This study investigated the hydraulic and mechanical behaviors of unsaturated coarse-grained railway embankment fill materials (CREFMs) using a novel unsaturated large-scale triaxial apparatus equipped with the axis translation technique (ATT). Comprehensive soil-water retention and constant-suction triaxial compression tests were conducted to evaluate the effects of initial void ratio, matric suction, and confining pressure on the properties of CREFMs. Key findings reveal a primary suction range of 0-100 kPa characterized by hysteresis, which intensifies with decreasing density. Notably, the air entry value and residual suction are influenced by void ratio, with higher void ratios leading to decreased air entry values and residual suctions, underscoring the critical role of void ratio in hydraulic behavior. Additionally, the critical state line (CSL) in the bi-logarithmic space of void ratio and mean effective stress shifts towards higher void ratios with increasing matric suction, significantly affecting dilatancy and critical states. Furthermore, the study demonstrated that the mobilized friction angle and modulus properties depend on confining pressure and matric suction. A novel modified dilatancy equation was proposed, which enhances the predictability of CREFMs' responses under variable loading, particularly at high stress ratios defined by the deviatoric stress over the mean effective stress. This research advances the understanding of CREFMs' performance, especially under fluctuating environmental conditions that alter suction levels. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

The plastic strain of calcareous sand is related to its stress path and particle breakage, rendering the hardening process complex. An expression for the stress-path-dependence factor was developed by analyzing the variations in plastic strain across different initial void ratios. A stress-path-independent hardening parameter was derived from the modified plastic work and was subsequently validated. Constant-proportion loading tests on calcareous sands confirmed the applicability of this hardening model. The results indicated that under isotropic compression, the plastic volumetric strain increased with increasing average effective stress, albeit at a decreasing growth rate. A positive linear relationship was observed between the volumetric strain modulus and relative breakage index. The proposed hardening parameter effectively captured the particle breakage and stress path effects in calcareous sand and was validated through theoretical calculations and laboratory tests, offering valuable insights into the mechanical behavior of fragile granular soils.

Considering fabric evolution effects is crucial for accurately describing the macroscopic mechanical behavior of cohesionless soil under cyclic loading. Building upon the nonlinear dilatancy equation established for sand-gravel composites under monotonic loading, a fabric-dilatancy internal variable, which accounts for fabric evolution during the dilatancy stage under cyclic loading, is introduced. An elastoplastic constitutive model based on the generalized plasticity framework is proposed to capture the full range of mechanical behaviors of sand-gravel composites under both static and liquefaction conditions. By comparing the liquefaction deformation, stress paths, and excess pore water pressure development of sand-gravel composites before and after considering fabric evolution effects, the significance of fabric evolution effects in simulating the liquefaction response of sand-gravel composites is demonstrated. The model's performance is validated through a series of large-scale triaxial tests on sand-gravel composites under both static and dynamic loading conditions, as well as by comparing with test results from relevant literature. The results show that the model generally provides a reasonable representation of the stress-strain-volume behavior of sand-gravel composites under static drained conditions, as well as the accumulation and dissipation of excess pore water pressure, stress path evolution, and liquefaction deformation during liquefaction. This model can serve as a powerful tool for numerical simulation in sand-gravel composites engineering.