Analysis Of Face Instability Of Shield Tunnels With Consideration Of Groundwater Seepage

Shield tunnel method has made rapid progress in many countries, which is becoming the most primary method in underground excavation with the minimum soil disturbance. At the same time, the application of the method has caused many construction accidents such as face instability, collapse, spew and leakage. These accidents lead to the surface collapse, destroy the surrounding environment badly, and cause huge economic losses, hinder the normal operation of the traffic, which have seriously affect people's lives. Some countries such as England, Germany and Japan have developed the Shield tunnel method to a new stage and made rapid progress in theoretical and experimental research. But the analysis of face stability and collapse at homes mainly depends on practical experience, there has been little research with respect to the stability of tunnel face in hard geological conditions. The paper developed the following work and research:(1) Slice method derived from slope stability analysis were used in face stability analysis of shield tunnels. Extend the Terzaghi theory to layered soil, the support pressure at the tunnel face is developed with the consideration of underground seepage flow. It was found that the existence of groundwater seriously affects the stability of a tunnel face, most part of the total support pressure is owing to the seepage pressure acting on tunnel face. The ratio of the seepage force to the total support pressure shows much change with the variation of water level;(2) EPB are widely used in tunnel excavation, some advanced problems such as slurry infiltration and filter cake creation were developed in this study. We schematize the soil in front of the tunnel as a one-dimensional semi-confined aquifer, the excess pore pressure distribution in the semi-confined aquifer is established. The wedge stability model with the consideration of slurry infiltration depth and excess pore pressures was proposed, which improved the method to calculate the limit support pressure. The result shows the excess pore pressure has great effect on the stability of tunnel face as they reduce the effectiveness of the support force from the slurry and lower the effective stress in the soil;(3) Advanced pipe grouting is now being widely used in hard geologic conditions, while the research of face stability considering the pipe grouting was poor in homes. In this study, a three-dimensional explicit finite-difference program was used to calculate the head distribution around tunnel face with the consideration of advanced pipe grouting, the limit theorem and limit equilibrium are incorporated in face stability analysis to quantify the influence of pipe grouting. It was found that the extent of the plastic zone does not propagate any further with the decrease of support pressure, the magnitude of vertical components of the displacement is well controlled. The support pressure varies with less amplitude when the length of pipe grouting is 2 times larger than the diameter of tunnel in tunnel construction;(4) When the tunnel is excavated with high advance velocity, the head distribution around the face is influenced greatly. While the velocity was not included in face stability analysis in many researches, which lead to large error in support pressure results. We derived the three dimensional steady-state saturated groundwater flow equations with the consideration of tunnel advance rate by weighted residual-Galerkin method. A finite element program to analyze the groundwater flow around a tunnel was made. It was found that the head distribution around the face is greatly influenced by the shield advance velocity, the increase of advance velocity lead to the head drops more significant in case of less permeable ground and it means the increase of the seepage pressures at the tunnel face.