Which of the zones requires that material be placed in layers not to exceed 12 inches in compacted thickness?

• A. Bottom Zone
• B. Cover Zone
• C. Top Zone
• D. Bedding Zone

Answer: (C)

The lift thickness rule is about how you compact backfill in stages. The material in the top zone is closest to the finished surface and will bear the most load from traffic and weather. Limiting each compacted layer to not more than 12 inches in this zone ensures the compactor can densify the entire lift, reducing voids and uneven density that lead to settlement or surface cracking. This controlled thickness helps achieve a uniform, stable finished surface.

Bedding and bottom zones have different roles (support under the pipe and good contact with the subgrade), so their lift requirements are governed by those needs. The top zone is the one where keeping lifts under 12 inches matters most for surface performance.