Negative Skin Friction on Piles and Pile Groups

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What is negative skin friction on piles and pile group?

Negative skin friction is usually a downward shear drag acting on a pile or pile group because of downward movement of surrounding soil relative to the piles. This shear drag movements are anticipated to occur when a pile penetrates into compressible soil layer that can consolidate. It is reported that, A small relative movement between the soil and the pile of around 10 mm may be adequate for the full negative skin friction to materialize. Moreover, the time of ending the negative skin fiction of piles is estimated to be around 2 years and the degree of consolidation of the soft soils reaches 90%. Finally, this article presents different aspect of negative skin friction on piles and pile group.

Factors that cause negative skin friction on piles and pile group

• Newly placed fill material on compressible soil before the completion of consolidation.
• If fill material is loose cohesionless soil
• When fill material is deposited over layer of soft soil or peat.
• Lowering groundwater which increases the effective stress causing consolidation of soil with resultant settlement and friction force being developed on the pile.

Effect of negative skin friction on piles and pile groups

• Negative skin friction contributes to the uneven settlement of piles or pile group.
• For piles in compressible soils where pile capacity is contributed by both point resistance and shaft adhesion, the problem of negative skin friction should be considered a settlement problem.
• In bearing piles where the settlement of the pile is negligible, negative skin friction becomes a pile capacity problem.

Negative skin friction in single pile

The negative skin friction in single piles can be computed using the following expressions:

Negative skin friction of piles in cohesive soil

Where: : negative skin friction : perimeter of the pile : pile length in compressible soil as illustrated in Fig. 2. : unit adhesion and can be computed using equation 2 Where: : adhesion factor : Undrain Cohesion of the compressible layer

Negative skin friction of piles in cohesion less Soils

Where: K: lateral earth pressure coefficient : unit weight of soil : angle of friction between pile and soil, which may vary from 1/2 to 2/3.

Negative skin friction on pile groups

Negative skin friction in pile groups equal to the greater of equation 4 and equation 5:   Where: n:number of piles in the group Pg: perimeter of the group : unit weight of the soil within the pile group up to a depth  Ag: area of pile group with in the perimeter Pg

The effect of negative skin friction on the factor of safety with respect to the ultimate load capacity of a pile or a pile group

The influence of negative skin friction on safety factor with respect to ultimate load carrying capacity of piles or pile group is considered by introducing the factor of safety, so: ultimate load capacity of a pile or a group of pile=(working load + negative skin friction load)( Factor of safety) 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 as demonstrated in Fig.2.