Trench width sets how much you dig, backfill and cart away on every metre of pipe. For stormwater, the minimum width comes from the pipe size, and the standard sets it. AS/NZS 2566.2:2002 gives it in clause 4.4.1 and Table 4.2.
The minimum trench width
For a pipe with an outside diameter from 300 to 450 mm, the table gives 100 mm of bedding, 200 mm of side clearance on each side and 150 mm of overlay. That makes a trench 700 to 850 mm wide, measured at the springline. A 375 mm pipe needs a trench about 775 mm wide before any allowance for joints, valves or working space.
What it does to the price
- Width times depth is the digging. Each extra 100 mm of width at 1.5 m deep adds about 0.15 m3 of excavation per metre of pipe.
- Check the width on the drawings. Is it the minimum or a design width? The difference is excavation, backfill and disposal on every metre.
- Depth drives the shoring and dewatering. This is our pricing practice rather than a figure the standard sets, so we price it as its own line.
Where the width comes from
Clause 4.4.1 notes that local widening may be needed at joints, valves and thrust blocks, so price those spots separately. For stormwater, AS/NZS 3500.3 clause 6.2.3 points the trench width to AS/NZS 2041, AS/NZS 2566.2, AS/NZS 3725 and AS 4060, depending on the pipe. Confirm which one governs your job before you price.
We price trenches layer by layer from the long sections and the pipe schedule. Send your RFQ with the drawings and we usually reply the same business day.
Common questions
What is the minimum trench width for a 375 mm stormwater pipe?
About 775 mm. Table 4.2 of AS/NZS 2566.2 puts a 300 to 450 mm pipe in a 700 to 850 mm band, before any allowance for joints, valves or working space.
Does a deeper trench always cost more?
Usually, yes. Excavation, shoring and dewatering all grow with depth, so we price each run in its depth band rather than averaging the whole line.
Who sets the trench width on my job?
The design, the pipe standard and the authority that governs the work. We check the drawings against AS/NZS 2566.2 and flag any difference before we price.