Bearing capacity of piles with negative skin friction based on field tests

In response to the significant post-construction settlement (up to tens of centimeters) persisting in coastal deep soft clay strata pretreated by the drainage consolidation methods, which triggers the negative skin friction (NSF) effects on pile foundations, the axial force distribution and bearing performance degradation mechanisms are investigated through the field tests. Two prestressed high-strength concrete (PHC) piles with different tip-bearing layers are selected, and the rebar stress gauges are embedded to monitor the real-time axial force evolution under two working conditions: no pile-top load (Condition I) and applied working load (Condition II). The results demonstrate that under Condition I, the axial force at the neutral point represents the maximum drag load on the pile, which increases progressively with soil settlement. The neutral point depth migrates downward from an initial 30.00 m (friction pile) to 44.66 m (end-bearing pile), corresponding to the drag loads of 2160 kN and 3640 kN, respectively (at a pile-soil relative displacement of 87 mm). After 55 days of sustained loading under Condition II, the proportion of pile-top load increment transmitting below the neutral point significantly attenuates, while the upper pile-soil interface transitions to a positive friction zone due to pile penetration. The study indicates that delaying pile construction until the post-treatment soil stabilization can reduce drag loads by over 40%, whereas the current codes overestimate drag loads by assuming the static neutral points, leading to conservative bearing capacity predictions. These findings provide quantitative experimental evidence for optimizing pile design in deep soft soil regions.