How trench collapses happen

Why you should never take soil stability for granted.

In the construction industry, working below grade is just as hazardous as working at heights. Any time you are in a trench, you are at risk of being in a trench collapse.

Even the most stable-seeming trench wall can cave in if it’s not properly sloped, supported, and inspected regularly. Collapsing soil can crush you beneath a weight equal to that of a mid-size car. The Occupational Health and Safety Act (OHSA) recognizes the severity of the risk: 20 sections of the Regulation for Construction Projects (O. Reg. 213/91) deal exclusively with safety requirements for excavations and trenching. And yet even with these laws in place, in 2024, multiple Ontario workers died in trench-related incidents.

Trenches are dug for a number of reasons, including the installation or servicing of underground utilities and waterproofing of building foundations. They’re dug in all seasons and in any location. They can fail for equally diverse reasons.

How soil conditions contribute to trench collapses

The type of soil you dig in, however, is a primary factor. It determines the overall stability of your trench walls and must be properly evaluated before any work occurs.

Section 226 of O. Reg. 213/91 describes four types of soil based on factors such as hardness, density, moisture content, and internal strength. Type 1 is the hardest soil—it’s almost like rock—while Type 4 is soft, loose, and flows easily, like sand. The higher the soil number, the more likely it is to collapse if not properly excavated and supported.

For trenching operations in Ontario, “The vast majority of work involves Type 3 soil, which has a low amount of internal strength,” says IHSA Health and Safety Consultant Ryan Smith. “In part that’s because any soil that has ever been previously excavated or otherwise mechanically disturbed must be automatically considered Type 3, if not Type 4 soil.”

A competent person must identify the soil type found at a project and plan to protect workers accordingly. (More on that below.) It’s common for multiple soil types to be present, even in a small area or in soil that’s never been excavated before.

“Utilities are often buried in sand, which is Type 4 soil, to protect the utility from contact with rocks and other sharp and abrasive objects,” Smith says, adding that when more than one soil type is identified, the law requires that the entire excavation be protected based on the highest number (i.e., the weakest) soil.

How changes to site conditions affect trench safety

But a soil’s assessed “type” is not the only determinant of its strength. And that strength can change. It can be negatively affected by changing environmental and jobsite conditions, such as:

Evaporation: Once a trench is dug, the sides of the open excavation are exposed to the air, which can decrease soil cohesion. “Moisture is what binds the soil particles together,” Smith says. “In a hot, dry summer, moisture will evaporate from the trench walls over time, making the soil looser and less consistent—and more likely to fail.” The longer a trench is open to the air, the greater the risk of a cave-in.

Water: Excess moisture from rain, melting snow, or thawing earth can weaken soil, too. Heavy rain is particularly dangerous: not only does it erode exposed trench walls, it can create unseen changes to the water table that further reduce soil cohesion. (Workers have also become trapped in trenches due to flash floods caused by torrential downpours.)

Vibration: Nearby vehicle traffic or vibrations from construction operations such as earth moving, compaction, pile driving, and blasting can all contribute to the collapse of trench walls.

Excessive load: Also called surcharge, any extra weight next to the trench puts pressure on its walls. This surcharge is most often caused by excavated earth (a.k.a. “spoil”) piled too close to the trench, but nearby vehicles, equipment, tools, and even workers can overburden a trench wall. “It’s something that I see regularly, even though the law says that you cannot have anything within one metre of the edge of an excavation,” Smith says.

How to protect against trench collapses

So how do you protect workers and prevent trench cave-ins? The hazard can be eliminated for most people on a jobsite by ensuring that only workers who are directly involved in the trench operation go near the excavated area.

For those who must be in the trench, the law says you can never enter it alone. Smith tells the story of a quality control technician who entered an excavation to get a soil sample without telling anyone: “It would’ve been a couple minutes of work for this person. But the excavation hadn’t been finished; the walls weren’t properly supported. And the workers were on break. So no one was aware that the technician had even been on site until they found him in the trench under collapsed soil.”

Sloping the trench walls is the preferred way to reduce the risk of a cave-in. This is done by cutting back the walls at an angle—to stabilize the walls, prevent spoil from sliding back, and to reduce the pressure it puts on the walls. The slope angle depends on the soil type. For example, for Type 3 soil, you slope the walls at a gradient of 1 to 1 from the bottom of trench: the walls of a three-metre deep trench would therefore be cut back by three metres.

But tight jobsites do not always allow for the proper amount of sloping. In that case, you have two options:

• Use a prefabricated, timber, or hydraulic shoring system to support the trench walls. Nowadays, hydraulic systems are often preferred by contractors as the shoring can be installed from above, without workers having to enter the trench. However, it cannot be used in Type 4 soil, which flows too easily and can collapse the hydraulic supports over time.
• Protect workers with a trench box, a steel structure designed to withstand the weight of collapsing soil if trench walls fail. “For smaller jobs, trench boxes are typically used due to their availability and convenience,” Smith says. “But it’s important to note that the box only offers protection when you’re in it.”

Whether you use shoring or trench boxes, either system must be designed by a professional engineer based on the identified soil conditions at your project—and installed per the manufacturer’s instructions and engineer’s drawings.

How continuous inspections can improve trench safety

Of course, the goal is to prevent a cave-in from happening in the first place. Even with proper sloping and other controls, it’s important to frequently inspect the trench walls for signs of weakness. The longer a trench has been exposed—to water, evaporation, vibration, and more—the greater the chance that its soil conditions could change.

Conduct a thorough inspection at the start of each workday and at every shift change. Workers who enter the trench should also be trained to spot the signs of soil weakening or failure and should be continuously looking for them.

“And the construction regulations also require a worker to be above the trench, monitoring conditions from there, even if a trench box is in place,” Smith says.

Cracking is the most obvious sign of trench weakness, as is water seeping from the walls (or, conversely, if the trench walls are drying out). Beyond that, you should also check any hydraulic supports being used in the trench for cracks, bends, and other defects. And monitor the pressure gauge, too. Under too much force, the hydraulic struts may leak internally and fail.

If soil weakness is identified, immediately evacuating anyone in the trench is job number one. Excavation equipment can be used to strip loose or cracking material from the wall, before further sloping is undertaken or additional supports are installed.

But remember that a support system has to be designed for the soil found in the trench. “What happens if you thought you were working in Type 2 soil and your system was designed for that, but then conditions change or you hit a pocket of Type 3 or 4 soil?” Smith asks. “Well, to ensure worker safety, the law says you need to redesign the support system for that new soil type—or at minimum, get it in writing from your engineer that the supports are still appropriate for the conditions.”