🚧 Civil · 6 min read
Trench Excavation, Bedding, and Backfill Takeoff by Pipe Size
Trenching is a per-metre calculation once you fix the trench cross-section for each pipe size and depth band.
Trench cross-section
Trench width W = pipe OD + 2 × working clearance (150–300 mm each side)
Trench depth D = cover + pipe OD + bedding thickness
Trench depth D = cover + pipe OD + bedding thickness
Minimum cover is typically 600 mm under landscape, 900 mm–1.2 m under roads. Add side slope or shoring below the safe vertical depth (~1.2 m in most soils).
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Volumes per metre of trench
| Item | Per metre |
|---|---|
| Excavation | W × D (+ slope wedge if battered) |
| Bedding | W × bedding thickness (100–150 mm) |
| Pipe surround | W × (OD + 150 mm) − pipe cross-section |
| Selected backfill | W × (300 mm above pipe) |
| Common backfill | excavation − (bedding + surround + selected + pipe) |
Spoil balance
Surplus spoil (loose) = [excavation − total backfill (bank equiv.)] × (1 + swell%)
Granular bedding and surround are usually imported, which displaces an equal bank volume of site spoil to disposal.
Extras that belong in the BOQ
- Dewatering where the trench is below the water table
- Trench shoring / trench box hire by length and depth band
- Marker tape and warning slabs
- Reinstatement — road pavement or topsoil and turf over the trench line
🚧 Using EngEst Pro
EngEst Pro’s Civil Calculator takes off trench works by pipe size and depth — excavation, bedding, surround, backfill, compaction, and surplus spoil — and links reinstatement quantities back into the BOQ.
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