Negative skin friction on piles: Which of the following statements about negative skin friction (downdrag) is/are correct?

Introduction / Context:
Negative skin friction (downdrag) occurs when the surrounding soil moves downward relative to the pile shaft. The interface shear then acts downward, opposite to positive shaft resistance, effectively increasing the load the pile must carry and reducing available capacity for superstructure loads.

Given Data / Assumptions:

• Soft, compressible clays or recently placed fills that consolidate under their own weight.
• Groundwater changes or surcharge that trigger settlement of the surrounding soil mass.
• Pile installed to a deeper, relatively stable stratum.

Concept / Approach:
When soil settles relative to the pile, the shear stress mobilized at the pile–soil interface acts downward. This is termed negative skin friction and contributes an additional downdrag load. It must be added to the structural load when checking axial capacity and settlement. The design may include bitumen sleeves, coatings, or ground improvement to reduce downdrag.

Step-by-Step Solution:

Verification / Alternative check:
Load test data often show larger apparent loads under the same jack pressure when negative skin friction is present due to the extra drag along the shaft.

Why Other Options Are Wrong:
Each of A, B, and C correctly describes negative skin friction; hence “All of the above” is the most accurate single choice.

Common Pitfalls:
Ignoring seasonal groundwater changes; assuming downdrag is negligible for bored piles—this depends on soil and construction sequence.

Final Answer:
All of the above