Negative Skin Friction on a Pile - Effect of Drainage Condition-Paper

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The study uses finite element modelling in PLAXIS 3D to investigate negative skin friction (NSF) on a single pile in consolidating soft soil. NSF occurs when the surrounding soil settles more than the pile, creating downward shear forces, or dragload. The model represents clay with the Modified Cam-Clay model, sand with the Mohr–Coulomb model, rock with the Hoek–Brown model, and the pile as linearly elastic. It was validated against an instrumented pile case history in Bangkok clay, with predicted settlement and axial-load patterns broadly matching field measurements.<br /><br />Parametric analyses examined one-way and two-way drainage, pile length, degree of consolidation, and fill thickness. Drainage condition significantly affects neutral-plane depth and dragload during consolidation, but both are similar for the two drainage conditions once consolidation is complete. Fill thickness has little effect on neutral-plane depth, while greater fill thickness increases dragload. Neutral-plane behaviour also varies with pile length and drainage condition.<br /><br />The findings indicate that drainage conditions need not be distinguished when designing for the final dragload, but their influence during consolidation may matter. The study assumes a wished-in-place pile and does not account for installation effects.

Keywords

finite element modelling

PLAXIS 3D

negative skin friction

single pile

soft soil consolidation

dragload

neutral plane

Modified Cam-Clay model

drainage conditions

pile settlement