Abstract: The shear deformation characteristics of the pile-soil interface is significantly influenced by the water content due to the structural strength and water-sensitive nature of loess, leading to strain-softening behavior during shear deformation. Effective saturation and Bishop’s effective stress were employed as direct driving variables to reflect the effects of saturation on the structural strength of loess, based on the water-stress coupling characteristics of the pile-loess interface. Structural parameters such as cohesion, friction angle, and compression index, along with their evolution equations, are developed to reflect the degradation of structural strength with plastic strain and effective saturation. On the basis, by equating the plastic deformation of unsaturated structural loess with saturated non-structural loess under lateral confinement, a load-collapse function is developed for the pile-loess interface in the effective stress-effective degree of saturation space. An elastoplastic hydro-mechanical coupling model for the pile-loess interface is developed by integrating a soil-water characteristic curve. The model is validated using direct shear test data from unsaturated structural Lanzhou loess and field pile test data from Shanxi unsaturated loess. The results show that the proposed model effectively represents the hydro-mechanical coupling behavior of the pile-unsaturated loess interface, reflects the effects of saturation on shear strength, and captures the variation of strain-softening characteristics at the pile-soil interface with saturation. The model offers an effective approach for disaster prevention design, analysis, and assessment of the load-carrying behavior of piles in unsaturated loess.

Key words: pile-loess interface, load-transfer, structural strength, water-stress coupling, strain softening