Trenching and Excavation
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Summary What is a trench or excavation? A trench is a narrow channel that is deeper than it is wide. A trench can be up to 15 feet wide. An excavation is any hole or trench that is made by removing earth. Who is at risk? AFSCME members that repair water, sewer, and other utility lines, build roads, or perform other digging tasks. What are the hazards? The most common serious hazard is death or serious injury resulting from a cave-in. Other hazards include contact with utility lines, exposure to toxic chemicals, or being struck by vehicles when working in or near traffic. What can be done to protect workers? A competent person must identify hazards before and during the operation. A protective system should be used to prevent the sides from collapsing or the sides of the trench should be sloped. What law applies to this type of work? The Occupational Safety and Health Administration (OSHA) standard for trenching and excavation work is 29 CFR 1926.650-652, subpart P. |
What is trenching and excavation work?
A trench is a narrow channel that is deeper than it is wide. A trench can be up to 15 feet wide. An excavation is any hole or trench that is made by removing earth.
What are the hazards of trenching and excavation work?
Cave-in
The most common serious hazard is a cave-in. Workers can be killed or seriously injured if the sides of a trench or other excavation collapse. Cave-ins are most often caused by:
- Vibration from construction equipment or traffic in the construction area that makes the soil come apart.
- The weight of equipment that is too close to the edge of the trench can cause a collapse. The weight of the earth that has been removed (spoil bank) can also put a dangerous strain on the trench walls.
- Soils that do not hold tightly together are more likely to collapse. For example, sandy soils are not cohesive and can easily slide back into a trench. By contrast, clay usually holds up well.
- Soil that has been dug before is not as stable as earth that has not been previously disturbed. Work on utility lines involves digging previously disturbed soil.
- Water weakens the strength of the trench sides. Rain can also be a hazard by either putting too much weight on the walls or filling the trench with water. Soil that is too dry will crumble easily.
- NOTE: the risk of a cave-in increases if more than one of these conditions is present!
Other Hazards
- Contact with electric, water, sewer, natural gas, or other types of utility lines can cause serious injuries or death from drowning, exposure to chemicals, or electrocution.
- Toxic gases can be released during digging. Trenches should be treated as a confined space and the air should be tested. This is very important for bell-bottom types of excavation.
- Being struck by vehicles when working in or near traffic.
Who is at risk?
AFSCME members that repair water, sewer, and other utility lines, build roads, and perform other digging operations are at risk. Each year, as many as 400 workers are killed and another 4,000 are injured during trenching and shoring operations.
What can be done to protect workers?
- Determine risks before the work begins. A competent person must evaluate the possible dangers before the work begins and until the operation is completed. The person must know the risks posed by the soil that will be disturbed. The operation must be watched at all times because the danger can increase when it rains or other conditions change.
- Use protective systems for any trench or other excavation that is 5 or more feet deep. Excavations that are less than five feet deep may also require a protective system if the competent person on site feels there is a possibility of cave-in. Protective systems include:
- Sloping means the sides of the hole open out from the excavation. The type of soil determines the required angle. Sloping is less practical for deeper digs.
- Benching is similar to sloping with steps cut into sides of the trench.
NOTE: a registered engineer MUST approve sloping and benching systems
in excavations greater than 20 feet in depth! - Shoring supports the walls of the excavation. Shoring is made up of wales, crossbraces, and uprights. The material can be metal or wood, but plywood and 2'x4's are not adequate shoring materials. The equipment can be hydraulic or pneumatic. Shoring must be installed from the top down and removed from the bottom up.
- Shielding, also called trench boxes or trench shields, are structures that are placed in the excavation to prevent the sides of a trench from caving-in. The worker is only protected while in the "box." Some trench boxes can be moved as the work progresses. Heavy equipment must always be used to place the box or shield in the trench. The shield must extend at least 18 inches from the top of the slope of the trench.
What laws apply to trenching and excavation work?
The Occupational Safety and Health Administration's (OSHA) standard for trenching and excavation work is 29 CFR1926.650-652, subpart P. The Standard can be found on the OSHA website.
August 2001
For more information about protecting workers from workplace hazards, contact the AFSCME Health and Safety Program at (202) 429-1228, or 1625 L Street, N.W., Washington, DC 20036.