Extension of an unsaturated bounding-surface isotropic formulation to triaxial stress space with consideration of varying shear stiffness

ABSTRACT Unsaturated soils exhibit complex behaviour under various stress paths, particularly at small strains. Current models often fail to capture nonlinearity. This paper extends a bounding-surface isotropic model to triaxial stress space, accurately predicting soil behaviour across different stress paths while accounting for small-strain shear stiffness. The model relies on a previously proposed mechanical law that utilizes scaled stress as the primary variable. A significant feature of this model is its ability to predict nonlinear shear stiffness by employing an empirical equation for shear modulus reduction. The model also supports both associated and non-associated flow rules, adding flexibility with only two extra parameters for the non-associated case. Validation against existing experimental data – covering isotropic and triaxial stress paths and small strains up to 1% – demonstrates the model’s high accuracy in reproducing stress–strain curves across different loading scenarios. It outperforms traditional elastoplastic models in capturing the strong nonlinearity at small strains.