For estimating negative skin friction on a pile, which of the following shear strength parameter is required?
- Ac′ and φ′Correct
- Bc and φ
- Cc u
- Dunconfined compressive strength
Solution
Concept
Negative Skin Friction on Piles
Formula
Fn = P × Ln × fs, where fs = K × σv' × tan(δ) or fs = c' + K × σv' × tan(ϕ')
Explanation
Option A (c′ and φ′) represents the effective shear strength parameters of the soil (effective cohesion c′ and effective angle of internal friction φ′). Negative skin friction develops as effective stress builds up over time due to long-term soil consolidation and settlement around the pile. Therefore, an effective stress approach using effective shear strength parameters (c′ and φ′) is required to accurately estimate the long-term downward drag force on the pile.
Shortcut
Remember "Effective for Long-Term": Negative skin friction occurs due to long-term consolidation and settlement as water pressure dissipates, which always requires effective shear strength parameters with primes (c′ and φ′).
Common mistake
A common mistake is selecting undrained or total shear strength parameters (such as c and φ, or cu) by assuming negative skin friction is a short-term phenomenon, forgetting that it develops over time due to soil consolidation, which requires effective shear strength parameters (c′ and φ′).
Correct answer: A) c′ and φ′