Modelling the behaviour of unsaturated soils using a kinematic hardening constitutive model

Long linear geotechnical structures (embankments and cuttings) are critical components of strategic road and railway networks. Environmental actions (seasonal variations in water content) induce cyclical swelling and shrinkage in the soil. Over time, these cycles can result in cumulative, irreversible deformations, reducing the strength and stiffness of the soil, ultimately contributing to the degradation and potential failure of these geostructures. The intensity of seasonal water content cycles is anticipated to increase due to more frequent and extreme wet and dry events, accelerating the degradation of geostructures. To model these effects, advanced constitutive models for unsaturated soils that are able to reproduce the observed behavior are essential. This study introduces a new advanced constitutive model for the hydro-mechanical behavior of unsaturated soils, formulated within the framework of elasto-plasticity with internal variables. The model incorporates the influence of recent stress history using a kinematic hardening approach, enhanced with elements of bounding surface plasticity initially developed for saturated soils. This model is further extended to unsaturated conditions by introducing a loading collapse curve. It allows for the retention of stress history, predicts irrecoverable losses in stiffness and strength, and captures the hysteretic response under cyclic loading. The model is implemented in a constitutive driver using an implicit numerical scheme, and its predictive capability is demonstrated through numerical simulations of laboratory experiments involving a complex sequence of loading stages.