Axial Pile Analysis: Episode 3 - Downdrag in Axial Pile Design: What It Is—and What It Isn’t

Downdrag, or negative skin friction, is one of the most misunderstood concepts in axial pile design.

It occurs when settling soil surrounding the upper portion of a pile drags down against it, mobilizing downward shear stresses. This happens in conditions like:

• Fill or embankments placed after pile installation

• Consolidating soft clays

• Organic or peat layers under load

Here’s the important distinction:

Downdrag does not apply an external load to the pile in the way structural loads do. It doesn’t mean your factored axial capacity needs to go up.

But it does affect settlement — and that’s where it matters most.

When designing for serviceability (SLS), downdrag increases total settlement by mobilizing shaft resistance in the wrong direction. This can result in:

• Larger displacements under service loads

• Potential overstressing of connected structures

• A need to extend piles deeper into stable soil

 I cover this topic in depth in my new Module,
“Module (3a) Deep Foundations – Axial Pile Analysis with RS Pile”