Intergranular-strain elastoplastic modeling of small-strain clay stiffness and deep excavation response

Soft clay has high initial stiffness at very small strain. Its stiffness decreases nonlinearly with increasing strain and the degradation relationship depends on previous stress history. The above small strain stiffness characteristics play an important role in surrounding ground deformation induced by underground excavation. Based on that, the intergranular strain (IGS) elastic model and the critical state anisotropic boundary surface model are combined to establish an effective stress elastoplastic model considering the small strain stiffness characteristics of clay. The proposed model can memorize soil deformation history through IGS and characterize stress direction change through variation in the angle between IGS and strain increment. It can also realize the stress path-dependent small strain stiffness evolution simulation through path-dependent stiffness evolution equation. This work further proposes the convert method between the parameters of the IGS model and those of the Overlay model (i.e., the small-strain module of the widely used HS-Small model), while perform a comparative analysis between the two models. The IGS model is applied and examined by analyzing boundary value problem of deep excavation case studies. The results show that (1) the IGS model can reasonably reflect the influence of previous stress history on the small strain stiffness of clay and the ground deformation induced by excavation. (2) When the previous stress history amplitude is small, both IGS and Overlay models can effectively reflect the effects of previous history on the degradation relationship of small strain stiffness. (3) With the previous stress history amplitude increasing, Overlay model could underestimate the small strain stiffness under the stress path with a small angle from the previous path.