Question

Negative skin friction occurs when ...........

• A. pile is in overconsolidated clay
• B. surrounding soil settles more than pile
• C. pile expands
• D. water table rises

Hint:

Negative skin friction adds extra downward force on a pile and must be accounted for in foundation design.

Answer 1

The Correct Option is B

Step 1: Understand the concept of skin friction
Skin friction is the resistance developed along the surface of the pile due to interaction with the surrounding soil. It generally assists in carrying axial loads.
Step 2: Define negative skin friction
Negative skin friction, also known as down-drag, occurs when the surrounding soil moves downward relative to the pile, thus exerting a downward force on the pile.
Step 3: Condition that causes this
This situation commonly arises when the soil around the pile consolidates or settles more than the pile itself, especially in soft clays or loose fills.
Step 4: Evaluate the options
(1) Overconsolidated clay resists settlement — Incorrect
(2) Surrounding soil settling more than pile causes down-drag — Correct
(3) Pile expansion is unrelated to skin friction direction — Incorrect
(4) Rising water table reduces effective stress but doesn't directly cause negative skin friction — Incorrect
Step 5: Conclusion
Therefore, negative skin friction occurs when the surrounding soil settles more than the pile.