Research Of Existing Shield Tunnel Deformation Caused By Adjacent Excavation

After the tunnel is built and put into normal use,it will inevitably face the influence and threat of adjacent construction.The adjacent construction disturbed the stress field of the stratum around the tunnel,resulting in the internal force change and structural deformation of the tunnel during operation.This will endanger the safety of the tunnel in normal operation.The common cases include:longitudinal non-uniform settlement,structural cracking and destruction,opening of segments joints in different degrees,water seepage and water leakage,etc.Therefore,it has become an important research topic to predict the influence of adjacent construction on the stability of existing shield tunnel structure quickly and accurately,and to formulate corresponding emergency protection measures to ensure the safe use of the tunnel and make its normal operation.Based on the existing achievements,this thesis explores the response of adjacent construction(ground loading,foundation pit excavation,close shield crossing)to the existing shield tunnel by means of field monitoring,finite element software analysis and analytical method.The main results of this thesis are as follows:A calculation method for the longitudinal deformation of the existing shield tunnel under ground heap load is proposed.An Euler-Bernoulli beam on Pasternak foundation is used as a simplified model for tunnel force analysis.Based on the Elastic solution of Boussinesq,the additional load matrix distribution of ground loading on the existing tunnel position is obtained.Combined with the modified Attewell foundation reaction coefficient,the differential equation of tunnel deformation is established,and the longitudinal displacement solution of tunnel is obtained by using the finite difference method.The feasibility of the proposed method is proved by comparing with the field monitoring data,the simulation results of finite element software,and the calculation results of three simplified elastic analysis methods with different foundation bed coefficients.On top of this,the influence of the parameters of the tunnel and the pile load is discussed,and the influence of the maximum displacement of the existing tunnel is studied from the aspects of the pile load size,the relative position relationship between the tunnel and the pile load,and the relative stiffness of the tunnel and the soil.A semi-empirical formula for the maximum longitudinal displacement of the existing tunnel is proposed,which can provide a theoretical basis for the response of the existing tunnel under the adjacent surface loading.A simplified calculation method is proposed to calculate the uneven lift of adjacent tunnels in foundation excavation engineering.Based on the spatial effect of tunnel,the shear layer of Pasternak foundation model is assumed to transfer the force between soil and tunnel in the horizontal direction of tunnel axis.By modifying the foundation bed coefficient proposed by Vesic,the equilibrium equation of the uneven uplift value of the existing tunnel caused by foundation excavation construction is established.Six simplified analysis methods of elastic model are proposed.The feasibility of the proposed method was verified by comparing the published field monitoring data with six simplified elastic calculation methods.The shear effect model is adopted to study the response of shield tunneling to the existing shield tunnel.Considering the additional stress caused by the soil loss of shield excavation at the axis of the existing shield tunnel,the existing shield tunnel is simplified as Timoshenko beam with shear effect,and the finite difference method is used to establish the equation to solve the longitudinal displacement solution and the maximum segment opening of the existing shield tunnel.The results show that the calculated longitudinal displacement and segment opening of the shield tunnel are in good agreement with the measured values considering the shear deformation of the tunnel.Increasing the shear stiffness of segment joints does not reduce the opening.On the contrary,it has a negative effect on the tunnel structure,resulting in the increase of segment joint opening.When the shield tunnel is driven parallel under the existing shield tunnel,the longitudinal deformation of the existing tunnel is the largest,and the segment opening is 0,compared with the oblique or vertical tunneling.Analyze the settlement of existing shield tunnel caused by shield excavation clearance.The three-dimensional finite element software is used to simulate the construction process of shield grouting between Meicun station and Shangmeilin Station of Shenzhen Metro Line 9.The numerical simulation can better reflect the deformation and surface settlement of existing shield tunnel caused by shield tunneling.The filling effect of tunneling clearance of underpass shield tunnel on existing shield tunnel is studied.The response of grouting grout with different elastic modulus under shield tunneling is analyzed.The surface settlement above the existing shield tunnel caused by shield excavation is smaller than that far away from the existing shield tunnel,and the width of the settlement slot is wider.The elastic modulus of grouting slurry affects the width of settlement trough.When the elastic modulus of the slurry is less than 10 MPa,the settlement of the existing shield tunnel is relatively sensitive to the elastic modulus of the slurry.The elastic modulus of the slurry has a great influence on the internal force of the existing shield tunnel.Considering the additional stress at the axis of the existing shield tunnel caused by the weight difference between the excavated soil and the segment lining system,the theoretical model of longitudinal deformation of the tunnel considering the shear effect of the tunnel,combined with the finite difference method,the calculation formula of longitudinal displacement of the tunnel is derived.The forecast results are compared with the measured data by analyzing the published engineering cases.The results show that when the distance between the tunnel and the shield tunnel is constant,the maximum deformation of the shield tunnel increases gradually when the upper span Angle changes from 90~o to 0~o.It is suggested that the upper span shield tunnel should be vertically traversed as far as possible to reduce the adverse impact on the operating tunnel when conditions permit.When the stiffness of shield tunnel is low,reinforcement is a limited measure to reduce the uplift of the tunnel.However,if the soil condition of the foundation is poor,improving the foundation is not the most economical and effective measure to reduce the uplift of the tunnel.