Field Tests on Influencing Factors of Negative Skin Friction for Pile Foundations in Collapsible Loess Regions

As a reliable building foundation form, piles are driven into collapsible soil layers to ensure stability of foundations. Because of water immersion, significant subsidence occurs on collapsible loess; then negative skin friction emerges on the pile surface, which eventually causes serious bearing capacity failures of pile foundations. Relying on water immersion tests of multiple piles in Lanzhou, China, this study analyzed the influencing factors of negative skin friction for pile foundations in collapsible loess regions. The main factors studied in this research are cumulative relative collapse amount, pile type, and change in loess collapsibility. The results demonstrate that the maximum negative skin friction has a negative correlation to the cumulative relative collapse amount, which is determined by the degree of difficulty of the emergence of the shear fracture surface. Owing to the compaction effect of the driven pile and surcharge load of the exploded pile, their negative skin frictions increase in varying degrees compared to that of the bored concrete pile. At the same test site, the changes in loess collapsibility are mainly affected by natural moisture content and dry density. Increases in both the natural moisture content and dry density reduce the loess collapsibility, immersion settlement rate, and negative skin friction of pile. The loess collapsibility can be improved by surcharge loading and pre-watering to reduce the adverse effect of negative skin friction on pile foundations in engineering applications.