Enhancements to the Coupled Hydro-mechanical Hypoplastic Model for Unsaturated Soils-Paper

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The paper enhances the three-dimensional hydro-mechanically coupled hypoplastic model developed by Tafili and Machaček (2023) for unsaturated soils. Unlike conventional approaches that may overlook suction changes during wetting and drying, the model represents suction-related effects on soil stiffness, strength, and volumetric collapse.<br /><br />Two modifications address limitations in the original model. First, the bulk modulus is revised to account for changes in degree of saturation during compression, reflecting the resulting variation in the unsaturated soil’s compression response. Second, the model’s overconsolidation-ratio (OCR) dependency is improved through a more accurate stress projection. The influence of saturation on stiffness is also controlled for overconsolidated states.<br /><br />The enhanced model was implemented in ABAQUS and assessed against experiments for Bonny silt, Loess, and Lower Cromer Till. It reproduced hysteresis in water-retention and bulk-modulus responses during wetting and drying, suction- and stress-dependent shearing behaviour, and wetting-induced collapse at different stress levels. In the collapse tests, it provided more reliable predictions than the reference model.<br /><br />Overall, the proposed changes improve prediction of unsaturated soils’ hydro-mechanical behaviour across varied stress paths. The model does not, however, account for small-strain effects or hydraulic residual states such as residual water and air entrapment.

Keywords

unsaturated soils

hydro-mechanical coupling

hypoplastic model

suction effects

degree of saturation

bulk modulus

overconsolidation ratio

wetting-induced collapse

water retention hysteresis

ABAQUS implementation