A new multi-surface plasticity model for cyclic hardening of unsaturated granular soils

This paper proposes a new multi-surface plasticity model for the cyclic hardening of unsaturated granular soils crucial for assessing their resilience jointly under repeated loading and changing climatic conditions. The model incorporates a generalised kinematic hardening law and a double logarithmic compressibility law, both of which are essential to accurately simulate soil cyclic behaviour, particularly when considering water retention hysteresis and volume change dependence. A tailored adaptive sub-stepping stress integration algorithm has also been developed, featuring a sub-algorithm for initial hydraulic state and path determination adaptable to various soil water retention curve formulations. The model is validated through experimental results obtained from repeated load triaxial tests and cyclic simple shear tests on unsaturated granular soils under diverse initial void ratios and suctions. The study underscores the importance of soil water retention properties in plastic strain development and offers predictive axial plastic strain contours on the compaction plane. These findings provide essential insights for soil performance optimisation and practical guidelines for effective soil compaction in field applications.