Shield Tunnelling Effects On Behavior Of Soil And Adjacent Existing Tunnel In Soft Clay

With the rapid development of urban underground traffic system,the construction of one or more shield tunnels adjacent to existing tunnels is becoming more and more common.The new shield tunnel will generate additional stress and displacement to the existing tunnel,making the existing tunnel have a longitudinal displacement and cross-section convergence displacement,which will have a negative impact on the safety,functionality and durability of the tunnel structure,resulting in structural damage,deformation and degradation,such as segment cracking,longitudinal differential settlement,circumferential joint dislocation,longitudinal joint opening,water leakage and so on.The influence of shield on existing tunnel is mainly due to the influence of shield on soil,and the tunnelling will cause soil displacement and excess pore water pressure.The soil excess pore water pressure will dissipate over time,and thus lead to soil consolidation settlement.Therefore,it is of great value to study the influence of shield tunnel on the surrounding soil and the existing tunnel.In general,the content of this paper is divided into two parts,namely the effects of shield on adjacent existing tunnel,and the soil stress and displacement,excess pore water pressure and consolidation settlement caused by shield.The main research and achievements are as follows:(1)Effects of shield underpassing at close distance on the displacement of existing tunnel and construction control.In this paper,a case of shields underpassing the existing tunnels in Hangzhou Metro is taken as the background.Through monitoring the displacement of the existing tunnels,according to the influence processes and characteristics of the shields underpassing with close range and small angle on the displacement of the existing tunnels,and combined with real-time record of shield parameters,the variation of the vertical settlement,horizontal displacement and convergence displacement of the existing tunnels has been studied when the relative positions of the shield machines and the existing tunnels are different.(2)Theoretical study on longitudinal deformation of existing tunnels by shield driving parameters.The Mindlin solution can be used to calculate the additional stress along the central axis of the existing tunnels caused by the shield driving parameters such as the compressing pressure of the cutter-head q,friction of the shield skin f,synchronous grouting pressure p.The additional stress caused by soil loss can be calculated by the image method theory.Winkler foundation model can be used to calculate the deformation of the existing tunnels.According to the different crossing processes,the total deformation of the existing tunnels can be calculated by combining the above four parts of deformation.Finally,comparing the theoretical results with the measured data of the metro construction project in Hangzhou,the valid of the method is confirmed.(3)A theoretical method for evaluating the longitudinal reinforcement of existing tunnels is proposed.The existing tunnels are simplified as two-parameter foundation beam,the soil vertical displacement of the existing tunnels is calculated by using Longanathan formula,the additional displacement is equivalent to the additional stress applied to the existing tunnels by using the two-parameter foundation formula,the tunnels after reinforcement with variable stiffness are discretized into N units,and the internal forces and displacements of the existing tunnels are calculated by using the initial parameters and the transfer matrix method.Based on the theory of tunnel longitudinal reinforcement proposed in this chapter,the deformation of the variable stiffness tunnels can be calculated,and then the reinforcement range and stiffness of the existing tunnels are studied and the longitudinal deformation behavior after the reinforcement is evaluated.(4)The stress and displacement of the soil caused by the elliptical shields in isotropic soil are deduced.Using Muskhelishvili's complex function method,the displacement and stress of the soil caused by the uniform convergence of the elliptical tunnel boundary are calculated,and the influence parameters are analyzed.Combined with image method,the ground deformation caused by elliptical shield tunnel in isotropic soil are calculated,the influences of soil parameters on ground deformation are studied.The theoretical calculation method of the ground displacement caused by the elliptical shield tunnels in soft soil are proposed,which will provide reference for the study of the ground displacement caused by various cross-section shields.(5)The stress,displacement of the soil and ground displacement caused by the elliptical shield tunneling in a linear elastic soil mass with cross-anisotropic stiffness properties have been deduced.Based on the analysis method of continuum mechanics to establish theoretical model,Referring to the complex function method proposed by Zymins,and combined with the elliptical hole deformation stress function proposed by Lekhnitskii,the displacement and stress changes of the surrounding soil caused by the uniform shrinkage deformation of whole plane elliptical tunnel boundary are calculated.Due to the lack of theoretical method of ground displacement caused by elliptical shield,the calculation method proposed in this chapter is mutually verified with the previous chapter,and the influences of the anisotropic physical parameters of soil are discussed.(6)The excess pore water pressure of the soil caused by the uniform convergence and elliptical deformation modes at the circular tunnel cavity wall has been studied by the complex function method.The variation of soil stress caused by the deformation(homogeneous deformation and elliptical deformation)of shield tunnel is solved by using complex variable function method(the exact solution)and displacement derivative method(The approximate solution),and then the excess pore water pressure of soil is deduced by Skempton theory.Comparing the results of the exact solution and and the approximate solution,the influence factors of the excess pore water pressure are obtained.(7)The soil excess pore water pressure and ground consolidation caused by the grouting pressure of TBM have been studied.The corresponding analytical solutions are derived by using the Terzaghi-Rendulic two-dimensional consolidation theory along with the conformal transformation,supposing the soil is linear elastic,and considering the tunnel with three idealized sealing boundary conditions as impermeable,partially permeable and fully permeable.The results show that the long-term consolidation settlement of ground is related to the initial excess pore water pressure distribution,the relative permeability of tunnel lining and surrounding soil.This study confirms that a tunnel acting as a drain can accelerate the consolidation of soil and enlarge soil deformation due to consolidation.