| With the advancement of urbanization,it is necessary to continuously rebuild or build new urban transportation infrastructure to meet people’s travel convenience.Urban rail transit is an important solution to this problem.The urban rail transit project is implemented in stages,and the gradual encryption of the transportation network will inevitably lead to the close construction of underground projects.It is necessary to properly handle the contradiction between new projects and existing projects,and at the same time meet their technical,economic and safety requirements.Based on the new Xinyue Tunnel near the existing Metro Line 2 project in Xiamen,this paper analyzes and studies the influence of foundation pit excavation construction parameters on the main structure of foundation pit and the existing subway tunnel by means of theoretical analysis and numerical simulation.(1)Summarize the deformation theory of foundation pit,and summarize the theoretical calculation methods of ground settlement around foundation pit,horizontal displacement deformation of retaining structure and rebound uplift of foundation pit bottom caused by foundation pit excavation.(2)Using the mechanical model of shield tunnel under the influence of foundation pit excavation,the calculation formula of additional stress caused by unloading stress at the bottom and side wall of foundation pit in the process of foundation pit excavation is derived,and the deformation calculation formula of existing shield tunnel is derived based on Winkler foundation beam theory.(3)Combined with finite element software,a three-dimensional numerical model is established under two spatial distribution States,the foundation pit spans the existing shield tunnel and the foundation pit and the shield tunnel are in parallel.Considering factors such as excavation sequence,excavation layered thickness,water retaining cofferdam,the deformation of the ground around the foundation pit,foundation pit support,foundation pit bottom and the settlement and deformation law of the existing tunnel are analyzed.Through the above work,the main research results are as follows:(1)when the cofferdam is piled up beside the foundation pit,the maximum settlement of the outer surface of the pit appears at the corresponding position of the crosssection centerline of the cofferdam;After the foundation pit excavation,slight uplift occurred at the outer slope toe of the water retaining cofferdams on both sides.(2)When the foundation pit(slope excavation)crosses over the existing shield tunnel and is in an oblique state,strengthening the bottom of the foundation pit with triaxial mixing piles can improve the uplift of the foundation pit bottom and the uplift of the existing shield tunnel,in which the uplift decreases by 3.04 mm,a relative decrease of32.3%;The lifting is reduced by 0.91 mm,which is relatively reduced by 17.9%.In addition,in the construction of foundation pit,the method of reducing the thickness of the first layer and the number of layers of excavation can be adopted to reduce the surface settlement around the foundation pit,the uplift of the bottom of the pit and the uplift of the shield tunnel below.(3)When each layer of foundation pit is excavated by divisions,the surface settlement,horizontal displacement of retaining structure and uplift of pit bottom caused by division jumping excavation are slightly less than those caused by sequential excavation.However,after the final excavation is completed,different excavation sequences have no significant effect on the above results.This rule is also applicable to the horizontal displacement of the existing subway shield when the foundation pit is in parallel with the existing shield tunnel.Considering the convenience of construction,in actual construction,it is suggested to adopt sequential excavation for foundation pit excavation.(4)When the foundation pit and the existing shield tunnel are in parallel state,the jumping excavation method is adopted to excavate the foundation pit.Because the reserved soil can play a similar internal supporting effect on the foundation pit structure,the right arch waist of the shield tunnel will produce "wavy" deformation along the vehicle driving direction,with the maximum value of 2.54 mm,which does not exceed the safety control standard of 10 mm,especially after the excavation of the first part of soil in each layer. |
