26 Jun Trenching Safety
A trench is a narrow channel (up to 15 feet wide), generally deeper than it is wide, made below the surface of the ground. An excavation is any man-made hole or trench that is made by removing earth. Trenching is recognized as one of the most hazardous construction activities. The greatest risk is a cave-in and even a small job can present serious safety hazards. The key to preventing this type of accident is good planning.
Each year in the United States trenching cave-ins result in hundreds of serious injuries and dozens of deaths. Trenches are needed to build roads, for the installation and repair of utility lines, water and sewer lines, television cable and many other uses. (The list of the types of workers that might be involved in working in or around a trench is too long to include here.) Anyone whose work requires them to work in or around a trench must be aware of the hazards so they are not involved in or cause an accident to happen.
Obtain a permit from Occupational Safety and Health if workers are required to enter an excavation that is five feet or deeper. OSHA requires a competent person to inspect, on a daily basis, trenches for possible cave-ins, failures of protective systems and equipment, hazardous atmospheres, or other hazardous conditions. Refer to the OSHA website listed below for the complete list of competent person requirements.
In trenching, soil is defined as any material removed from the ground to form a trench or hole. Soil can weigh more than 100 pounds per cubic foot. Most soil is thought of in terms of cubic yards. One cubic yard of soil may weigh more than 2700 pounds. OSHA classifies soil into four groups: solid rock, Type A, Type B, and Type C. Solid rock is the most stable, with Type C soil being the least stable. If you are unsure of the soil type, always assume it is Type C. Soil removed from a trench must be kept at least two feet back from the edge of the trench.
- Cave-ins – can be caused by:
- Vibration of nearby construction equipment or vehicle traffic.
- Weight of equipment that is too close to the edge of the trench.
- Soils that do not hold tightly together.
- Soil that has been dug in before is not as stable as undisturbed earth.
- Water weakening the strength of the trench sides.
- Hazardous atmospheres – may be generated as toxic gases can be released by the digging or accumulate in the trench bottom.
- Underground utilities – call 811 at least 2 days before digging to determine the location of any utility services.
Protective systems are methods of protecting workers from cave-ins of material that can fall or roll into an excavation/trench or from the collapse of nearby soil structures. Protective systems include shoring, sheeting, shielding, sloping, and benching. For trenches between five feet and 20 feet deep, protective measures must be taken. It is up to the planners of the construction project and the competent person on site to determine which systems will work best. If an excavation is greater than 20 feet deep, a registered professional engineer must design the protective system.
Trenches deeper than four feet must have a way to get in and out (usually a ladder) for every 25 feet of horizontal travel within the trench.