The negative skin frictional caused by loess collapse will decrease the bearing capacity of single pile, which is essential to the design of pile foundations in loess areas. In this study, a method for estimating the subsidence of soil layer at any depth is firstly proposed based on the total self-weight collapse value. Secondly, a new load transfer constitutive model for pile-soil interface is developed, which considers the nonlinear stress-strain relationship and the ultimate shear strength of soil. Then, a load transfer calculation model for pile foundation is established, which can calculate the pile axial force, the pile skin frictional, neutral point position and the settlement of a single pile. The calculation results are compared with the test data that obtained from a pile foundation on-site immersion test and the effectiveness of the calculation method is verified well. This calculation method may be useful for designing pile foundations in collapsible loess regions.

The water sensitivity and structural characteristics of collapsible loess are two typical factors that significantly influence its mechanical behaviors. This paper presents a simple and practical elastic-plastic model based on the modified Cam-Clay model to well capture the essential behavior of unsaturated intact loess. The model employs deviator stress and spheric stress as the stress variables, with the water content serving as the moisture variable. The critical state surface of the model can be determined by utilizing the shear strength parameters of unsaturated soil under axisymmetric stress conditions. An initial yield surface equation is established by incorporating structural strength into the elliptical yield surface equation, which is used to determine the starting point for elastic-plastic deformation calculations under different humidity and stress combinations. The model comprises several parameters, each of which has a clear physical interpretation and can be conveniently obtained through conventional triaxial tests. The validity of the model for unsaturated intact loess is confirmed through a comparison with the stress–strain relationship of unsaturated intact loess in the axisymmetric stress state. This work has the potential to significantly enhance our ability to predict and mitigate potential geotechnical disasters, such as foundation deformation under axisymmetric conditions and slope stability problems under non-axisymmetric conditions. Ultimately, the application of this model could contribute to the safety and stability of infrastructure and construction projects in loess regions.

The mechanical properties of soil–concrete interfaces directly impact the bearing capacity and structural stability of underground projects. Characterizing mechanical responses and quantifying multi-factor influence mechanisms are fundamental to geotechnical design, numerical simulation, and safety assessment. To reveal the mechanical properties of the unsaturated loess–structure interface, this study conducted a series of direct shear tests on loess–concrete interfaces under varying moisture contents. The effects of interface roughness, soil dry density, normal stress, and soil moisture content on the interfacial shear strength were quantitatively evaluated. The results show 20–35% shear stress variation with dry density, up to 35% shear strength reduction upon wetting, less than 10% shear stress difference due to interface roughness, and normal stress controls, shear stress magnitude, and initial failure sliding displacement. Based on the test results, moisture content was introduced as an additional variable to establish a modified hyperbolic model for unsaturated soil-structure interfaces. This model contains six parameters, all of which can be determined through interface direct shear tests at different moisture contents. These findings advance the quantitative understanding of unsaturated loess–concrete interface mechanics and provide a critical theoretical foundation for the design, numerical analysis, and stability assessment of unsaturated loess–structure interfaces under multi-factor coupled conditions in practical geotechnical engineering.