| The new subway tunnel undercrossing the existing subway tunnel has the characteristics of high construction risk,complex surrounding environment and high requirements for deformation control.In this paper,based on the engineering background of a new tunnel in soil rock composite stratum obliquely passing through the existing subway tunnel,the deformation law of the existing structure and the new tunnel structure during the tunnel construction stage is studied,and the influence of the location of the new tunnel on the deformation of the existing structure is analyzed.The main conclusions are as follows:(1)During the construction process of new tunnel oblique crossing existing tunnel,the complex mutual position relationship leads to great difference of deformation at different positions of stratum,while the change rate of vault settlement of new tunnel increases at the initial stage of excavation,and decreases with the excavation settlement rate of tunnel.(2)Based on the real engineering case,a three-dimensional finite element model is established to analyze the surface deformation,new tunnel deformation,existing tunnel deformation and bias effect in the construction process.Because of the increase of ground settlement caused by tunnel excavation,the existence of existing tunnel support structure plays a bearing role on soil load,which weakens the soil gravity effect caused by the excavation of new tunnel.Therefore,the soil settlement directly above the underpass position in the direction of tunnel excavation is smaller than that in other locations.(2)The total displacement of tunnel structure develops towards the inner side of the tunnel.And the horizontal displacement of the structure is most significant on both sides of the tunnel,while the vertical displacement is larger in the arch and invert.(3)The horizontal bias effect of the existing tunnel increases gradually before the new tunnel is excavated to the undercrossing section,and reaches the maximum value when the tunnel is excavated to the undercrossing section;when the new tunnel is excavated to the existing tunnel location,the axial bias bending moment of the existing tunnel support structure will also change.In the range of-2.5～ 2.5 times the tunnel diameter from the underpass location,the new tunnel excavation has a significant impact on the bias effect of the existing tunnel.(3)The influence mechanism of the horizontal distance between the left and right lines of the new tunnel and the vertical distance between the existing tunnel and the new tunnel on the structural deformation is studied.With the increase of vertical spacing of new tunnels,the thickness of overburden soil disturbed by tunnel excavation increases,which leads to the increase of surface settlement.The surface disturbance caused by new tunnel excavation gradually transits from "local disturbance" to "overall disturbance".(2)With the increase of the vertical distance between the new tunnel and the existing tunnel,the horizontal bias effect of the existing tunnel decreases,while the axial bias effect increases at first and then decreases with the increase of the distance.When the vertical distance between the new tunnel and the existing tunnel is greater than 1 times of the height of the new tunnel,the bias effect of the new tunnel construction on the existing tunnel support structure is gradually weakened.(3)With the increase of the vertical distance between the existing tunnel and the new tunnel,the surface settlement curve evolves from "single settlement tank" to "double settlement tank".The horizontal distance between the new tunnels increases,which has different influence on the horizontal and axial bias effects of the existing tunnel structure,Finally,the bias effect of the existing tunnel is significant after the new tunnel passes through the existing tunnel. |
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