The most common hazard associated with trenching or excavation work is a cave-in, according to the American Federation of State, County and Municipal Employees. People at risk of being caught in a cave-in include workers who repair water, sewer and utility lines; road builders; and workers who perform digging operations.
AFSCME states that cave-ins can be caused by:
- Loose soil due to vibrations from construction equipment or traffic in the construction area
- The weight of equipment too close to the edge of a trench
- Soil that does not hold tightly together, such as sandy soil
- Water, which weakens the strength of trench sides
Before performing trenching or excavation work:
- A competent person must evaluate possible dangers before work begins and until the operation is completed.
- The operation must be monitored at all times.
- Protective systems must be used for any trench or excavation that is 5 feet or more deep. (Excavations that are less than 5 feet deep also may require a protective system if the competent person onsite thinks a cave-in is possible.)
Protective systems include:
- Sloping. This means the sides of the hole open out from the excavation. The type of soil determines the required angle.
- Benching. This system is similar to sloping with steps cut into the sides of the trench. (Note: A registered engineer must approve sloping and benching systems in excavations more than 20 feet deep.)
- Shoring. This supports the walls of the excavation. Shoring is made up of wales, cross-braces and uprights, and must be installed from the top down and removed from the bottom up.
- Shielding. Shielding involves trench boxes or trench shields 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 boxes can be moved throughout the work process. The shield must extend at least 18 inches from the top of the slope of the trench.