Pile foundations — conditions leading to negative skin friction (Identify when downward drag develops along the pile shaft.)

Introduction / Context:
Negative skin friction (downdrag) is a geotechnical condition where the soil moves downward relative to the pile, imposing additional compressive drag on the shaft. Designers must account for this extra load in axial capacity checks and settlement analysis.

Given Data / Assumptions:

• Single or group piles embedded in stratified soils.
• Possible consolidation or fill placement above the pile's neutral plane.
• Pile tip may be in a firm bearing stratum.

Concept / Approach:
Downdrag occurs when the soil mass experiences settlement (e.g., consolidation of soft clay under new surcharge or lowering of groundwater) that exceeds the settlement of the pile. Relative downward movement mobilizes shear along the pile in the downward direction, adding to structural load and reducing the available safety margin.

Step-by-Step Solution:
1) Identify mechanisms that cause soil settlement (surcharge, consolidation, liquefaction effects).2) Compare movements: if soil settles more than the pile shaft, drag develops downward.3) Include downdrag as an additional permanent action in design load combinations.4) Mitigate via sleeves, bitumen coating, or placing pile tips deeper into non-settling strata.

Verification / Alternative check:
Load-transfer (t–z) curves show negative skin friction where relative displacement is downward; neutral plane lies where shaft friction changes sign.

Why Other Options Are Wrong:
(a) Upward drag is the opposite of downdrag; (c) firm continuous soil without settlement does not create negative friction; (d) driving onset does not by itself cause downdrag.

Common Pitfalls:
Ignoring downdrag in serviceability; double-counting drag as both load and resistance; not checking group effects.

Final Answer:
the surrounding soil settles more than the pile