Negative Skin Friction in Pile Foundations: Mechanisms and Mitigation Techniques in Collapsible Soils - A Review

This review examines the mechanisms, controlling factors, and mitigation methods associated with negative skin friction (NSF) on pile foundations, with particular emphasis on collapsible loess and other weak soils. Experimental, field, numerical, and design studies, including recent work published up to 2026, are synthesized to clarify how soil settlement, pile stiffness, slenderness, pile-end restraint, surface condition, groundwater change, and wetting govern downward shaft shear and dragload. The literature shows that NSF develops when the surrounding soil settles more than the pile and that the neutral plane is the level at which the relative pile-soil movement, and hence the direction of shaft shear, reverses. Wetting of collapsible soils intensifies relative displacement by degrading interparticle bonding, although the mobilized interface stress also depends on the accompanying reduction in soil strength. Three principal mitigation mechanisms are identified: low-friction coatings reduce interface adhesion and promote slip; casings and sleeves provide more direct mechanical isolation; and under-reamed or modified pile geometries redistribute load toward deeper resistance. Ground treatment, which removes the cause of settlement rather than its interface effect, is assessed alongside these measures. A critical comparison of the techniques is presented, together with summary tables that link each NSF mechanism to its influencing factors and corresponding mitigation measures and that rate the applicability of each technique under different soil conditions. The reviewed evidence indicates that no single technique is universally superior, because effectiveness depends on the soil profile, protected length, pile-end condition, rate of settlement, and the required contribution of positive shaft resistance. Reliable assessment therefore requires the complete shaft-friction profile, dragload accumulation, neutral-plane position, and settlement response rather than a single peak stress. The review also identifies a need for standardized design procedures, longer-term field validation, and direct comparative testing under consistent soil and loading conditions.