What is Negative skin friction and how it works.

 

Negative skin friction (NSF) refers to the phenomenon that occurs when the friction between a soil or rock surface and a pile or foundation element creates an opposite force, pulling the foundation down into the ground rather than resisting its movement. This is commonly observed in piles or deep foundations that are embedded into soft soils, especially in areas where the soil undergoes consolidation or settlement.

Here’s how it works:

1.     Soil Consolidation or Settlement: In soft, compressible soils, like clay or silt, when the soil undergoes settlement (due to load or natural compaction), it can cause downward movement of the soil around a pile or foundation.

2.     Friction Development: As the surrounding soil consolidates or moves downward, the friction (skin friction) between the pile surface and the soil increases. Normally, this friction resists the movement of the pile, providing support.

3.     Negative Skin Friction: In cases where the settlement of the soil occurs faster than the pile can move (such as in the case of excessive consolidation or water drainage), the pile is effectively "dragged down" by the frictional forces between the pile and the settling soil. Instead of the pile being pushed upward or supported by friction, the friction now acts downward, effectively pulling the pile deeper into the ground.