Slip effect at the pile-soil interface on dragload

Negative skin friction is perhaps one of the most common problems in the design of pile foundations in soft soils. The development of dragload on single piles and piles in groups in consolidating ground was investigated from 2D and 3D elasto-plastic numerical analyses. Conventional no-slip continuum analysis and slip analysis were conducted for comparison. The parametric analyses showed that dragload for single piles and their reductions in pile groups were substantially over-estimated by the continuum analyses. Axial loading of about 125-325% of the maximum dragload was required to eliminate dragload, depending on the stiffness of the bearing layer. Dragload development heavily depended on the interface friction coefficient, surface loading, and axial load, governing soil slip behavior at the pile-soil interface. The application of axial loads on piles in groups resulted in reductions in group effects. An example analysis and two previous experimental observations of dragload and group effects were back-analyzed. The slip analysis estimated reasonable dragload and group effect, whereas unrealistic computations were obtained from the no-slip continuum analysis.