Axial Pile Analysis – Episode 5- Alpha and Beta Methods for Ultimate Axial Resistance

One of the most common sources of confusion in axial pile design is that the same pile can end up with very different ultimate capacities depending on the code used.

If you apply the AASHTO approach and then repeat the analysis using the Canadian Foundation Engineering Manual (CFEM), the difference in axial resistance can be significant. This happens even though the underlying soil mechanics concepts are similar. The difference lies in how each code implements the theory.

In the CFEM framework, axial resistance is evaluated using:

• Beta method for drained analysis
  This applies to cohesionless soils and long term effective stress conditions in cohesive soils.

• Alpha method for undrained analysis
  This applies to short term total stress conditions in cohesive soils.

In my axial pile analysis course, I use RS Pile because it is the only software I am aware of that directly implements the CFEM Alpha and Beta methods. At the moment, this implementation is available for bored piles, not driven piles.

For driven piles, the commonly adopted approaches are:

• Nordlund 1963 and 1979, which form the basis of the FHWA method for cohesionless soils

• Alpha method, as adopted by FHWA, for cohesive soils

An important point that is often overlooked is that CFEM allows the use of Alpha and Beta methods for both bored and driven piles, provided the assumptions are justified.

From a practical standpoint:

• If you are practicing in Canada, RS Pile is a strong option for axial pile analysis, either as a primary design tool or to verify in house spreadsheets.

• If you are practicing in the United States, Ensoft software is more commonly used, with SHAFT for bored piles and APile for driven piles. That said, RS Pile also includes AASHTO based approaches.

This episode is about understanding why the numbers change and making sure the method you use aligns with the governing code and the design intent.