Which thickness is allowed for a layer when FDP is used?

• A. 2 inches.
• B. 4 inches.
• C. 3 inches.
• D. 1 inch.

Answer: (C)

Thicker lifts aren’t allowed because the FDP process relies on a uniform, thoroughly mixed mixture produced in a limited depth. Keeping the layer to three inches per pass ensures the pulverized material, binder, and any additives are evenly blended, with consistent moisture and aggregate distribution, so the mix can be properly compacted and achieve the desired strength. If you went deeper than three inches, blending and compaction become unreliable and the layer may have weak spots. If you go much shallower, you don’t gain the intended structural improvement and you’d need additional passes. Three inches is the balance that gives a stable, workable base.