Abstract: Due to its unique composition and fabric characteristics, loess exhibits structural and hydrological sensitivity, with its mechanical properties, including yield stress, shear strength, and deformation characteristics, being closely related to saturation and structural integrity. Considering the influence of saturation on the structural strength of loess, structural parameters such as yield stress, friction strength, and cohesion strength are introduced, and a functional relationship between these parameters and initial saturation was established. An evolution equation for structural parameters during plastic deformation was derived based on the structural degradation characteristics observed in shear deformation of loess. On this basis, the yield function for structural loess was developed by drawing inspiration from the Cambridge model. By taking yield stress and structural parameters as hardening parameters, an elastoplastic constitutive model for loess, accounting for saturation and structural evolution, was proposed using the associated flow rule, and the corresponding constitutive matrix was derived. Based on triaxial consolidation drainage test data for undisturbed Q₃ loess from Dongjiahe Town, Tongchuan City, Shaanxi Province, stress-strain curves under varying confining pressures and saturations were predicted, and the model’s rationality was verified. The results indicate that under low saturation conditions, loess exhibits high structural strength, which decreases significantly after yielding, accompanied by pronounced strain softening characteristics. As soil saturation increases, structural strength and corresponding yield stress decrease, with stress-strain curves becoming less steep, indicating increased plastic deformation and progressively less pronounced softening characteristics. The model effectively captures the relationship between loess structure and initial saturation, enabling a more accurate description of mechanical behavior under varying saturation conditions and reflecting how strain softening characteristics vary with saturation.

Key words: unsaturated loess, initial saturation, structural, softening characteristics, elastic-plastic model