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.