STZ-Clay: A shear-transformation-zone theory based constitutive model for clay

Abstract

The shear-transformation-zone (STZ) theory is a mesoscale-based approach that attributes material macroscopic plastic deformation to the flipping, creation, and annihilation of mesoscopic structures known as STZs. In this study, the potential of the STZ theory in soil constitutive modeling is demonstrated through the development of a model coined STZ-Clay, by extending the conventional STZ theory to incorporate two essential characteristics of clay, i.e., plastic volumetric strain and pressure dependence. The STZ-Clay model assumes that the plastic volumetric strain arises from the creation and annihilation of STZs, while the pressure-dependency is accounted for by the dissipation rate function and evolution law of two internal variables, namely STZ density and orientational bias. Remarkably, the STZ-Clay model achieves a unique critical state under different loading paths and reasonably reproduces the influence of consolidation history on the hardening and dilatancy of clay. Overall, the STZ-Clay model offers a fresh perspective for understanding the yielding mechanism and critical state of clay through the lens of mesostructural evolution.