Modeling negative skin friction development in a single pile in collapsible loess subjected to progressive wetting

Negative skin friction (NSF) along a pile could arise from soil collapse, leading to additional axial force on the pile and potential settlement. However, the influences of soil density and soil–pile shear strength variations due to soil wetting on NSF and axial forces remain poorly understood. This study employs numerical analysis to explore pile–soil interactions subjected to wetting in collapsible loess areas. A hydromechanical coupled model is developed and implemented by considering the progressive collapse and changes in the pile–soil shear strength due to wetting. The pile–soil shear strength parameters are determined through suction-controlled direct shear testing on unsaturated loess, with the collapse behavior of unsaturated loess verified by field immersion tests. A two-dimensional axisymmetric pile–soil interaction model was established in the numerical model to investigate the impact of the pile head load, friction coefficient, and embedment depth on the negative skin friction during the wetting process in collapsible loess regions. To gain deeper insights into the evolution of pile negative skin friction under collapsibility conditions, the simulation results were compared with the negative skin friction values calculated via the β-method. The simulation results show that the axial force and NSF are influenced by the pile head load and friction coefficient, with two neutral points observed at a 5 m embedment depth but only one neutral point observed at an embedding depth of 0 m. The back-calculated value of β at the same depth increased over simulation time, questioning the validity of using a fixed β value in the β-method to estimate the NSF, considering the "hang-up" effect of piles and soil stress redistribution. The results of this study could offer valuable insights for estimating the NSF in the design of piles in collapsible loess regions.