The stability of long unsupported tunnel excavated in normally consolidated (NC) and heavily over consolidated (HOC) saturated clay under undrained condition has been assessed. The analysis was performed using finite element formulation of lower bound limit analysis in conjunction with second order cone programming. The influence of both anisotropy and non-homogeneity in undrained shear strength of NC clay and only anisotropy in undrained shear strength of HOC clay was examined. A dimensionless stability factor is defined to assess the tunnel stability in terms of self weight and undrained shear strength of soil mass, and tunnel cover. The variation of stability factor as a function of anisotropic and non-homogeneous strength parameters of soil, surcharge on the ground surface and tunnel cover was established. The anisotropy and non-homogeneity in undrained strength of clay have been found to have significant effect on the stability of tunnels. With reduction in the values of anisotropic and non-homogeneous strength parameters, the magnitude of stability factor decreases.

在不排水条件下，评估了在正常固结（NC）和高固结（HOC）饱和黏土中开挖的无支撑长隧道的稳定性。使用下限极限分析的有限元公式结合二阶锥规划进行分析。研究了各向异性和非均质性对NC粘土不排水抗剪强度的影响，仅考察了各向异性对HOC粘土不排水抗剪强度的影响。定义了无因次稳定性因子，以根据自重和土壤质量以及隧道覆盖层的不排水抗剪强度评估隧道稳定性。建立了稳定系数随土体各向异性和非均匀强度参数，地表和隧道盖上的超载而变化的函数。已经发现粘土不排水强度的各向异性和非均质性对隧道的稳定性有重要影响。随着各向异性和非均匀强度参数值的减小，稳定性因子的大小减小。