Research on Bearing Characteristics of Pile Foundations Considering Negative Skin Friction Induced by Loess Seismic Subsidence

Negative Skin Friction (NSF) induced by loess seismic subsidence poses a significant threat to the safety of pile foundations in seismic loess regions. To address this problem, a theoretical calculation model for the vertical bearing behaviour of piles under the combined action of pile–top load and seismic loading is developed based on the shear displacement method and pile–soil interaction theory. The depth-dependent seismic subsidence of loess is obtained through numerical simulation and fitted using an exponential function, which is incorporated into the analytical framework. The proposed method is validated through finite element simulations and an engineering case in Lanzhou, China. Results indicate that seismic intensity significantly increases pile axial force, drag load, and pile–top settlement, while the neutral point position remains nearly unchanged. Increasing pile–top load weakens NSF and shifts the neutral point upward. Larger pile diameters slightly reduce drag load but have limited influence on the neutral point. Increasing pile length increases drag load and shifts the neutral point downward, while reducing pile–top settlement. Sensitivity analysis reveals that seismic intensity and pile length are the dominant control parameters. The proposed model provides a practical analytical tool for evaluating NSF in seismic subsidence loess and offers theoretical support for seismic design and mitigation of pile foundations in loess regions.