Negative friction of single pile under different supporting conditions

The wetting-induced settlement and deformation of collapsible loess can generate negative pile–soil friction, thereby reducing the bearing capacity of the pile foundations and posing a threat to the superstructures. Pile-side frictional resistance is controlled not only by the properties of the pile itself but also by the support conditions at the pile base. In this study, In this study, indoor single-pile immersion model tests were conducted using an artificially prepared collapsible loess analog to investigate the evolution of negative skin friction and the neutral point under different pile base support conditions. The results shows the artificially prepared loess-like material exhibits a high degree of consistency with the collapsibility behavior of natural loess; when the pile bottom is not supported, no negative frictional resistance and neutral point are generated; when the pile bottom support is non-collapsible soil holding layer, pile bottom slurry expanding head, and end bearing pile, the measured neutral point depth ratio is 0.455 to 0.673, 0.658 to 0.855, and 0.913 to 0.975, respectively. The pile-side negative frictional resistance gradually increases to reach a peak value, then decreases to zero and eventually transitions to positive friction. Greater stiffness and bearing capacity of the pile base support result in a wider distribution zone of negative skin friction and shift the neutral point closer to the pile tip. This study provides new insights into optimizing pile foundation design under varying pile base conditions and offers a reference for analyzing the mechanical behavior of pile foundations in collapsible loess regions.