When the bottom of a trench does not form the bed for the pipe and is below the installation level, backfill with sand or fine gravel in layers of maximum depth how many inches?

• A. 6 inches
• B. 4 inches
• C. 8 inches
• D. 12 inches

Answer: (A)

When the trench bottom doesn’t provide a proper pipe bed and sits below the planned installation level, you raise the pipe bed by backfilling with sand or fine gravel in shallow lifts. Each layer should be no more than six inches thick. This approach gives a smooth, uniform support around the pipe, minimizes voids, and reduces the risk of point loads that could damage or misalign the pipe during final grading and backfill. Layered filling also makes it easier to compact the material evenly and to bring the bed up to the correct installation level without creating gaps or settlement. If layers were thicker than six inches, voids or uneven compaction could occur, potentially leading to movement or damage once the trench is buried.