Application of a Non-Coaxial Soil Model with an Anisotropic Yield Criterion in Tunnelling

Abstract

Soil strength isotropy and coaxiality of the principal axes of stress and plastic strain rate tensors are often assumed in modelling the soils around tunnels, in the conventional plastic theory. However, the complexity of the soil response during the excavation of tunnelling cannot be well exhibited. In this paper, a recently developed two-dimensional (2D) elastoplastic constitutive model incorporating both soil strength anisotropy and non-coaxiality is firstly reviewed and then implemented in the finite element platform ABAQUS through the user-defined material subroutine (UMAT). This model is then numerically applied to analyze tunnel excavation problems. Two case studies, i.e., centrifuge testing and field testing, are carried out and compared with the numerical simulations to investigate the influences of soil anisotropy and non-coaxiality on the stress paths of soils around the tunnel crown and the subsurface settlement induced by tunnel excavations. The results show that the representative soil elements around tunnel crown experience severe principal stress orientations. The predictions of normalized subsurface settlement troughs can be improved by considering initial soil strength anisotropy compared to the conventional Mohr–Coulomb model. A larger value of the non-coaxial coefficient results in a larger magnitude of the maximum vertical displacement. The normalized subsurface settlement troughs are better improved in the case of centrifuge testing than that of the field testing.