The effect of negative skin friction on piles and pile groups

Piles are often designed to resist axial loads acting on the pile head through the development of positive shaft resistance (PSR) and end-bearing resistance. In addition to the pile head loading condition, shear stresses are mobilized at the pile-soil interface when the soil settles more than the pile in consolidating ground owing to the developmentof relative movement between the soil and the pile. A proportion of the weight of the surrounding soil is transferred tothe pile causing additional compressive load (dragload) on the pile shaft and pile settlement (downdrag). In this situation, the mobilized shear stresses act downward and are called negative skin friction (NSF). Although the basic mechanismand the solution to the NSF problem are well established, there are, among engineers, debates regarding various crucial aspects of the NSF phenomena, which can lead to confusion and misunderstanding (Lee 2001). Hence the basic terms relatedto the NSF problem are adopted according to the recent definitions proposed by Fellenius (1999): (1) Negative skin friction (NSF): soil resistance acting downward along the pile shaft as a result of a downdrag and inducing compression in the pile (2) Downdrag: downward movement on a deep foundation unit due to NSF and expressed in terms of settlements (3) Dragload: load transferred to a deep foundation unit from NSF (4) Neutral plane (NP): location where equilibrium exists between the sum of the downward-acting permanent loads applied to the pile and dragload due to negative skin friction and the sum of upward-acting positive shaft resistance and mobilized toe resistance. The neutral plane is also where the relative movement between the pile and the soil is zero.