Workplace Safety Blog Post

Trenching & Excavation Safety

Excavation Safety
Photo: Oregon Department of Transportation

Trenching and excavation accidents in the construction industry aren’t a new occurrence. Fatalities have happened for decades, in large part because of cave-ins. OSHA even began a National Emphasis Program for trenching and excavation safety in 1985 to reduce the number of serious accidents.

Violations, serious injuries and fatalities related to this type of work continue to occur, though. For example, a worker installing sewer lines at a residential home in Missouri was killed when a trench collapsed in late 2013.

According to OSHA, “The fatality rate for excavation work is 112% higher than the rate for general construction.” Yet many employers choose to work without proper safety measures in place either because they do not understand regulations or they think working without safeguards will save time and money.

On average, two workers are killed each month in trenching and excavation accidents. To avoid these fatalities, it’s important for employers and workers to understand the hazards posed by trenches and excavations and the methods for alleviating those hazards.

Trenches & Excavations – Definitions

The terms trench and excavation are often used for the same construction situations, but they have slightly different meanings. An excavation refers to any type of hole dug in the ground, according to Oregon OSHA, while a trench is a type of narrow excavation that is usually deeper than it is wide. A trench has soil on all sides, while an excavation might only have soil on one or several sides.

Trenches and excavations are used for many purposes such as constructing basements, performing utility work and installing pipes.

Both excavations and trenches can be dangerous, largely because of the soil that is being dug up. Depending on the type of the soil (such as clay, sand or gravel) and its moisture content, a particular trench or excavation might be more prone to collapse.

Normally, the pressure created by soil is balanced because the soil is distributed evenly in the ground. When a hole or trench is dug, however, there is no longer even pressure on the soil from all sides, so the soil can cave into the open space.

A pile of dirt may not look incredibly dangerous, but that soil is heavy, weighing up to 114 pounds per cubic foot. A cubic yard can weigh over 3000 pounds, which can quickly crush a person. Even if a cave-in doesn’t completely cover a worker in an excavation or trench, the pressure from the soil can suffocate a person because the lungs cannot expand enough for the person to breathe.

Rescue attempts are often dangerous because further cave-ins are possible, making it difficult to extract the trapped person fast enough.

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Methods of Protection

Whenever an excavation is five or more feet deep, protective measures must be used to prevent a collapse. A cave-in can happen quickly, not leaving workers time to escape, so preventative steps must be taken ahead of time.

Multiple options exist for protecting workers in excavations. The most common are benching, sloping, shoring and shielding.

Benching involves creating a series of levels on each side of an excavation. Imagine a set of steps descending into a pit; benching looks a bit like those steps, and it prevents the soil from collapsing.

A trench box protects workers from falling soil. Photo: NIOSH

Sloping is a similar technique where the sides of an excavation are angled rather than vertical. This prevents the soil from caving in at the bottom of the excavation, which is what often leads to the most serious accidents.

The additional two options—shoring and shielding—require equipment to reinforce the walls of an excavation.

Shoring prevents the soil from moving and caving in, and a shoring system can be made of wood or metal. Aluminum hydraulic systems are often installed.

Shielding, on the other hand, focuses more on protecting the worker. A typical type of shield is a trench box, which fits into a trench and creates a space between the person and any soil that falls.

The type of protection you use at a worksite depends on a number of factors including the type of soil, the work that will be performed and even the weather conditions. A competent person (defined by OSHA as “an individual who is capable of identifying existing and predictable hazards or working conditions that are hazardous, unsanitary, or dangerous to workers, soil types and protective systems required, and who is authorized to take prompt corrective measures to eliminate these hazards and conditions”) must assess the site to determine what protective measures are needed. This person should assess the soil, moisture levels and weather conditions. He or she should also inspect the site daily and whenever environmental conditions change.

Additional Rules for Trenching & Excavation Safety

Even if you use an appropriate method for preventing a cave-in at your worksite, the soil can still become unstable. If heavy machinery comes too close to the edge of a trench, for example, the weight of the machine could cause the soil to collapse. Similarly, if piles of soil removed from the excavation are put too close to the top of the excavation, the weight of the soil could create problems. To avoid these types of collapses, keep all materials and machinery at least two feet from the excavation.

Workers also need to be able to quickly exit an excavation if the site begins to look unstable. Employers must provide ladders, ramps or other means of egress within 25 feet of all workers in excavations.

Finally, consider any additional hazards that could exist in or around an excavation:

  • In some deep trenches, the atmosphere could be hazardous, especially in areas near landfills or where hazardous substances could exist. These trenches should be tested for dangerous gases. Oxygen levels should also be tested.
  • If digging occurs near power lines, electrocution is possible. Always consult local utility companies to learn about power and pipe locations before beginning a project.
  • Fall hazards exist for people working above an excavation. Barriers near the edges can prevent workers from falling.
  • Construction equipment like backhoes is often used to dig large excavations. These machines pose struck-by hazards for workers on the ground. Make sure to post signs on all sides of this equipment warning workers to stay out of the equipment’s swing radius.

For more information about trenching and excavation regulations, consult OSHA standards 29 CFR 1926.651 and 29 CFR 1926.652. Workers can consult OSHA’s Quick Card for basic trenching safety tips.

Looking for more safety advice for your construction worksite? Take a look at our Ladder Safety infographic.

Click to see the entire infographic.

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