Continuous Geometry, Continuous Flow, Continuous Compression: A Numerical Component-Interaction Assessment for Fractional Clay Plasticity

Constitutive models for clays have historically treated yield geometry, plastic-flow direction, and compression as separate problems, with little regard for their interaction. This paper tests that assumption by coupling Chatwong et al.’s verified teardrop yield surface with a stress-fractional flow rule and an AJOP-derived hardening modulus in a 2×2×2 factorial design, integrated incrementally along a shear-strain-controlled path. Using real Boston Blue Clay and London Clay parameters, no single main effect or interaction dominates: the flow main effect is consistently largest, the flow×compression interaction is comparably large wherever defined, and compression’s role grows substantially with overconsolidation ratio. Two structural singularities are identified: a phase-transformation point in the teardrop surface’s non-associated flow rule, absent from the fractional rule, and a hardening singularity in the AJOP-based modulus, whose tangent falls to the swelling index at a finite, soil-dependent preconsolidation stress, bounding the evaluable overconsolidation range of the compression-related interactions; a proportional-κ variant removes this singularity by construction while preserving the factorial ranking, identifying it as a property of the constant-κ embedding, not of AJOP itself. Under an approximate undrained path the geometry×flow interaction carries over unchanged, while compression’s role is suppressed several-fold. Chatwong et al.’s validation of the borrowed surface against real undrained triaxial data for all four calibrated soils is reproduced; the incremental framework built on it is not yet validated to the same standard.