times the width of the foundation pit.And the optimal position of the second support position is near the net height of the 2/3 support system.Finally,based on the above analysis,this paper puts forward the suggestions of selecting the parameters of the mixed support system of the long and narrow metro station,and provides reference for the engineering practice.