A time-dependent nonlinear model for unsaturated red clay considering temperature effects

To quantitatively analyze the complex mechanical behavior of unsaturated red clay fill along railway lines in western China under different environmental conditions, this study develops a refined nonlinear model that accurately integrates the effects of temperature, matric suction, and time. A series of triaxial shear tests with controlled strain rates were conducted under controlled temperature and suction conditions to reveal the significant influence of temperature, matric suction, and strain rate on key physical and mechanical parameters such as the initial shear modulus and peak stress difference of unsaturated soils. The results show that temperature significantly increases the shear modulus at high strain rates, while high matric suction reduces the strain rate effect on the strength of unsaturated soils. Additionally, the coupling effect of matric suction and temperature alters the relationship between peak stress difference and strain rate. The proposed nonlinear model, by introducing the evolution relationship of the above-mentioned physical and mechanical parameters with matric suction, temperature, and strain rate, comprehensively reflects the evolution of the stress–strain behavior of unsaturated red clay subgrade fill under complex environmental conditions. This model provides an important theoretical framework for engineering applications in similar complex environments and offers critical insights for engineering problems in analogous geological contexts.