Face Stability Analysis Of Shield Tunnel Using Slip Line Method

Traffic congestion and environmental factors put forward higher requirements for the utilization of urban underground space.Tunnel excavation is often unavoidable under severe geological conditions,especially in urban areas.The stability of the tunnel face is an important problem in the design and construction of tunnel engineering.The instability of the tunnel face is a common problem of soil stability destruction,causing huge losses to people's lives and property.In the process of earth-pressure balanced shield tunneling,the high of the limit support pressure of the tunnel face often leads to the surface uplift,while the low of pressure will lead to the sudden collapse of the tunnel face and eventually lead to the surface subsidence.Both of these miscalculated pressures can lead to time and financial losses.In order to assess the risks and design appropriate countermeasures to ensure the safety of tunnel excavation and prevent damage to adjacent structures,it is necessary to conduct a reliable analysis of tunnel face stability and predict the necessary support pressure.In the early stage,soil stability analysis methods mainly focused on the limit equilibrium method,the slip line method and the limit analysis method.It is assumed that the soil at the excavation site is an ideal rigid-plastic material following the Mohr-Coulomb yield criterion,and is assumed to be an isotropic,homogeneous,and incompressible ideal continuum material.Due to deformation of the ground is not being taken into account,thus the soil is idealized as a rigid-plastic material.The slip line theory assumes that the soil has achieved the plastic equilibrium state with Mohr Coulomb criteria,then a mathematical model of the static equilibrium with solvable limit equilibrium boundary is established.In the calculation model,the number and thickness of overburden,the physical characteristics such as soil cohesion and internal friction Angle,and the influence of tunnel construction line slope on tunnel face stability are considered.The slip line method is used to solve the slip line field and stress field of the soil behind the tunnel face,and then the limit support pressure and failure mechanism of the tunnel face are solved.The results show that the slip line solution of the limit support pressure of the tunnel face is reliable.After the combination of several variables,the influence of each parameter on the limit support pressure of the tunnel face is obtained,and its accuracy is verified by FLAC3D simulation.Under the same condition of homogenizing soil and controlling other conditions,the limit support pressure of the tunnel face decreases linearly with the increase of internal friction angle or cohesion,and the influence of internal friction angle is greater than that of cohesion;the increase of external friction angle and cohesion will lead to the increase of the limit support pressure of the tunnel face,and the influence of external friction angle is significant,and the influence of external cohesion is very small.In the case of multiple layers of soil,the physical parameters of each layer of soil have different effects on the limit support pressure of the tunnel face:for example,the influence of internal friction angle of 1 layer(corresponding to c1??1??1)of soil on the limit support pressure of the tunnel face is greater than that of 0 layer(corresponding to c0??0??0)of soil.Considering that the thickness of the overburden layer changes with the slope of the construction line,the limit support pressure of the tunnel face decreases with the increase or negative increase of the slope of the tunnel construction line.Considering that the thickness of the overburden layer is constant,the limit support pressure of the tunnel face increases as the slope of the tunnel construction line increases.