Research Of Face Stability For Shield Tunnels In Saturated Ground

Face stability of shield tunnels in saturated ground is important and difficult for the control of construction safety.This thesis investigates the face stability of shield tunnels constructed in saturated ground by using theoretical analysis and numerical simulation.The time effect of groundwater seepage,which is simplified to two ideal conditions: the undrained condtion and the drained condition,is considered.The undrained condition refers to the case of low soil permeability and fast advance rate,while the drained condition refers to the case of high soil permeability and slow advance rate.For the undrained condition,the collapse failure mechanism of tunnel face is proposed to obtain the limit support pressure by the upper-bound theorem.A series of numerical simulations are conducted to verify the analytical solution of limit support pressure.For the drained condition,the analysis method for total limit support pressure of tunnel face in collapse failure condition is put forward.The fitting formulas of total seepage pressure are given on the basis of numerical simulation.Comparison of limit support pressure between analytical solution and numerical solution is presented.In addition,the influences of fluid time and soil permeability coefficient on pore pressure are discussed to study the method of the judgement between undrained condition and drained condition.Finally,combined with the practical engineering,the research results in this thesis for face stability of shield tunnels in saturated ground are applied to the analysis of practical engineering.The main research contents are illustrated as follows:(1)For the face stability of shield tunnels in undrained condition,a new collapse mechanism applied to homogeneous soil and a new collapse mechanism applied to multilayered soils are proposed.The former mechanism is composed of five rigid truncated elliptical cylinders and a vertical elliptical cylinder,while the latter is made up of five rigid truncated elliptical cylinders on which a distributed force acts.Based on the upper-bound theorem,the theoretical solutions of limit support pressure for the two kinds of collapse mechanisms are derived.A series of numerical simulations are conducted to verify the analytical solution.The proposed methods are compared with the exsiting method to manifest the superiority than the exsiting method.(2)For the face stability of shield tunnels in drained condition,the inadequacy of the exsiting method is described.Then,an analysis method of total limit support pressure in collapse failure condition is put forward.It is considered that the total limit support pressure is composed of three sections,that is,the water pressure acting on tunnel face,the effective support pressure and the total seepage pressure.According to the magnititude of water pressure,the problem of tunnel face stability in drained condition is divided into three cases(The water pressure of tunnel face is zero or between zero and initial hydrostatic pressure or initial hydrostatic pressure).For every case,combined theoretical analysis with numerical simulation,the fitting formulas of total seepage pressure acting on tunnel face is obtained and the analytical method of calculating total limit support pressure is presented.Comparison of total limit support pressure between analytical solution and numerical solution is conducted.The influence of water pressure of tunnel face on total limit support pressure is analyzed.(3)By numerical simulation,the influences of fluid time and soil permeability coefficient on pore pressure are discussed.The idea of the judgement between undrained condition and drained condition is given by the contrast of pore pressure variation fore and after tunnel excavation.Then,combined with the practical engineering,the research results in this thesis for face stability of shield tunnels in saturated ground are applied to the analysis of practical engineering.