Hypoplastic Model for Unsaturated Soils With Degree of Saturation Dependent Stiffness

ABSTRACT This paper presents an enhanced hydro‐mechanically coupled hypoplastic constitutive model for unsaturated soils, developed by extending an existing framework, with particular emphasis on improving the stiffness formulation to better capture the influence of the degree of saturation. The existing framework links Bishop's effective stress to the strain rate and incorporates a degree of saturation‐dependent normal consolidation line (NCL), enabling it to reproduce the volumetric collapse behavior observed during wetting. Although this framework has successfully reproduced key aspects of unsaturated soil behavior while remaining consistent with established theoretical and thermodynamic principles, recent numerical investigations have revealed the occurrence of inadmissible stress states, indicating the need for further refinement. To address these limitations, two key enhancements are introduced in this study: (i) an improved formulation of the bulk modulus (K) to incorporate the effects of degree of saturation, and (ii) the inclusion of overconsolidation ratio (OCR) dependency in the unsaturated soil response. The revised bulk modulus formulation is validated with experimental measurements and previous models at very small strains, using data from four different soil types. The enhanced model is implemented within the INCREMENTAL DRIVER platform using a user‐defined material subroutine. Simulations of triaxial tests from the literature, involving various stress paths for two different clays, were also conducted. Comparative analyses with experimental results and earlier model versions clearly demonstrate the improved predictive performance of the proposed enhancements.