BEM and FEM approaches to the analysis of negative skin friction on piles

Negative skin friction (NSF) may be a relevant problem in the design of piles in soft soils, when significant areas around the piles are loaded at the ground surface. The downdrag on piles is traditionally evaluated by reversing a part of the shaft resistance in an applied load and considering it in the evaluation of a safety factor against a bearing capacity failure. Such an approach is totally inadequate, because NSF is actually a problem of soil-pile interaction. Two methods of analysis of a pile subjected to both an external load and NSF are presented: Boundary Element Method (BEM) and Finite Element Method (FEM). The former method is based on a BEM approach and concentrates non-linearity effects at the pile-soil interface while the latter is a FEM approach using the package PLAXIS 2D. The accuracy of the methods is checked back analysing a well-documented case history of bored piles in soft soils.