Negative skin friction: causes, effects, remedies

Negative skin friction (NSF), also called down-drag, is the downward drag force on a pile shaft when the surrounding soil settles more than the pile. The soil then moves down relative to the pile and acts as an extra load instead of supporting the pile.

Typical situation

A pile is driven through a soft compressible clay layer to a firm stratum (rock, dense sand). If a fill is placed over the clay after driving, the clay consolidates under the fill and settles, dragging the pile shaft down. Down-drag acts down to the neutral point, below which the pile moves more than the soil and normal (positive) skin friction acts.

  Fill  ////////////////
  GL ---------------------   soil settles
  Soft |  | <- soil moves down
  clay |P |   NSF acts down
  - - -|  |- - neutral point
  Firm |  |   positive friction
  layer|__|   end bearing

Causes

• Fill placed on soft clay after driving the piles.
• Lowering of the water table (increase in effective stress, consolidation).
• Consolidation of remoulded clay disturbed by pile driving.
• Heavy surcharge or loads near the pile, and settlement of loose sand under vibration.

Effects

• Adds a downward load $Q_{nsf}$ on the pile, so the net capacity is reduced: $Q_{all} = \dfrac{Q_{ult} - Q_{nsf}}{FS}$ (or the working load plus $Q_{nsf}$ must be less than the capacity).
• Excessive settlement or structural overstress of the pile, especially for end-bearing piles.
• Tilting or cracking of the structure above, and settlement of the piles relative to the surrounding ground.

Remedial and preventive measures

• Coat the pile shaft over the compressible layer with bitumen (or use a sleeve) to reduce friction.
• Use a casing or a larger-diameter driven hole filled with bentonite slurry.
• Preload or drive the piles after the consolidation of the fill is complete.
• Use lightweight fill, or avoid surcharging, and keep the water table from being lowered.
• Increase the pile capacity (longer or larger piles) to carry the drag load.