What material should I use for trench backfill?
Use what the project specification and trench detail require. Pipe installation guides split the trench into zones: bedding, haunching and initial backfill around the pipe, typically crushed stone or clean sand and gravel, and final backfill above it, any material the spec and road requirements accept that is free of large rocks, organic material and debris.
Trench backfill zones from bottom to top
- Foundation: only where the trench bottom is not a firm platform.
- Bedding: brings the bottom to grade and supports the pipe evenly.
- Haunching: fill under the lower half of the pipe, which carries load.
- Initial backfill: around the pipe and over its crown, to the depth the detail shows.
- Final backfill: the rest of the trench to subgrade or the surface.
Trench backfill material to keep out of the pipe zone
The Plastics Pipe Institute says Class IVB and Class V soils offer hardly any support and are normally not recommended as embedment. Measure the embedment zone with the pipe bedding calculator, then the fill above it in the trench backfill calculator with pipe diameter set to zero so the pipe is not deducted twice.
Sources
Checked 2026-11-02.