When trench bottom is below the installation level of the pipe, the backfill must be composed of what material placed in layers?

• A. Sand or fine gravel
• B. Crushed rock
• C. Topsoil
• D. Concrete

Answer: (A)

When the trench bottom is below where the pipe sits, the backfill needs a soft, uniform bedding that can be layered and compacted without harming the pipe. Sand or fine gravel provides that cushion around the pipe, fills voids, and compacts smoothly in layers to prevent hollows or point loading from sharp rocks. This layered, gentle bedding helps protect the pipe as the surrounding soil settles and allows proper drainage away from the pipe. Concrete would overly stiffen the surround and hinder drainage, topsoil would settle unevenly and offer poor support, and crushed rock can have sharp edges and doesn’t compact as predictably in layered installations. So, sand or fine gravel placed in layers is the appropriate backfill material.