Research On Influence Of Existing Tunnel Deformation Caused By Foundation Pit Excavation Above Subway

With the rapid development of the national economy,underground rail transit is being constructed by many large and medium-sized cities.However,commercial buildings around the subway have also increased along with the subway.The foundation pit is usually constructed earlier than the main structure.Excavation of the foundation pit will change the stress field of the surrounding soil and may affect the safety of the tunnel below.Therefore,it is of great value to study the influence of foundation pit excavation on the deformation of the tunnel below.In this article,the author systematically studied tunnel deformation caused by excavation of the existing tunnel above the existing tunnel through engineering statistics,theoretical analysis,finite element simulation,and orthogonal test.The main research contents of this article are as follows:(1)By referring to the literature,the author sorted out the cases of excavation of foundation pits above existing subway tunnels in China.In order to analyze the deformation law of tunnels and the influencing factors of tunnel deformation in such projects,the author summarizes the excavation depth of the foundation pit,the positional relationship between the foundation pit and the tunnel,and the deformation displacement of the tunnel.In the 21 engineering surveys counted in the article,after the excavation of the foundation pit is completed,the vertical deformation of the existing tunnel below appears upward,and the maximum vertical displacement of the tunnel is between 3.2 and 16.92 mm.Over 76% of engineering cases have a maximum displacement between 5mm and 15 mm,and this displacement interval accounts for the highest proportion.(2)A two-stage analysis method based on Mindlin solution and Winkler foundation model is used to establish a longitudinal deformation equation for solving the effect of excavation of existing tunnel above the existing tunnel.This method comprehensively considers the shielding effect of the envelope structure when the foundation pit is excavated and the restraining effect of the soil above the tunnel.The author draws on the modified idiom model based on the elliptic parametric equation to establish a calculation model of the longitudinal equivalent stiffness of the tunnel considering the lateral deformation of the tunnel.Combined with the parameters of the tunnel segment of Zhengzhou Metro Line 1,the reduction coefficient of the longitudinal stiffness of the tunnel is calculated to be 0.186,it provides a reference for similar projects.(3)Using the finite element software ABAQUS,the author analyzes the whole process of excavation of the foundation pit to analyze the displacement changes of the bottom of the foundation pit,the envelope structure and the tunnel.Compare the theoretical calculation results with the numerical simulation results,and analyze the influence of single factor changes on tunnel deformation.The author adopts the method of combining numerical simulation and orthogonal experiment to carry out numerical orthogonal experiment,and uses the finite element software to calculate the maximum uplift deformation of the tunnel under each test condition.In this paper,the importance order of each influencing factor is studied through the analysis of variance and range,and obtain the parameter combination when the displacement of the tunnel uplift is the smallest.