This study addresses a key limitation of conventional clay constitutive models, which often assume linear stress paths at low stress ratios and lack a systematic link between plasticity and yield surface shape. A symmetry-consistent bounding surface plasticity framework is proposed, introducing two shape parameters, Ψ and Ω, to control curvature and scaling of the yield surface under low stress ratios. The formulation preserves a unified, smooth yield function with continuous gradients, ensuring compatibility with standard numerical integration schemes. To enhance practical applicability, a three-level calibration strategy is established, ranging from direct triaxial interpretation to empirical correlations based on oedometer-derived indices. Model performance is validated against experimental data for clays with varying plasticity, demonstrating improved representation of curved stress paths without increasing formulation complexity. The proposed approach provides a transparent and reproducible extension to existing frameworks, bridging the gap between theoretical consistency and engineering-oriented calibration.
Chatwong et al. (Fri,) studied this question.