A study on negative skin friction of pile in collapsible sandy loess considering infiltration level

To clarity the negative skin friction characteristics of pile in sandy loess of the northern Loess Plateau considering infiltration level, large-scale water injection tests in in-situ pit to simulate rainfall infiltration and saturated infiltration were conducted, numerical simulations were combined to extend the range of infiltration level. The relationships among sequential behaviors of infiltration, collapse and dragging load, as well as regulating laws of negative skin friction by infiltration level were investigated. The results show that collapse activating depends on both infiltration intensity and water retention time, a slight collapse can activate significant negative skin friction. Negative skin friction develops in a phased manner with infiltration level. Current design method completely adopts parameters under saturated condition without distinction is conservative in some cases. Revised effective stress method and revised experience method for negative skin friction calculation were proposed. The conclusions are of great significance for promoting pile design optimization.