Modified unified hardening model for breakable coarse-grained soils with nonlinear critical state behavior under low confining pressure

Abstract

This investigation develops a nonlinear Unified Hardening constitutive model (N-UH-L) for brittle coarse-grained soils (CGSs), e.g., siliceous mudstone coarse-grained soils (SMCGSs), that incorporates the nonlinear behavior of the critical state line (CSL) in the mean stress − deviatoric stress (p − q) space at low confining pressures. In detail, an improved modified Cam-Clay (MCC) strength criterion is integrated with the model, and a nonlinear exponent (δ) is introduced to capture the evolution of the CSL. Then, the effect mechanism of particle breakage on the nonlinear behavior of CSL in p − q space is clarified. Meanwhile, the empirical expressions for the δ were provided for both non-cohesive and cohesive coarse-grained soils (CGSs), respectively, anchored in the relationship between the weighted average improved relative breakage potential and the input plastic work. Furthermore, the model’s predictive capability was confirmed through three cases of consolidated drained triaxial tests (i.e., tests on basalt ballast with Group A and B, latite basalt ballast in New South Wales, and penetrating eroded SMCGS), and one case of Consolidated undrained triaxial test (i.e., test on rockfill materials of Changhe Dam). The results show that the N-UH-L model effectively describes dilatancy, hardening/softening, and their dependence on the initial state of CGSs. Moreover, the nonlinear exponent has minimal impact on the deviatoric stress at the early and late stages of shear strain, but induces significant deviations in deviatoric stress during the mid-stage. Additionally, as the initial void ratio increases, the sensitive zone of deviatoric stress shifts towards earlier stages of strain, while the effect of nonlinear exponent on deviatoric stress decreases with higher confining pressure.