Trenches that are overexcavated and require backfilling below the level of the pipe shall be backfilled in layers not exceeding how many inches?

• A. 12 inches
• B. 18 inches
• C. 6 inches
• D. 3 inches

Answer: (C)

When backfilling trenches that are overexcavated and go below the pipe level, you need to control compaction very carefully around the pipe. Using small lifts—no more than six inches—lets you compact each layer thoroughly and evenly, reducing the risk of voids, pipe movement, or deformation as loads are applied. Larger lifts beneath the pipe, like twelve or eighteen inches, are harder to compact properly in that critical zone and can trap voids or create uneven support. A much smaller lift, such as three inches, would be unnecessarily slow and doesn't provide extra benefit for typical soils. So, the layer thickness should be six inches.