The requirements below come from Ontario Regulation 213/91 (Construction Projects) under the Occupational Health and Safety Act (OHSA), specifically Part III.1 covering Excavations, sections 222 through 242. These are the rules that apply every time a shovel or backhoe breaks ground on an Ontario construction project where workers may enter the excavation.
A cubic metre of soil weighs between 1,100 and 1,800 kg depending on type and moisture content. At 1.2 metres deep, a trench wall collapse buries a worker from the waist down in soil weighing more than a small car. The force is enough to cause crush injuries to the pelvis, legs, and internal organs within seconds, and asphyxiation within minutes as the chest compresses under the weight. There is no time to dig someone out by hand. This is not hypothetical. Between 2010 and 2024, Ontario recorded multiple excavation fatalities, most in trenches between 1.5 and 3 metres deep. The 1.2-metre threshold in O. Reg. 213/91 section 222 exists because the physics of soil collapse become lethal at that depth. Below 1.2 metres, a worker can typically climb out unassisted. Above it, they cannot. The regulation makes the protection trigger at the point where self-rescue becomes unlikely. Section 222(1) states that no worker shall enter an excavation unless the walls are sloped to the required angle, shored, or protected by a trench box. The key phrase is "where a worker may enter" — if nobody enters the excavation (machine-dug utility trench, for example), the shoring requirements under s.222 do not apply, though barriers around the opening under s.223 still do.
Not all soil behaves the same way when you cut into it. O. Reg. 213/91 does not explicitly name four numbered types the way some jurisdictions do, but the standard industry practice in Ontario and the classification system used by competent persons on site recognizes four categories that determine the safe angle of repose. Type 1 soil is hard, cemented material — shale, sandstone, hardpan, cemented sand and gravel. It holds a near-vertical face (75 degrees, or 1:0.25 horizontal to vertical) because the particles are bonded together. Type 2 is firm cohesive soil — stiff clay, silty clay, glacial till. It holds a 45-degree face (1:1) because the clay particles stick together, but a sustained rain or freeze-thaw cycle can turn Type 2 into Type 4 overnight. Type 3 is soft or granular — sand, gravel, loamy sand, uncompacted fill. These particles have no cohesion and will slump to their natural angle of repose, about 34 degrees (1:1.5). If you have ever seen a sand pile at a gravel pit, that slope is roughly 34 degrees — that is physics, not regulation. Type 4 is the most dangerous classification: previously disturbed soil (backfill) or soil that is saturated with water. The slope is 18.5 degrees (1:3), meaning for every metre of depth you need three metres of horizontal cut-back on each side. A 3-metre deep trench in Type 4 soil needs to be 18 metres wide at the top if you are relying on sloping alone. That is why Type 4 conditions almost always require shoring or a trench box instead of open-cut sloping — the land area required for sloping is impractical on most sites. The critical rule that experienced excavation workers know: soil type can change within the same trench. You might start in firm clay (Type 2) and hit a pocket of sand (Type 3) or an old backfilled utility trench (Type 4) without warning. A competent person must be present to reclassify soil as conditions change. Treating the entire trench as the weakest soil type encountered is the safest approach.
Section 222 gives three options for protecting workers in an excavation. Sloping (or benching) means cutting the walls back to the required angle for the soil type. It is the simplest method — no equipment to install, no engineering calculations — but it requires the most space. A 2-metre deep trench in Type 3 soil (34 degrees, 1:1.5) needs 3 metres of horizontal cut-back on each side, making the trench 6 metres wider at the top than at the bottom, plus the trench width itself. In a residential lot with 1.5 metres of sideyard clearance, sloping is physically impossible. Shoring is a structural system that holds back the excavation walls. Timber shoring uses walers, struts, and sheet piling. Hydraulic shoring uses aluminum or steel frames with hydraulic cylinders that press against the trench walls. Shoring prevents the wall from moving — it actively supports the soil. For excavations between 1.2 and 6 metres, a competent person designs the shoring. Over 6 metres, a Professional Engineer must design it under s.228(2). A trench box (trench shield) is a prefabricated steel or aluminum structure that sits inside the trench. Unlike shoring, a trench box does not prevent cave-in — it protects the workers inside the box if a cave-in occurs. The box must be rated for the depth of the excavation and the soil type. Workers are only protected while inside the box; the area outside the box is unprotected. On most residential and light commercial construction sites in Ontario, hydraulic shoring or a trench box is the standard method because lot sizes do not allow the sloping option. The constructor rents the equipment, a competent worker installs it per the manufacturer's instructions, and work proceeds inside the protected zone.
At 6 metres (approximately 20 feet), O. Reg. 213/91 section 228(2) requires a Professional Engineer to design the excavation support system. This is not optional. At 6 metres, the soil pressure on the trench walls exceeds the capacity of standard off-the-shelf shoring and trench box equipment. The forces involved are enormous: the lateral earth pressure on a 6-metre trench wall in saturated clay can exceed 50 kN per metre of wall length. A miscalculation results in a catastrophic wall failure that moves thousands of kilograms of soil in seconds. The PE must produce signed and sealed engineering drawings specifying: the type of support system, member sizes and spacing, installation sequence, maximum allowable depth, and soil conditions assumed in the design. These drawings must be on site at all times while the excavation is open. The constructor cannot modify the PE design without the engineer's written approval. If an inspector finds a 6-metre-plus excavation without PE drawings on site, expect a stop-work order immediately. The same PE requirement applies to shaft excavations (depth exceeds width) and tunnel excavations regardless of depth, because these geometries create stress concentrations that standard shoring tables do not address.
Section 226 requires a competent person to inspect excavations daily before work begins and after every rainfall, thaw, or change in conditions. "Competent person" under OHSA means someone qualified by knowledge, training, and experience to do the work, and familiar with the Act and regulations. For excavation inspection, this means someone who can identify soil types, recognize signs of wall instability (tension cracks, bulging, sloughing, water seepage), verify shoring is properly installed and undamaged, and make the call to evacuate if conditions deteriorate. The inspection is not a checkbox exercise. The competent person walks the excavation, looks at every wall face, checks the shoring for any displacement, looks for water accumulation at the base, checks the spoil pile distance from the edge (minimum 1 metre per s.225), verifies that no heavy equipment has moved closer to the edge than the shoring design allows, and confirms that ladder access is still in place (every 15 metres of travel distance per s.224). After a heavy rain, the inspection must be repeated before any worker re-enters. Rain saturates the soil, increases its weight, reduces cohesion in clay, and can turn a stable Type 2 trench wall into a collapsing Type 4 failure surface in hours. Freeze-thaw cycles have the same effect — frozen soil appears stable, but as it thaws, it loses all cohesion and can slough off in blocks. Experienced excavation supervisors know that Monday morning is the most dangerous time — a trench that was stable on Friday may have deteriorated over the weekend due to rain, temperature changes, or vibration from weekend traffic.
Section 228 requires locating all underground utilities before any excavation begins. In Ontario, this means contacting Ontario One Call (1-800-400-2255 or online at ontarioonecall.ca) at least 5 business days before digging. Ontario One Call notifies the utility owners (gas, electric, water, telecom, sewer), who then send locators to mark the approximate position of their buried infrastructure with paint or flags. The locates are approximate — the regulation and industry practice require hand-digging within 1 metre of any marked utility. No machine digging within the hand-dig zone. The consequences of hitting an underground utility range from inconvenient (cutting a cable TV line) to fatal (puncturing a high-pressure natural gas main, striking a live electrical conductor). A natural gas leak in a confined trench creates an explosive atmosphere in seconds. An electrical contact at distribution voltages (typically 14.4 kV to 44 kV in Ontario) is instantly fatal. Even striking a water main can flood the trench fast enough to drown a worker in a matter of minutes. The locate marks fade over time and can be disturbed by equipment. If the excavation work extends beyond 30 days, the locates must be refreshed. If marks are missing or unclear, stop work and call for re-location. The cost of a re-locate is zero. The cost of a gas main strike or electrocution is incalculable.