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).

NEGATIVE SKIN FRICTION ON PILES

The net ultimate load carrying capacity of the pile is given by the equation

clip_image003

clip_image005 = negative skin friction

clip_image007 = 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

clip_image009

Where n – number of piles in a group

clip_image011 – negative skin friction on each pile

clip_image011[1]= S x p x L (for cohesive soils)

clip_image013 (for granular soils)

clip_image015 (where c – cohesion, clip_image017- reduction factor)

Where p – perimeter of the pile

L – depth of fill

clip_image019 = earth pressure coefficient

clip_image021= unit weight of fill

f – coeffieicent of friction = clip_image023

clip_image025= 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.

clip_image027

S – shear resistance of soil

L – depth of fill

p – perimeter of pile group

clip_image021[1] – unit weight of soil

A – area of pile group enclosed in perimeter p.