Lateral Pile Analysis: Episode 10 - Lateral Pile Analysis – Fixity Depth

Let’s Talk About “Fixity Depth”

One of the biggest challenges in lateral pile analysis is not the loads, not the soil model, and not even the p-y curves.

It is simply:
Where does the pile actually “fix” into the ground?

On paper, we often see two numbers:

Pile cut off elevation – decided by the structural team.
Pile tip elevation – what we as geotechnical engineers recommend.

But in practice, things get more interesting.

During driving, a pile can achieve its axial capacity at a shallower depth than expected. Contractors might ask:
“Can we shorten the pile? The hammer shows refusal early.”

For axial capacity only—maybe.
But for lateral performance—that can be dangerous.

Because lateral fixity has nothing to do with where the hammer stops. It depends on how deep the pile needs to be to achieve zero rotation and almost zero lateral movement.
And that depth is almost always deeper than the depth needed to satisfy axial capacity.

And the key message:
Never shorten a driven pile below the geotechnical fixity depth, even if axial capacity is achieved earlier. This means you have to clearly define the maximum pile tip elevation.

The fixity depth is what provides the lateral restraint needed to keep the pile behaving safely under real-world loads.

This is why in my lateral pile analysis course, I dedicate a full chapter to this concept and demonstrate how to determine geotechnical fixity directly from the slope and deflection profiles.

If you are designing driven piles, this is one topic that you must master.