Which Proctor test is typically used for base, subgrade, and MSE wall backfill materials?

• A. Standard
• B. Modified
• C. Heavy-Duty
• D. Quick

Answer: (B)

The test used to determine how dense a soil can become at a given moisture content comes in two common forms, and the one with higher compaction energy is chosen for these materials. The Modified Proctor uses a heavier hammer and more energy, which pushes the soil to a higher maximum dry density and changes the moisture content at which that density occurs.

Base and subgrade layers and the backfill behind a mechanically stabilized earth (MSE) wall must achieve high density to support loads, minimize settlement, and improve stability against lateral pressures. Because you’re aiming for a denser, more tightly packed fill in these situations, the Modified Proctor provides the most appropriate moisture-density relationship and compaction target. That higher energy compaction mirrors what field crews apply with heavy equipment, ensuring the lab results match what will be achieved in practice, which is why this version is typically specified for these materials.