Abstract: Small-strain stiffness of saturated clay and its directional dependence are critical for the design of deep excavations in soft soils and the evaluation of excavation impacts on adjacent infrastructure. Though many constitutive models have been proposed to capture the behavior of fine-grained soils at small strains, they are in general too complex to be incorporated into the simplified analyses of excavation-induced ground movements and soil-structure interaction. Moreover, in these models, small-strain moduli are normally assumed to be isotropic and hence independent of loading directions. In this study, based on the concept of intergranular strain (IGS), we propose a simple and easily calibrated hypoelastic model for saturated clay with anisotropic small-strain stiffness. The model considers direction-dependent moduli at very small strains, non-linear degradation of stiffness with strain levels, recent stress history effects, and stress-dilatancy relation. By comparing against the results of stress probe tests on natural Chicago clay, it shows: 1) The proposed model can reasonably represent the degradation of stiffness at small strains, the stress-strain relationships at large strains, as well as the development of excess pore pressures and soil dilatancy. Also, the model can well approximate advanced elastoplastic models. 2) Compared with the existing Overlay model, the IGS model can better replicate the dependence of small strain stiffness on the intermediate stress path rotations between monotonic and full reversal loading. 3) Anisotropy can noticeably influence the soil stiffness at very small strains such that soil specimens characterized by smaller stress path rotations can feature higher initial shear modulus. With the increase in strain levels, nevertheless, recent stress history becomes the dominant factor in affecting small-strain characteristics of clay.

Key words: saturated clay, small-strain stiffness, anisotropy, intergranular strain, constitutive models