Backfill

After a hydrovac excavation is completed and the exposed utility has been inspected, documented, or repaired, the hole must be backfilled and the surface restored. The type of backfill material used depends on the project specifications, the type of utility exposed, and the surface conditions. Common backfill materials include native soil, imported granular fill, sand, and flowable fill (also called controlled low-strength material or CLSM).

Flowable fill is often the preferred backfill for pothole and bell hole excavations because it self-levels, requires no compaction, and provides consistent support around the utility. However, it is more expensive than granular fill or native soil. Many specifications prohibit using the excavated slurry as backfill because the water content makes it impossible to compact to required density specifications.

Backfill must be placed in lifts and compacted to the specified density, typically 90 to 95 percent of maximum dry density as determined by a Proctor test. Failure to properly compact backfill results in settlement, which can crack pavement, damage utilities, and create safety hazards. Some specifications require compaction testing by an independent testing laboratory.