Study On The Impact Of Shield Tunnel Underpass Construction On Existing Tunnels

With the rapid development of urban rail transit,shield tunnels often cross existing underground tunnels,and the safety of underground tunnels has become the focus of construction attention.Based on the background of a water conveyance tunnel crossing an section of No.8 line of Xi’an Metro,this paper combines theoretical analysis with FLAC3D numerical simulation to analyze the ground surface and existing tunnel deformation caused by shield tunnel underneath construction.The main research achievements in this paper are as follows:（1）The mechanism of soil deformation caused by shield construction is elaborated.Twostage method is used to introduce the unloading coefficient of soil arch,consider the influence of angle between new tunnel and existing tunnel,regard existing tunnel as Eular-Bernoulli beam placed on Pasternak elastic foundation model,establish tunnel deformation control equation and solve it by superposition method principle,and obtain the analytical solution of existing tunnel deformation caused by inclined crossing construction of double-line tunnel.（2）FLAG^3D is used to establish three-dimensional model and analyze the influence of different construction parameters on the deformation of existing tunnel and ground surface in loess strata.The influence of tunnel excavation on the deformation of near tunnel is studied by taking grouting pressure,silo pressure,equivalent layer elastic modulus,existing tunnel elastic modulus,vertical spacing between two tunnels,soil deformation modulus,friction angle in soil and soil cohesion as simulation variables.The results show that the disturbance to existing tunnels can be reduced by properly increasing grouting pressure or grout strength in actual projects.（3）Summarize the stipulations on stress,strain and joint opening value of underground pipeline in domestic and international codes,put forward comprehensive safety control criterion of existing tunnel,introduce Peck formula and relative rigidity of pipe and soil,derive the calculation formula of settlement tank width of existing tunnel,put forward a sim ple algorithm for calculating water stop elongation between tunnel sections based on this,and verify it by numerical simulation and graphic method.App Designer in MATLAB makes the derived theoretical formula into an interactive interface form,which can easily and quickly obtain the deformation values of existing tunnels and ground.（4）Through a water conveyance tunnel project under Line 8 of Xi’an Metro,numerical simulation analysis is carried out to analyze the surface deformations and existing tunnel deformations at different construction stages of left and right lines,and to analyze the soil mass and stress of water conveyance tunnel after completion of construction of left and right lines.Finally,the comprehensive safety control criterion proposed in this paper is used to judge the safety of the water conveyance tunnel.The results show that the settlement deformation of water conveyance tunnel is gradually generated according to certain gradient and continuous slow change.The settlement deformation caused by shield construction will not cause cracking or water stop pulling apart of water conveyance tunnel.