Trench Safety and Utility Trenching

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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The rules and habits that decide when you can step into a trench, and how the walls are kept up.

The big idea

Trench safety is a worker-safety matter set by your provincial or territorial OHS regulator (WorkSafeBC in BC), not by NBC 2020 Part 9. So trigger depths, spoil distances and engineer thresholds differ by province. This page uses BC and Alberta as the worked examples, because their regulations were read for this page, and the By Province box shows what the others say.

Most trench deaths happen in trenches that were "only a few feet deep". Everything below is about keeping the walls where they belong and keeping people out until they are safe.

When the rules kick in

• BC: a trench is an excavation less than 3.7 m (12 ft) wide at the bottom, over 1.2 m (4 ft) deep, and of any length (OHS Reg. s. 20.1). Workers may not enter a trench deeper than 1.2 m unless the sides are sloped or supported as Part 20 requires (s. 20.81).
• Alberta: protection is required before entry into a trench over 1.5 m (about 5 ft) deep unless it is cut back (OHS Code Part 32; the regulator's explanation guide cites s. 446 and s. 450).
• Other provinces: 1.2 m (4 ft) in SK, ON, NB, NS, NL, PE and YT; 1.5 m in AB and MB. See the By Province box.
• Under the trigger depth the hole still has to be safe. Soft, wet, flooded or loaded ground can fail at any depth.

When you need an engineer

Some excavations need written instructions from a qualified registered professional, usually a geotechnical or civil engineer. In BC (OHS Reg. s. 20.78) that includes an excavation:

• deeper than 6 m (20 ft)
• next to a building or other structure
• subject to vibration or hydrostatic (water) pressure that could cause hazardous movement
• where the ground slopes away from the edge steeper than 3 horizontal to 1 vertical

In BC, shoring that is a support structure needs an engineer, and end shoring with walers longer than 1.8 m (6 ft) needs engineer design (s. 20.78 and 20.81(4)). Yukon has a similar list. Alberta requires a professional engineer to design and certify temporary protective structures for excavations deeper than 3 m (10 ft) (s. 456).

Note the second item: digging beside an existing house, garage or retaining wall puts you in engineer territory even in a shallow trench. That catches a lot of renovation crews out.

Call before you dig

• Locate underground utilities before you excavate. Start with your province's one-call centre.
• Lead times: BC 1 Call, 3 full business days, 1-800-474-6886. Alberta One-Call, 3 full business days, 1-800-242-3447. Manitoba, 1-800-940-3447, contact within 3 business days. Quebec, Info-Excavation, 4 working days. Saskatchewan, 2 working days per SaskEnergy.
• Private lines on the property are not covered by a locate request.
• Never probe for buried gas or electrical lines with pointed tools. Hand-dig or vacuum-excavate within the utility owner's required distance.
• Remove or secure trees, poles and boulders next to the dig if they could endanger workers.

Soil under the trench rules

Alberta's code classifies trench soil into three types (s. 442): hard and compact, likely to crack or crumble, and soft, sandy or loose. Ontario uses four types, Type 1 to Type 4. Whatever the system, the soil is described by how it behaves, not by what it is called.

• Firmest: hard and solid, holds its shape
• Middle: likely to crack or crumble
• Weakest: soft, sandy, filled or loose

Wet, disturbed, previously dug or vibrated ground is weaker than it looks. Treat fill and backfilled ground as the weakest class. Soft or saturated clay can be as bad as loose sand. When in doubt, treat the soil as the weakest class, or get a competent person to assess it.

Your options: slope, bench, shore or box

Before anyone goes in, the sides must be sloped, benched or supported. Common ways:

• Sloped as a qualified professional specifies, or at the angles your regulation allows for the soil.
• Benched as your regulation shows.
• Shored to the minimum requirements in your regulation's tables, or to an engineer's design.
• Protected by a manufactured trench box or shoring cage, used within the maker's limits.

Choosing and building any of these is a job for a qualified person or an engineer, not for a homeowner. Install shoring from the top down and remove it in reverse order.

Getting in and out

Workers need a safe way in and out of every trench, reachable quickly from where they are working, and a ladder must extend above the edge so you can step off.

• Alberta: in a trench over 1.5 m (5 ft) deep, the exit may be no more than 8 m (26 ft) from the worker (s. 455).
• NB and NS: ladder at least 1 m (3 ft 3 in) above the edge and within 15 m (50 ft) of the worker. PE: at least 3 ft (914 mm) above and no more than 50 ft (15 m) from workers. NL: at least 0.91 m (3 ft) above.
• Keep the ladder clear of debris, tools and hoses.

Worked example (Alberta). A trench 1.8 m (6 ft) deep and 40 m (131 ft) long. A ladder serves 8 m either side, so one ladder covers 16 m (52 ft). 40 ÷ 16 = 2.5, so you need at least 3 ladders, spaced so no one is more than 8 m from one.

Why trenches collapse

• Water softens walls and builds pressure behind them
• Spoil piles, stored materials and equipment load the edge. Alberta requires spoil at least 1 m (3 ft 3 in) from the edge, and the pile slope no steeper than 45 degrees (s. 453).
• Vibration from machines, traffic and compactors shakes loose soil down
• Frost leaves hidden cracks that open as the ground thaws
• Digging next to older trenches, utilities or fill (soil already disturbed)
• Undercutting the wall by over-excavating at the bottom
• Soil changing partway down without anyone noticing

Daily checks

• Check the trench before the first person goes in, every shift
• Check again after rain, thaw or heavy frost
• Check after anything vibrates the ground or loads the edge
• Check when the soil changes or water starts to seep

Cracks parallel to the trench, sloughing, bulging walls or new seepage mean everyone gets out and nobody goes back in until the problem is fixed. Alberta also requires that workers not enter until hazardous accumulations of water are eliminated (s. 445). Keep a short written log of what you checked and when. It is cheap protection if anyone ever asks.

Utility trenching

Depth of cover, separation between services and bedding for water, sewer, gas, electrical and communications lines are set by the utility owner and your municipality, not by one national number. Cover depth depends on local frost depth, traffic loads and pipe material.

• Get the servicing requirements from the municipality or utility before you cut the trench.
• Plumbing that serves the building follows your provincial plumbing code and the municipality's requirements.
• Electrical service depths come from the Canadian Electrical Code as adopted in your province, and the utility. Gas work needs a certified gas contractor.
• Bed pipe as the manufacturer and the municipality specify. Keep sharp rock out of the pipe zone.
• Backfill in lifts, free of organics, and protect the pipe zone while compacting (see Backfill and Lift Thickness).

Notes from the trade