Stability Analysis Of Tunnel Face In Soils Based On The Upper Bound Method Of Limit Analysis

To ensure the face stability is the key issue of the safety control of tunneling.Buildings adjacent to the collapse of the ground surface due to the face failure are usually damaged during the tunneling process all over the world.A certain difference between the theoretical model and realistic situation of tunnel face exists due to the insufficient consideration on complex conditions of tunnel and soil and simplificaiton on solving process,which can’t realize the requirement of safety control of the practical tunneling project.Therefore,this study focuses on the influences of the soil arch effect,the imhomogeneity and anisotropy of soil strength parameters,the groundwater seepage,the geometric shapes of tunnel face and the deformation characteristics of the purely cohesive soils on the stability of tunnel face.Based on the evolving process and failure characteristics of tunnel face,the upper bound method of limit analysis is adopted to to construct the stability analysis models of the tunnel face.The main research contents and conclusions of this study are given as follows:（1）To consider the soil arch effect,this study adopts the ellipsoid theory and the kinematically admissible velocity field of soil arc zone constructed by the spatial discretization technique to establish the stability analysis models of tunnel face.The results show that the upper bound solutions of the limit support pressure are significantly improved by two proposed analytical models.The limit support pressure of tunnel face is more sensitive to the friction angle.The loosening soil pressure of soil arch zone can be effectively estimated by the ellipsoid theory,which also provides a more conservative estimate on the stability of tunnel face.The boundary equation of soil arch zone can be obtained from the propsoed kinematically admissible velocity field of soil arch zone.（2）To consider the spatial variability of soils,the position functions for cohesion and friction angle are presented,and the stability analysis model of tunnel face is established based on the rotational failure mechanism and the spatial discretization technique.The results show that the limit support pressure increases nonlinearly with the ratio of the inhomogeneity coefficients n₀/n₂ and the anisotrpic coefficient k of cohesion.The anisotropy of cohesion is beneficial to the face stability when the anisotropic coefficient of cohesion k<1.While the anisotropy of cohesion will lower the face stability when k>1.The range of the failiure zone of tunnel face decreses in both the longitudinal and transverse directions with the anisotropic parameter of friction angleΩ₃ when the isotropic friction angleφ₀ is adopted.（3）The analytical equations for the hydraulic head distribution of the groundwater ahead of tunnel face is proposed,and the effect of the groundwater seepage on the face stability is studied.The results show that the analytical equation for the hydraulic head distribution corresponds well with the result of numerical simulation,which has a sufficient accuracy.The limit support pressure increases linearly with the groundwater level h_w/D,and the failure zone is prone to extend away from tunnel face with h_w/D.The hydraulic head ahead of tunnel face decreases with the ratio of the soil permeability k_h/k_v,and the face pressure decreases nonlinearly with k_h/k_v.（4）The influences of the geometric shapes of the equivalent horseshoe-shaped,circular,rectangular and elliptic tunnel faces on the failure mode,the limit support pressure and stability factor of tunne face are studied by adopting the spatial discretization technique.The results show that the required face pressures of the circular and rectangular tunnel faces are relatively larger than that of the horseshoe-shaped and elliptic tunnel faces.With the same face area,the face stability will firstly decrease and then increase with the ratio of the face height to the face width D/W.The face shape changes the geometric shape of the optimal failure mechanism.The ranges of the failure zones of the horseshoe-shaped and elliptic tunnel faces are relatively smaller than that of the circular and rectangular tunnel faces.（5）Based on the failure characteristics of the purely cohesive soils,a continuous velocity field for the horseshoe-shaped tunnel face is built to obtain the equations of the limit support pressure and the load factor.And the related dimensionless parameters for estimating the face stability in the purely cohesive soils are provided.The results show that the horseshoe-shaped tunnel face can significantly improve the face stability in the purely cohesive soils compared with the circular tunnel face.The neglect of the soil inhomogeneity will lead to the overestimate on the face stability.Finally,an improved velocity field is proposed to solve the shortcoming that the original continuous velocity field continuously expands its failure region with C/D,which effectively improves the upper bound solutions of the limit support pressure and stability factor.