Analysis Of The Uplift Deformation Of The Longitudinal Obliquely Passing Metro Tunnel Below Due To Excavation Of Foundation Pit

Foundation pit engineering is widely used in the current urban underground space construction in China.With the continuous development of urban rail transit,the distance between foundation pit engineering and existing metro tunnels is getting closer and closer,and the excavation of the soil will inevitably break the force balance of the existing metro tunnel induces uneven settlement of the metro tunnel,which can cause leakage,bolt failure,and segment cracking in severe cases.As a means of transportation for people’s daily travel,the metro runs safely and comfortably.The importance of it is self-evident.However,for the metro tunnels with longitudinal slopes,traditional algorithms generally use the average buried depth of the metro tunnel to simplify the calculation,and cannot accurately predict the true uplift deformation of the tunnel.Therefore,how to accurately and quickly calculate the uplift deformation of the longitudinal obliquely passing metro tunnel below due to excavation of foundation pit is a subject of important engineering significance.This article uses case statistical analysis,theoretical analysis,numerical simulation and other methods to carry out research on this subject.The specific work completed and the main conclusions obtained are as follows:(1)By consulting the literature,we collected 22 cases of foundation pit engineering adjacent to existing tunnels in China,and sorted them according to the characteristics of the project.Through statistical analysis,the influence of the three factors,the horizontal distance between the tunnel and the foundation pit,foundation pit excavation depth,the height between the tunnel and the foundation pit,on the tunnel deformation is studied,summarizing the deformation law of the adjacent existing tunnels caused by the excavation of the foundation pit.(2)Starting from the theoretical analysis,based on the two-stage method,a theoretical solution to the uplift deformation of the lower longitudinal diagonal metro tunnel caused by the excavation of the foundation pit is proposed: firstly,when calculating the additional stress of the foundation pit unloading on the lower longitudinal diagonal metro tunnel,the longitudinal included angle β of the tunnel is introduced,the case where the buried depth H of the tunnel changes linearly with the length of the tunnel is considered and then the tunnel is regarded as a Timoshenko long beam resting on the Pasternak foundation model,the tunnel uplift deformation equation is obtained,and the finite difference method is used to reduce the order to solve.Numerical simulations and engineering examples are used to verify the correctness of the theoretical solution,and the results are compared with the calculation results of Euler-Bernoulli beam model and Winkler foundation model algorithm and traditional algorithm.The errors of the traditional algorithm in the calculation of tunnel uplift value and internal force can reach 10.5% and 30.14%,respectively.Finally,a parameter analysis is carried out based on the theoretical solution proposed in this paper.(3)Relying on the foundation pit excavation project of a square on metro line 3 in Nanning,using FLAC3 D finite difference software to establish a three-dimensional numerical model to simulate the entire process of foundation pit excavation and analyze foundation pit excavation During the construction process,the uplift deformation of the tunnel below was compared with the calculated results of the theoretical solution.The results show that the unloading effect of the excavation of the foundation pit will not only cause the uplift and deformation of the tunnel,but also cause the tunnel cross section to be vertically stretched.The maximum uplift value of the tunnel and the uplift range of the tunnel given by the theoretical solution are basically the same as the numerical simulation results,which can be used to preliminarily judge the maximum uplift value and the uplift range of the downward longitudinal oblique tunnel caused by the excavation of the foundation pit.Providing reference for future design and construction.