Foundation Engineering – Understanding negative skin friction on piles In geotechnical engineering, what is meant by “negative skin friction” on a pile, and under which site condition(s) does it commonly develop?

Introduction / Context:
Negative skin friction (also called down-drag) is a critical concept in pile foundation design. It occurs when the surrounding soil moves downward relative to a pile, imposing an additional downward shear along the pile shaft. Designers must recognize when it develops and account for the extra load it adds to the pile beyond the structural service load.

Given Data / Assumptions:

• Pile embedded in compressible strata such as soft clay, silt, or loose fill.
• Possible changes in the stress state of surrounding soil due to groundwater lowering or placement of surcharge fills.
• Pile material (concrete/steel/timber) is assumed to be sufficiently rough for shaft interaction.

Concept / Approach:
Skin friction can be positive (upward) when surrounding soil tends to drag the pile up, or negative (downward) when soil settles more than the pile. Negative skin friction develops when the soil's effective stress increases and it consolidates or settles relative to the pile. Two classic triggers are: (1) consolidation resulting from lowering of the groundwater table (which raises effective stress), and (2) consolidation under new surface loads (e.g., embankment or structural fill).

Step-by-Step Solution:
Identify mechanism: downward soil movement relative to pile causes downward shear along the shaft.Relate to site changes: groundwater lowering increases effective stress ⇒ consolidation ⇒ settlement.Recognize surcharge fill or natural consolidation of soft deposits as equivalent triggers.Conclude that both the definition in (a) and the condition in (b) are valid.

Verification / Alternative check:
Design methods include adding the drag load (often called negative skin friction load) to the pile's axial demand and checking capacity and settlement. Neutral plane analysis locates where pile and soil settle equally; below that, shaft resistance is typically positive, above it negative.

Why Other Options Are Wrong:

• (d) contradicts well-established mechanisms.
• (e) misstates hydraulic direction; negative skin friction relates to relative downward soil movement, not upward flow.

Common Pitfalls:
Ignoring negative skin friction leads to underestimating axial demand; conversely, double-counting both negative and positive friction over the same length is also incorrect. Always consider construction staging, groundwater control, and long-term consolidation.

Final Answer:
Both (a) and (b)