Negative skin friction (NSF) on a pile occurs when:
Choose the correct answer
The pile is in tension
Surrounding soil settles more than the pile, causing downward drag on the pile shaft, which ADDS to the axial load on the pile
The pile is too long
Groundwater table is very high
Correct Answer
B. Surrounding soil settles more than the pile, causing downward drag on the pile shaft, which ADDS to the axial load on the pile
Why is this the answer?
NSF (drag-down force): occurs in piles passing through consolidating soft clay overlying firm bearing stratum. The settling soil drags the pile down, adding to the structural load. NSF = K_s x sigma_v x tan(delta) x perimeter x length of settling layer. Must be subtracted from pile capacity in design.
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