Abstract: Loess exhibits distinctive properties, including notable macroporosity, water sensitivity, and structural characteristics, with its mechanical behavior being significantly affected by saturation levels. This study focuses on the constitutive modeling of the coupled hydro-mechanical elastoplastic behavior in structured loess, employing Bishop's effective stress and saturation as the primary driving variables within the model framework, while considering the corresponding soil skeleton strain and suction as conjugate variables. Accounting for the variation in structural properties with saturation, we introduce saturation-dependent yield stress structural parameters and frictional strength structural parameters, and formulate an evolution equation for these structural parameters in relation to plastic strain and saturation. Utilizing the yield equation for remolded saturated loess as a foundation, we construct the LC (loading-collapse) yield equation for unsaturated structured loess within the effective stress-saturation space, based on the correlation between saturation and yield stress. A volumetric strain-dependent soil-water characteristic curve is adopted to delineate the hydraulic behavior of loess. By integrating volumetric strain with its mechanical response, we establish a coupled hydro-mechanical elastoplastic constitutive model for unsaturated loess. The model encompasses 14 parameters, which can be calibrated through standard unsaturated triaxial tests and isotropic consolidation tests. The model's validity is confirmed by predicting the outcomes of compression and shear tests on loess under conditions of constant suction and constant moisture content, and comparing these predictions with established experimental data. The findings demonstrate that the model accurately captures the yield strength of loess across varying saturation levels, forecasts the strain softening behavior of unsaturated loess under low confining pressures, and adequately describes the coupled hydro-mechanical behavior of structured loess, thereby offering essential theoretical underpinnings for deformation and stability analyses in loess engineering.

Key words: unsaturated loess, structural parameter, hydro-mechanical coupling, softening characteristics, structural evolution