Study On The Influences Of Top-down Method Excavation And Dewatering On Adjacent Metro Station And Tunnenls

Foundation pits which adjacent to the metro station and tunnels are not uncommon in current cities,it is crucial to correctly evaluate the impact of lateral excavation on them.At present,most of the researches focus on the influence of excavation on the lateral or pit tunnel,the research on the influence of top-down method excavation on metro station is still rare.Also,a majority of related researches have neglected the influence of foundation pit dewatering on subway structure.Relying on the Foundation pit construction of Kunming YueHui shopping mall,this paper studies the force and deformation of the metro station and tunnels under the action of dewatering and top-down method excavation based on the means of finite element analysis,the main work includes the following aspects.(1)The influence mechanism of the foundation pit construction on the subway structure is analyzed in this chapter.On the basis of Flamant theory and Mindlin theory,the calculation method of the additional stress on tunnel structure caused by bottom unloading and lateral unloading of foundation pit excavation is summarized,especially,the effective self-weight stress change on the tunnel structure caused by dewatering and seepage force are considered.Based on winkler elastic foundation beam theory,this paper introduces the method to solve the longitudinal deformation,bending moment and shear force of tunnel.(2)Based on the investigation of the engineering and hydro-geological characteristics of this area,the 3D-finite element model about the whole process of the top-down method excavation is established by using Midas/GTS finite element software.Without considering the Baiyun road Metro station and tunnels,this chapter analyzes the variation law of the soil and supporting structure displacement in the case of different construction schemes and whether the dewatering is considered.The results shows that the model simulation result is reasonable;top-down method construction has little impact on the surrounding soil displacement.The displacement of the soil outside the pit and the deformation of the retaining structure are obviously greater when the dewatering is considered.There is little disparity in the results of different excavation schemes.The displacement of the soil outside the pit and the deformation of the retaining structure are obviously larger than those without considering dewatering.(3)On the basis of the previous chapter,a three dimensional finite element model is established which includes the foundation pit of the Yuehui and the Baiyun road metro structure.Considering dewatering of foundation pit,the influence of top-down method construction on adjacent metro station and tunnel is analyzed,the results show that the seepage field and displacement field is changed by the existence of the metro structure.In the initial stage of foundation pit excavation,the uplift of the excavation surface causes the deformation of the metro structure to rise up and deviate from the direction of the foundation pit,under the dewatering of the foundation pit,the deformation gradually changes from upward to subsidence,and the structure is offset to the foundation pit.when neglects dewatering,the metro structure increases gradually with the excavation of the foundation pit,which is inconsistent with the actual situation.Due to the different stiffness of the tunnel and station,differential deformation at the junction is also produced,which can be predicted by non-uniform coefficient.Reverse foundation pit construction has little influence on the deformation and stress of adjacent metro structure,the simulation results are much lower than the alarm value.The pore water pressure of soil surrounding tunnels decreased due to dewatering,the bending moment of tunnel increases and the deformation becomes more serious,the closer part to the foundation pit takes the greater influence.The influence on the tunnel which caused by the depth of the curtain wall,the dewatering scheme and the permeability coefficient of the soil layer are analyzed,the result reveals that the law of the deformation and stress of the tunnel are basically the same under the influence of different factors,however,the depth of cut-off curtain has greater influence on tunnel than the the dewatering scheme and the permeability coefficient of the soil.