Trench Backfill and Compaction

The layer-by-layer backfill procedure, material sizes and compaction targets that keep an installed duct in position.

A service trench is dug 300–450 mm wider than the outside diameter of the pipe or duct so that 75 mm of granular surround can be placed on every side. That surround is the duct's first line of protection against point loads from the soil above.

The first 300 mm of backfill above the duct is placed by hand, using sand or fine granular material with a maximum particle size of 40 mm. Only above that layer may excavated soil be returned to the trench.

Backfill is compacted in loose layers of 200–300 mm, each layer compacted before the next is placed. Most specifications require at least 95% of the Proctor maximum dry density within 450 mm of finished level — the zone that carries traffic.

Where extra protection is needed, a multi-duct bank is typically wrapped in 100 mm of concrete or bedded on 50 mm of blinding sand, with a minimum of 75 mm of cover between the duct and the concrete.

The test of a good backfill is time: a trench compacted to specification survives traffic and frost without the tell-tale settlement line along the surface, while a poorly compacted one announces itself within the first winter.

• 75 mm granular surround on every side of the duct.
• First 300 mm above the duct: hand-placed, particles no larger than 40 mm.
• Lifts of 200–300 mm, compacted to at least 95% Proctor within 450 mm of finished level.
• Concrete wrap of 100 mm, or 50 mm blinding sand, for multi-duct banks.

Further reading

• Conduit Burial Depths by Location and Ground Type
• Utility Locates and the Hand-Dig Zone