Episode 6 – Mechanically Stabilized Earth Walls: Select Granular Reinforced Fill Requirements

The reinforced soil zone is the heart of every MSE wall, and the choice of backfill material is one of the biggest factors in long-term performance. The des

Here’s why:

• If the wall design assumes a high friction angle (say 40°), then the selected fill must be capable of mobilizing that strength. That means a well-graded, coarse material compacted to about 98–100% of Standard Proctor MDD, and ideally verified with direct shear testing.

• Using weaker fill than assumed in design is one of the fastest ways to compromise wall stability.

 What do the guidelines say?

• FHWA NHI 10-024 suggests a typical gradation envelope (0–15% fines, PI < 6, durable particles).

• Materials must also pass durability checks such as sulfate soundness tests to ensure long-term performance.

 Electrochemical requirements matter too:

• For steel reinforcement, resistivity, pH, chlorides, sulfates, and organic content all need to be within limits to control corrosion.

• For geosynthetics, pH is the key parameter (with specific ranges depending on polymer type).

 In short, backfill is not “just fill.” It is a structural component of the wall, and its properties must match the assumptions made in design.