The ACUH model: A unified hardening constitutive model for anisotropically consolidated loess considering intermediate principal stress effects

As the main material, loess plays a crucial role in the construction of highways, railways and airfields in the Loess Plateau, China. However, the infrastructure construction in the transport sector in this region is susceptible to flow instability. In this study, a constitutive framework incorporating a unified hardening (UH) parameter is proposed for anisotropically consolidated loess (ACUH). It features a yield surface aligned with the anisotropic consolidation line, and an anisotropy stress ratio is integrated into the UH parameter to account for the effects of anisotropic consolidation on the dilatancy behaviour, strain hardening, and softening of loess. The ACUH model is extended to three-dimensional stress conditions, and the adjustment of nonlinear critical state lines (CSLs) in the e-lnp plane due to changing Lode angle can be estimated with a simple and practical calibration procedure. The model is validated against test data reported in the literature. Constitutive simulations are carried out to evaluate the influence of anisotropic consolidation and the intermediate principal stress on the instability characteristics of a silty loess. The results indicate that these factors significantly affect the peak strength, strength loss, instability stress ratio, and time period between the preceding anisotropy consolidation and the subsequent instability, which can be directly compared and quantified by an index, instability period angle θ, proposed in this study.