Pile Groups with Negative Skin Friction-Paper

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The paper presents a practical method for analysing single piles and pile groups affected by negative skin friction (NSF), which occurs when consolidating soil moves downward relative to a pile. NSF is primarily a serviceability concern involving pile settlement; the resulting drag force is an internal force relevant to structural strength, not an external load that should be added to the pile-head load when checking geotechnical capacity.<br /><br />The method separates the analysis into two stages: first, estimate free-field ground movements caused by consolidation; then apply those movements, together with any pile-cap loads, in PGROUPN. This program uses a three-dimensional nonlinear boundary element method to model soil and pile behaviour, including pile-group interaction, soil nonlinearity, and slip at the pile–soil interface. Comparisons with published boundary element and finite-difference analyses show generally good agreement, while requiring little computational time.<br /><br />The results demonstrate that assuming full shaft resistance is mobilized can substantially overestimate drag force and settlement, particularly when ground movements are modest. Interface slip can develop at relatively small movements and limits the shear stress transferred to the pile. In one example, accounting for nonlinear behaviour reduced the maximum drag force by 88% compared with a linear elastic analysis. Drag force and settlement also approach limiting values as ground movement increases.<br /><br />Unlike under conventional pile-head loading, interaction between piles generally benefits groups under NSF. Compared with an isolated pile, a 3×3 group at three-diameter spacing showed reductions in maximum drag force of 14% for a corner pile and 31% for a centre pile; the benefit diminished as spacing increased. Group interaction also tended to reduce the depth of the neutral plane.<br /><br />The paper concludes that pile–soil interaction analysis provides a more realistic and potentially more economical basis for NSF design than simplified full-friction assumptions, especially when settlement and pile-group effects matter.
Keywords
negative skin friction
pile settlement
pile groups
consolidation
drag force
pile-soil interaction
boundary element method
interface slip
neutral plane
geotechnical design