Methods for Backfilling Around Vaults

Excavation Envelope

• .The excavation envelope refers to the amount of space left around the vault when workers place the vault in the excavated hole. To provide proper support to the vault and prevent it from collapsing when in use, the backfill must have an envelope 500mm or 19.68 inches wide. However, the envelope also must be wide enough to accommodate the compaction machine to be used. If the envelope must exceed 500mm, non-structural class you can use backfill in the area outside the 500mm width to reduce costs.

Backfill type

• The only type of backfill not recommended for use around a concrete utility vault is flow-fill. Flow-fill is a type of concrete mix that will not allow the vault and connecting pipes to settle easily and can cause both to float. Floating means a space has been created beneath the concrete due to the ground settling and underground water drainage. This can cause the vault to go out of drainage slope and disrupt the flow in the pipes. Class 1 or 2 backfill is recommended. Both are soil-based and allow for proper settling. The site plans or building code will define which class is acceptable.

Lifts

• Backfill is placed in lifts measuring 300mm or 11.8 inches deep. A lift is a layer of backfill. Measure its depth after compaction, not when someone first places the material. Backfilling in lifts allows for a more thorough compaction around the entire vault, as every lift can be brought to the defined compaction standard.

Compaction

• The standard of compaction is defined by the project engineers according to the local building code. If the project has no engineer, such as is common with home projects, the local building code society can provide the standard. Use a hand tamp or tamping machine for tamping on dampened backfill material. It condenses the material to lessen the amount it will settle and to provide support for the vault. Temperature, weather and the class of backfill will affect the amount of time it will take to get a passing rate on a compaction test.