Analysis And Numerical Simulation Of Influence Of Foundation Pit Excavation On Shield Tunnel

In recent years,with the large-scale urbanization construction and the gradual improvement of underground transportation network,more and more engineering cases of the above-ground construction projects adjacent to metro tunnels have been reported.The unloading process of foundation pit excavation of various construction projects will have a great impact on the safe operation of tunnels.At present,the research on the excavation of shield tunnels adjacent to foundation pits is not perfect enough.This paper summarizes the calculation methods of additional deformation and stress of soil caused by foundation pit construction,and expounds the mechanical model of shield tunnel under the influence of foundation pit excavation.Based on the engineering background of a foundation pit excavation adjacent to Hefei Metro Line 2,using finite element software,a three-dimensional numerical model is established to study the influence of foundation pit excavation on the deformation of adjacent shield tunnels.The deformation of shield tunnel under the change of horizontal distance between foundation pit and tunnel,depth of tunnel,orientation and size of tunnel and other construction factors are discussed.The main research results of this paper are as follows.(1)The numerical results show that the main influence of foundation pit excavation on ground settlement is 2 times the excavation depth around the foundation pit.The existence of tunnel restrains the displacement field of surrounding soil layer.The ground settlement near the tunnel side is reduced by 10%~15% compared with that at the same location on the far tunnel side.The rebound increases nonlinearly with the increase of excavation depth,and the maximum value is at the center of the foundation.(2)The horizontal displacement of the retaining wall near the tunnel side is smaller than that of the far tunnel side,and the deformation and axial force of the retaining structure are related to the horizontal displacement of the connecting wall.The transverse section of shield segment shows irregular flattening deformation.The vertical extension is larger than the transverse contraction.The tunnel moves toward the foundation pit.The deformation of the tunnel near the middle and both ends is greatly affected by the foundation pit.(3)The influence law of shield tunnel deformation under the change of the spacing between foundation pit and tunnel,the excavation depth and the size of foundation pit is obtained.The segment deformation decreases exponentially with the increase of pit and tunnel spacing,and the influence range of horizontal spacing is about 12 m,beyond which the deformation range of shield segment is very small.The influence range of segment vertical displacement along tunnel axis is about 2 times of the length of foundation pit and 1.2 times of the length of foundation pit with horizontal displacement.The influence of foundation pit excavation on surrounding soil layers is regional.With the increase of tunnel burial depth,the transverse deformation of tunnel segments decreases gradually,and the variation range of vertical and transverse displacements is the same.And the change of foundation pit length has no significant influence on the horizontal and vertical deformation of tunnel segment.