Episode 3 – Mechanically Stabilized Earth Walls: What Friction Angle Should You Use for the Reinforced Backfill?

A common question in MSE wall design is:

What friction angle should we assume for the reinforced backfill?

Technically, you can specify any friction angle you like, as long as the on-site material can actually meet it.

But in practice, design codes offer limits based on experience and performance expectations.

 Here’s what major references say:

• AASHTO LRFD 2020 and FHWA NHI-10-024 recommend assuming a friction angle of 34° for design if the material will not be lab-tested, provided it meets gradation requirements.

• The Canadian Highway Bridge Design Code (CSA S6-19) allows the use of 35° as a default design value for well-graded granular backfill.

Now, here’s a question I hear often: Can I use a friction angle of 45°?

From a lab testing perspective, yes, it’s possible to get that value for very angular crushed rock with minimal fines.

But should we use it in design?

No. All three codes—AASHTO, FHWA, and CSA—cap the design friction angle at 40° for MSE walls.

 Why the cap?
It’s not just about conservatism. It’s also about:

• Variability in field compaction
• Long-term performance under drainage and load
• Compatibility between design assumptions and construction tolerances