Geotechnical analysis involving strain localization of overconsolidated soils based on unified hardening model with hardening variable updated by a composite scheme

Strain localization simulation of overconsolidated soils with high overconsolidation ratio (OCR) has been a long-standing challenge. Some critical state soil models, including the modified Cam-clay (MCC) model, have been widely applied, but they may not predict the shear dilatancy of overconsolidated soils well in some cases. Hence, the unified hardening (UH) model, which may be viewed as a generalized version of the MCC model, is implemented. It has been recognized, nonetheless, that without resorting to the regularization mechanism, the standard finite element method (FEM) or the second-order cone programming optimized finite element method (FEM-SOCP) often experiences instability or interruption of calculating the hardening-softening responses of overconsolidated soils. To resolve the aforementioned difficulty, the UH model is developed and implemented in the framework of FEM-SOCP based on the micropolar continuum (mpcFEM-SOCP) to predict strain localizations of overconsolidated soils. Furthermore, to obviate non-convexity of mpcFEM-SOCP induced by material softening, an effective composite update scheme of hardening variable pc, which refers to the implicit variable (IV) scheme for the hardening stage and then refers to the explicit variable (EV) scheme for the softening stage, is proposed. Based on one biaxial compression problem and one rigid strip footing problem, numerical analyses disclose that by applying mpcFEM-SOCP in conjunction with the composite update scheme of pc, the UH model of micropolar continuum can effectively predict the strain localization behavior of overconsolidated soil during its failure stage, and the stable hardening-softening responses of overconsolidated soils can be readily attained.