Excavation kills more bluntly than almost any other site hazard. A cubic metre of soil weighs over a tonne, a collapse happens in seconds with no warning, and a buried worker cannot dig themselves out. Two rules keep people alive: locate the underground services before the bucket touches the ground, and protect the walls of any trench a worker enters by sloping, shoring or shielding. Skip either one and you are gambling with a death, your own or a worker's, plus a struck gas or power line.
The mandatory locate: call before you dig
Before you break ground, you must find out what is buried beneath you: gas, electrical, water, sewer, telecom. Hitting a gas main or a live cable can kill instantly and cost a fortune. The locate is requested through your province's one-call service, and it is free.
In Ontario the locate runs through Ontario One Call, the single point of contact created under the Ontario Underground Infrastructure Notification System Act. By law you must request a locate before excavating, and you must wait for it to come back before you dig; starting without it is an offence as well as a hazard. Other provinces have their own one-call services (often under the "Click Before You Dig" or provincial damage-prevention banner). Request the locate well ahead of your start date, keep the locate documents on site, hand-dig (daylight) carefully around marked services, and never assume a service is where a drawing says it is.
When a protective system becomes mandatory
Once a trench reaches a defined depth, you may not enter it unless the walls are made safe, by sloping them back to a safe angle, shoring them with a braced system, or shielding workers with a trench box. The trigger depth is set per province, commonly around 1.2 metres (4 feet). British Columbia requires a protective system for any excavation over 1.2 metres unless it is cut entirely in stable rock. Ontario (O. Reg. 213/91) requires shoring, sloping or benching below 1.2 metres. Alberta sets protective measures based on soil conditions and depth, with egress requirements once a trench reaches 1.5 metres. Quebec requires excavations to be shored under the Safety Code, with the CNESST enforcing. Confirm the exact trigger and method for your province before anyone climbs in.
A competent person must classify the soil before the work, because the right slope angle and shoring depend on whether the ground is hard, firm or soft and wet. Previously dug or backfilled ground, and ground near water, is far less stable than it looks.
The supporting rules that prevent the collapse
The depth trigger is the headline, but several practical rules do the real work of keeping a trench standing:
- Keep the spoil back. Excavated material and equipment must be set back from the edge (Ontario requires at least 1.2 metres). A spoil pile at the lip adds load and is a leading cause of cave-in.
- Provide a safe way out. A ladder or ramp must be within reach (Ontario requires access within about 15 metres of any worker), so a buried or trapped worker, or a rescuer, is never far from an exit.
- Test the air in deep or confined trenches. Heavier-than-air gases and oxygen deficiency collect at the bottom; atmospheric testing before entry is part of the national standard.
- Watch the weather and the traffic. Rain, vibration from nearby machinery and passing traffic all destabilise a wall that looked fine an hour ago. Re-inspect after any change.
The killer hazard: cave-in
The hazard that actually kills in excavation is the cave-in, the trench wall collapsing onto a worker inside it. It is so dangerous because it gives no warning, it happens in seconds, and the weight is crushing: a person buried even to the chest cannot breathe against the pressure and cannot be dug out by hand in time. This is why entering an unprotected trench at or beyond the trigger depth is never an acceptable shortcut, not "just for a minute" to lay a pipe or take a measurement. The protective system goes in first, every time, before anyone enters. Struck utilities (gas, power) are the other lethal excavation risk, which is why the locate is non-negotiable.
Common mistakes
- Digging before the locate is back. In Ontario, starting before Ontario One Call responds is both an offence and a way to hit a live service.
- Entering an unprotected trench "just for a second". Most trench fatalities happen in shallow, brief entries with no protective system.
- Piling spoil at the edge. It loads the wall and triggers the collapse; keep it set well back.
- Trusting old ground. Backfilled or water-affected soil is unstable; a competent person must classify it before work.
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