Pile Foundations MCQ with answer and solution

Concept

Downward drag and negative skin friction on piles

Formula

Q_nf = P × L × α × c_u

Step-by-step solution

1. Negative skin friction is an unintended downward shearing force exerted along the perimeter of a pile shaft. 2. It occurs when a soft, highly compressible soil stratum around the pile settles more rapidly or by a greater amount than the pile. 3. Common triggers include surcharge loading on compressible fills, recent consolidation, or lowering of the water table. 4. The downward moving soil drags the pile down, reducing its net allowable axial capacity. Answer: B

Explanation

Negative skin friction (downward drag) develops along a pile shaft when the surrounding soft, compressible soil layer settles more than the pile itself. This downward movement of soil imparts an additional downward load onto the pile instead of providing load support.

Why the other options are wrong

A) Dense sand provides positive upward skin resistance and does not settle excessively. C) Structural overloading leads to pile plunging failure, not negative skin friction. D) Rising water table reduces effective stress and does not cause soil settlement drag.

Shortcut

Surrounding soil settles more than the pile = Negative skin friction (downward drag).

Common mistake

Assuming negative skin friction occurs in dense non-settling sand.