Question

Negative friction may develop if the material is loose _______ soil.
1. Cohesive
2. Cohesionless
3. Both (A) and (B)
4. None of the above

Answer 1

Correct Answer - Option 2 : Cohesionless

Concept:

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.

Factors that cause negative skin friction on piles and pile group

1. Newly placed fill material on compressible soil before the completion of consolidation.

2. If fill material is loose cohesion-less soil.

3. When fill material is deposited over layer of soft soil or peat.

4. 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

1. Negative skin friction contributes to the uneven settlement of piles or pile group.
2. 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.
3. In bearing piles where the settlement of the pile is negligible, negative skin friction becomes a pile capacity problem.