In the Cover Zone, what density is required when the top of the pipe is 15 inches below the bottom of the base?

• A. 100% of AASHTO T-99
• B. 95% of AASHTO T-99
• C. 80% of AASHTO T-99
• D. 90% of AASHTO T-99

Answer: (A)

In the area immediately around buried pipes, the fill must be compacted to the maximum density specified by AASHTO T-99 to provide a stiff, uniform foundation for the overlying pavement. A top-of-pipe depth of 15 inches places the material squarely in the Cover Zone, where the requirement is full 100% relative compaction of the AASHTO T-99 maximum dry density. Achieving this density helps prevent differential settlement and pipe movement under traffic loads by ensuring the surrounding soil is as dense as the lab-determined maximum. Lesser percentages are used in other zones or circumstances, but not in this Cover Zone condition.