Deformation Mechanism Of Shield Tunnel Considering Joint Nonlinearity In Soft Soils And Its Bearing Capacity Evaluation

At present,as a kind of the urban rail transit,metro shield tunnel has the characteristics of large traffic volume,high speed,safety,and so on,which plays an important role in the urban public transport.It greatly alleviates the traffic pressure on the urban ground,continuously widens the urban space and drives the urban development.Both in the preliminary design stage and in the operation stage,it is of vital importance to obtain the deformation and internal force of tunnel segments.As the weakness of the shield tunnel,the joint mechanical properties have a significant influence on the tunnel overall stiffness.What’s more,under the erternal loading,joints often behave nonlinear,which makes the determination of the deformation and internal force of tunnel segments difficult.Besides,the field monitoring data indicates that tructural damages(such as leakaging,segment cracking,etc.)occur with tunnel deformations,which will threaten the serviceability of metro tunnels and increase the maintenance cost.So,how to consider the influence of joints on the tunnel deformation and internal force? How to quantitatively analyze the bearing performance of tunnel structure according to the monitoring data? Therefore,taking the Shanghai Metro shield tunnel as the research background,this thesis systematically studies the deformation mechanism and treatment technology of the shield tunnel.Firstly,the mechanical properties and damage laws of the joint are studied in detail,and the safety index based on tunnel deformation is established to evaluate the health status of shield tunnel;Then,the calculation models for the transverse and longitudinal deformations of tunnel are proposed respectively,which takes the joint nonlinearity into consideration.Finally,a case study about an overdeformed metro tunnel caused by the road construction and corresponding rehabilitation measures are described in detail.The research results can provide reference for the tunnel structural design and performance evaluation in soft soil areas.The main works are drawn in the following aspects:1.The simplified model of flexural performance and the analysis method of ultimate bearing performance for the longitudinal joint are proposed.Through the three-dimensional refined numerical model and theoretical analysis model,the mechnical properties of longitudinal joints under the combined load of the axial force and bending moment are investigated in detial.Then,a theoretical calculation method is established for the longitudinal joint bending performance are proposed and a simlified model for the joint flexural performance is proposed based on the curve fitting.The ultimate bearing capacity assessment method for the longitudinal joint is also established,which provides a theoretical analysis method for the joint safety evaluation.By comparing with the test results,the validities of proposed simplified models are verified.2.A semi-analytical model is proposed to study the deformations and internal forces of jointed tunnel,where the joint nonlinearity and soil nonlinearity is considered.Firstly,the segment structure and the longitudinal joint are simplified into the Euler Bernoulli beam and the spring element respectively,and the soil around the tunnel is equivalent to Winkler foundation model.On the basis of the research in Chapter 2,the calculation approach for the tunnel transverse response is proposed.With the combination of the triaxial shear tests,the nonlinear stiffness of surrounding soil is determined,and the reliability of this proposed method is verified with relevant cases.It is obtained that considering the nonlinearity of soil and joint under the change of external environmental conditions is necessary.Furthermore,parametric studies are carried out to reveal the influence of the coefficient of earth pressure at-rest,the soil stiffness on the tunnel responses.3.The evaluation model of tunnel bearing capacity based on the convergence is establised,and the optimization strategies for the assembly and steel plate reinforcement are put forward.To investigate the tunnel deformation law,a refined three-dimensional numerical model is established,where the structural characteristics of a longitudinal joint is detaily moded.Then,tunnel structural responses under different buried depthes,different coefficients of earth pressure at-rest and different soil stiffnesses is analyzed.The influences of the assembly effect and steel plate on the tunnel performace are also carried out,and the corresponding optimization strategies are put forward based on the numerical results.4.The nonlinear model of tunnel circumferential joint is established and applied to the calculation of tunnel longitudinal deformation.Based on the equivalent stiffness model and the existing test data,the flexural and shear properties of transverse joint are analyzed,and a nonlinear model is furtherly proposed.Besides,the evaluation index for the tunnel serviceability in longitudinal direction is proposed accroding to the relationship between tunnel longitudinal deformations and transverse joints properties.Based on the Timoshenko beam,the Pasternak foundation model and the symplectic elasticity theory,a calculation model for the tunnel longitudinal deformation is established.With the combination of Biot’s consolidation theory,the time effecting is taken into consideration in the tunnel longitudinal deformation analysis resulting from the surcharge loading.5.Using the evaluation model proposed in the Chapter 5,the tunnel performance caused by the road construction is quantitatively analysed,and the corresponding rehabilitation measures are described in detail.Firstly,based on the field monitoring data,the tunnel deformation responses and structural damages under ground surcharge are analysed.Then,mutiple repair measures,e.g.,the lateral micro-disturbance grouting technology,ground load replacement and backfilling,steel plate reinforcement,are proposed to repair this overdeformed tunnel in the tunnel sections that have large convergences.Through the tunnel monitoring data during repairing,the quantitative relationship between the grouting efficiency and the tunnel deformation is obtained.Finally,the causes of tunnel large deformation are comprehensively discussed,and the disposal strategy for the tunnel large deformation under ground surcharge is put forward for engineering reference.