A Rotational Hardening Model Capturing Undrained Failure in Anisotropic Soft Clays

Induced anisotropy is known to play a major role in the undrained strength of clays, especially in case of weakly consolidated deposits. This paper discusses undrained failure in soft clays in light of material stability principles. For this purpose, a strain-hardening elastoplastic model widely used to study undrained instability in isotropic soils is enhanced to account for stress-induced anisotropy. A versatile yield function able to flexibly control the shape of the elastic domain is augmented through a hardening variable related to the evolving fabric, while rotational hardening is used to replicate the reorientation of the surface as a function of the loading history. Parametric analyses are used to illustrate the model capabilities, and instability indices for undrained failure are derived in analytical form. Finally, the model performance is tested against experimental evidence available for two widely tested soils: soft Chicago clay and Boston blue clay. The analyses illustrate how the proposed model allows an accurate representation of soil responses under extension and compression paths. In addition, it enables the identification of undrained failure resulting from the monotonic growth of shear stresses, as well as from a post-peak strength decay.