NEGATIVE SKIN FRICTION ON PILES AND PILE GROUP
When a fill or load is placed on compressible soil deposit, consolidation of the compressible material takes place. When the piles are driven through compressible soils before consolidation is complete or the site has newly placed fill or will be placed in future, the compressible soil will downward relative to the pile. The downward movement of soil develops skin friction between the pile and the surrounding soil and it is termed as negative skin friction. Negative skin friction can be developed from lowering on water level in compressible soils such as clay, peat, mud and soft soil and also due to increase in stress by some means (e.g. filling).
The net ultimate load carrying capacity of the pile is given by the equation
= negative skin friction
= net ultimate load
Where it is anticipated that negative skin friction would impose undesirable, large downward drag on a pile, it can be eliminated by providing a protective sleeve or a coating for the section which is surrounded by the settling soil.
To compute negative skin friction on group of piles, the minimum value from the following equations should be used.
(i) The negative skin friction as the sum of individual piles
Where n – number of piles in a group
– negative skin friction on each pile
= S x p x L (for cohesive soils)
(for granular soils)
(where c – cohesion, - reduction factor)
Where p – perimeter of the pile
L – depth of fill
= earth pressure coefficient
= unit weight of fill
f – coeffieicent of friction =
= angle of friction between pile and soil
(ii) The block skin resistance:
When the piles are placed close to each other, the negative skin friction resistance may act effectively on the block perimeter of the pile group.
S – shear resistance of soil
L – depth of fill
p – perimeter of pile group
– unit weight of soil
A – area of pile group enclosed in perimeter p.