Abstract：Accurate prediction of shear behavior in structured soils is crucial for geotechnical engineering design and construction. Existing research indicates that the initial structure during shearing often does not completely deteriorate, preventing the critical state line from returning to that of the reconstituted state in the void ratio-stress plane. To address this characteristic, a fractional non-orthogonal elastoplastic constitutive model for structured clay with interparticle bonding has been developed. By introducing parameters that describe critical state properties and combining them with the evolution law of structural state variables, a state-dependent yield surface evolution equation is established. Utilizing fractional calculus theory, the non-orthogonal gradient is derived by directly solving the fractional derivative of the yield surface, which determines the direction of plastic flow. The sub-loading surface theory is incorporated to account for the influence of stress history while facilitating smooth transitions in stress-strain relationships. The relationship between equivalent yield stress and structural state variables is established through isotropic compression behavior, thereby constructing a comprehensive constitutive framework. In this model, the yield surface shape, fractional order, and structural state variables are interconnected, allowing for dynamic adjustments of the plastic flow direction as structural degradation occurs. The model comprises ten parameters, all of which can be determined through conventional geotechnical tests. The model?s validity is verified through parametric sensitivity analysis and comparisons with test results from Corinth marl, Wenzhou marine clay, and Osaka clay. Results demonstrate that the model effectively captures the nonlinear compression characteristics, deviatoric stress peak strength, and volumetric deformation behavior of soils. Compared to the associated flow rule, the model exhibits superior applicability in predicting the mechanical response of structured clays, particularly showing advantages in peak strength and volumetric deformation predictions.
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2026, Vol. 45 
