Thermodynamics-Based Two-Surface Model for Natural Clays Considering Bonding Degradation and Fabric Evolution

Naturally deposited clays generally exhibit interparticle bonding and fabric anisotropy, which provide additional strength beyond the remolded state and lead to direction-dependent mechanical responses. During loading, microstructural rearrangement, manifested as progressive bond breakage and fabric evolution, enhances energy dissipation and induces irreversible deformation, even when the stress state remains within the yield surface defined by the preconsolidation pressure. This study presents a thermodynamically consistent two-surface model for anisotropic natural clays, in which an enlarged and shifted outer surface represents the bonds-enhanced bounding surface, and an inner surface undergoes a kinematic hardening to capture the plastic deformation occurring within the outer surface. The model incorporates a fabric tensor from the anisotropic critical state theory (ACST) and two isotropic variables related to bonding, with modified evolution laws linked to plastic strain. This framework enables the model to capture the combined effect of bond degradation and fabric evolution on the macroscopic mechanical responses when the stress state lies within the overconsolidated regime. With only nine model parameters, the proposed formulation reproduces experimental results for three natural clays with high fidelity, successfully simulating smooth stress–strain curves, hysteresis loops, orientation-dependent behavior, and strength degradation under monotonic, cyclic, and postcyclic loading.