Axial Pile Analysis – Episode 6 - Why Axial Resistance Must Be Capped

In the previous episode, I discussed how axial pile resistance can vary significantly between design codes depending on the adopted method and resistance factors.

There is another critical concept that deserves attention and is often misunderstood in practice: axial pile resistance does not increase indefinitely with pile length.

For piles in cohesionless soils, both shaft friction and end bearing are typically functions of effective stress. As depth increases, effective stress increases, and it may appear logical to assume that axial resistance keeps increasing with depth.

In reality, this does not happen.

At greater depths, local failure mechanisms develop around the pile. Beyond that point, additional depth does not translate into additional usable resistance. This is why axial pile resistance must always be limited by maximum shaft friction and maximum toe bearing values.

These limits are not calculated automatically, they are design inputs, and the responsibility sits with the pile designer.

If these limits are ignored, design software will happily report very large axial capacities that look impressive on paper but are not achievable in the field.

The American Petroleum Institute provides practical guidance on this issue by defining limiting shaft friction and limiting end bearing values for different cohesionless soils based on soil type and density. These limits act as a reality check and prevent unconservative designs driven purely by increasing effective stress with depth.