Negative skin friction on a pile develops when
- Athe soil in which it is driven is sandy soil
- Bthe soil so surrounding it settles more than the pileCorrect
- Cthe groundwater table rises
- Dthe soil near the tip is clay
Solution
Concept
Development of negative skin friction on piles
Formula
NONE
Step-by-step solution
1. Negative skin friction is a downward drag force exerted on a pile. 2. This occurs when the surrounding compressible soil layer undergoes settlement greater than the settlement of the pile itself. 3. This downward movement reverses the direction of friction forces, thereby reducing the load-carrying capacity of the pile. Answer: B
Explanation
Negative skin friction occurs when the surrounding soil layer settles more than the pile itself. This downward movement of soil creates friction forces acting downwards along the pile surface. As a result, it adds an extra downward load to the pile instead of supporting it.
Why the other options are wrong
A) Sandy soil alone does not cause negative skin friction unless surrounding soil settles relative to the pile. C) Rising groundwater table decreases effective stress, whereas lowering it causes consolidation settlement leading to negative skin friction. D) Clay near the tip affects end bearing capacity rather than inducing negative skin friction along the shaft.
Shortcut
Negative skin friction occurs when surrounding soil settles more than the pile.
Common mistake
Assuming negative skin friction is caused by downward pile movement rather than relative downward settlement of the surrounding soil.
Correct answer: B) the soil so surrounding it settles more than the pile