Evaluation of generalised soil water retention models and their role in hydro–mechanical modelling of unsaturated soils

The soil–water retention curve (SWRC) is fundamental in geotechnical engineering, influencing the hydraulic and mechanical response of unsaturated soils. This study evaluates three generalised SWRC models that account for density effects and/or hydraulic hysteresis: the Gallipoli, Sun, and Gao formulations. Their simulation performance is assessed against experimental data from a wide range of soils, including compacted till, Pearl clays, Barcelona silt, and silty sands. The analyses highlight the strengths and limitations of each model in reproducing main wetting and drying branches, scanning curves, and density-dependent shifts of the SWRC. Among the tested formulations, the Gao model demonstrates the most robust capability to represent hysteresis and density effects across broad suction ranges. Finally, selected hydraulic models were coupled with the hypoplastic constitutive model proposed by Tafili and Machaček (2023) to evaluate hydro–mechanical interactions of unsaturated soils under various stress and hydraulic conditions, highlighting that the choice of SWRC formulation strongly influences predictions of volumetric response, stiffness evolution, and suction-dependent strength. This underlines the importance of selecting an appropriate SWRC model for reliable hydro–mechanical modelling of unsaturated soils.