The rules below come from Ontario Regulation 213/91 (Construction Projects) under the Occupational Health and Safety Act, Part III — Excavations, sections 222 through 242. They apply every time a shovel or backhoe breaks ground on an Ontario construction project where a worker may enter the excavation. Numbers here are verified against the O. Reg. 213/91 Part III text; confirm the current consolidation at ontario.ca/laws before relying on it.
A cubic metre of soil weighs roughly 1,100 to 1,800 kg depending on type and moisture. At 1.2 metres (4 ft) deep, a wall collapse buries a worker from the waist down under more weight than a small car — enough to crush the pelvis, legs and internal organs in seconds, and to suffocate as the chest can no longer expand. You cannot dig someone out by hand in time. Below 1.2 m a worker can usually climb out unassisted; above it they cannot, which is why the regulation sets the line there. According to O. Reg. 213/91 s. 234(2)(a), a support system is required whenever a worker may enter an excavation 1.2 m (4 ft) or deeper. The trigger turns on the phrase "a worker may enter" — a machine-dug utility trench nobody enters does not need a support system, but you must still keep the edge clear (s. 233(1)) and, over 2.4 m, guard against falls (s. 233(4)).
O. Reg. 213/91 s. 226 sets out four soil types, and s. 234(2) sets the maximum slope for each. Type 1 is hard, very dense soil with no water seepage that only comes out with machinery — hardpan, consolidated clay, some glacial tills. Type 2 is very stiff, dense soil that a pick enters with moderate effort — silty clay, less-dense tills. Type 1 and Type 2 walls may be sloped to within 1.2 m (4 ft) of the bottom at a gradient of 1:1 (45°) — one metre of cut-back for every metre of height — with the bottom 1.2 m left vertical (s. 234(2)(e)). Type 3 is stiff-to-firm or compact-to-loose soil, or any previously-excavated (backfilled) soil, showing surface cracking and seepage; it must be sloped 1:1 (45°) from the bottom — no vertical portion (s. 234(2)(f)). Type 4 is soft, very loose, wet soil that runs or flows and exerts fluid pressure — muskeg, quicksand, saturated silty clay; it must be sloped 1:3 (18.4°) from the bottom, three metres of cut-back for every metre of height (s. 234(2)(g)). A 3 m deep cut in Type 4 needs 9 m of cut-back on each side, which is why Type 4 almost always means a support system instead of open-cut sloping. Soil can change within one trench — firm clay giving way to a pocket of sand or an old backfilled service run — so where more than one type is present, s. 227(3) requires you to classify to the highest (weakest) number present.
Section 234 gives three ways to protect a worker in an excavation. Sloping (or benching) cuts the walls back to the soil's required gradient — the simplest method, but the hungriest for space. A 2 m deep cut in Type 3 (1:1) needs 2 m of cut-back on each side; on a residential lot with 1.5 m of side-yard, that is impossible. Shoring actively holds the walls back: timber shoring uses sheathing, wales and struts (the prescribed member sizes live in the Table to s. 238, good to 6 m depth); hydraulic shoring uses aluminum or steel rams that pre-load the walls and can be installed from the top without anyone entering the trench. Every prefabricated, hydraulic or engineered support system must be designed by a professional engineer, with the drawings kept on site (s. 236). A trench box (trench shield) does not hold the walls back — it protects whoever is inside it if the wall lets go. It must be rated for the depth and soil, and workers are only protected while inside the box. The support system must extend at least 0.3 m (about 12 in) above the top of the excavation (s. 241(1)), and a ladder for access and egress must sit within the protected area (s. 240).
Under O. Reg. 213/91 s. 235(2), a trench deeper than 6 m (about 20 ft) or wider than 3.6 m must use an engineered support system designed for the specific location and project. At that depth the lateral earth pressure outruns off-the-shelf shoring and trench-box ratings, and a miscalculation moves thousands of kilograms of soil in seconds. The engineer's signed and sealed drawings must specify the system type, member sizes and spacing, installation sequence, maximum depth and the soil conditions assumed — and they must be on site while the system is in place (s. 236(7)). Installing the system in a way that departs from those drawings is an OHSA offence (s. 236(2)); a variation has to be approved in writing by an engineer (s. 236(5)). Separately, where an excavation may affect an adjacent building or structure, a professional engineer must specify the required precautions in writing (s. 229). Note that shafts and tunnels are not Part III excavations at all — they fall under Part IV (Tunnels, Shafts, Caissons and Cofferdams, ss. 243-331), which adds a Notice of Project, ventilation, atmospheric monitoring and rescue requirements.
O. Reg. 213/91 does not set a fixed general inspection interval — a point often misstated. What it does say: no worker may enter an excavation that does not comply with Part III (s. 224), and where a support system relies on a professional engineer's written opinion, the engineer or a competent worker must inspect it as often as that opinion specifies (s. 234(3), (5)). In practice, IHSA guidance — and good sense — is to have a competent worker inspect before each shift and after any event that changes the ground: rainfall, freeze-thaw, blasting or traffic vibration, or a surcharge load placed at the edge. Rain saturates clay, adds weight and strips cohesion, turning a stable Type 2 wall into a Type 4 failure surface within hours; frozen soil looks solid but sloughs off in blocks as it thaws. Monday morning after a wet weekend is the classic danger window. The competent worker checks every wall face for tension cracks, bulging and seepage, confirms the support system is undamaged, and keeps the 1 m (3 ft 3 in) edge clear zone (s. 233(1)) and equipment back from the wall (s. 233(3)).
O. Reg. 213/91 s. 228 requires all gas, electrical and other services in and near the dig to be located and marked before excavation begins, and any service that may pose a hazard to be shut off and disconnected. In Ontario that means contacting Ontario One Call (1-800-400-2255 or ontarioonecall.ca) before you dig; the utility owners then mark the approximate centre line of their buried plant with paint or flags. IHSA treats locate accuracy as 1 m either side of the mark, so hand-dig within 1 m of any marked utility — no machine in the hand-dig zone — and support exposed pipes and cables so they cannot fail (s. 228(3)). The stakes are absolute: a punctured high-pressure gas main fills a trench with an explosive atmosphere in seconds, distribution-voltage electrical contact (14.4 kV to 44 kV across much of Ontario) is instantly fatal, and a broken watermain can flood a trench fast enough to drown a worker. If marks are missing, faded or disturbed, stop and call for a re-locate — it costs nothing.
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O. Reg. 213/91 s. 234 requires a support system — sloping, shoring or a trench box — wherever a worker may enter an excavation 1.2 m (4 ft) or deeper (s. 234(2)(a)); below 1.2 m a support system is not required unless a hazard exists.
By the ToolFluency team · Updated September 2026
Free Ontario excavation safety checker. Enter depth, soil type, and site conditions to determine sloping angles, shoring needs, trench box requirements, and inspection checklists under O. Reg. 213/91 ss. 222-242. Key thresholds: 1.2m (4 ft) trigger, 2.4m, 6m (engineer required).
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