Trenching and excavation are widely recognized as among the most hazardous construction operations, resulting in an average of two deaths per month and hundreds of injuries each year due to trench collapses. OSHA has addressed construction-related trenching and excavation hazards by developing specific safety standards for the construction industry, making the requirements easier to understand and providing construction employers with various options for classifying soil and selecting employee protection methods.
OSHA defines an excavation as any man-made cut, cavity, trench or depression in the earth’s surface formed by earth removal. A trench is defined as a narrow excavation made below the surface of the ground with a depth greater than its width and a maximum width of 15 feet at the bottom.
The primary hazard of trenching and excavation is the potential for collapse. Since one cubic yard of soil can weigh as much as a car, cave-ins pose the greatest risk and are much more likely than other excavation-related accidents to result in worker fatalities or serious injury. An unprotected trench can literally become an early grave. Additional excavation hazards include the use of heavy machinery for digging; electrical hazards from overhead and underground power lines; underground utilities, such as natural gas; falling loads; and hazardous atmospheres.
Excavation Protective Systems
Trenches 5 feet deep or greater require a protective system unless the excavation is made entirely in stable rock. Trenches 20 feet deep or greater require that the protective system be designed by or based on data prepared and/or approved by a registered professional engineer.
A variety of protective systems can be utilized to reduce or eliminate trenching and excavation hazards, including:
- Benching – cutting the sides of an excavation to form one or a series of horizontal levels, or steps, usually with vertical or near vertical surfaces between levels;
- Sloping – cutting back the trench wall at an angle inclined away from the excavation;
- Shoring – installing aluminum hydraulic or other types of supports to prevent soil movement;
- Shielding – using trench boxes or similar types of supports to prevent soil cave-ins.
Among the many factors to be considered in the design of protective systems are: soil classification, depth of cut, water content of soil, changes due to weather or climate, surcharge loads (e.g., other materials to be used in the trench) and other operations in the vicinity.
OSHA requires safe access and egress for employees working in trench excavations 4 feet or deeper, such as ladders, steps, ramps or other safe means of to be located within 25 feet of all workers.
General Trenching and Excavation Rules
When working in or near a trenching and excavation site, the following steps should be taken:
- Keep heavy equipment away from trench edges
- Keep surcharge loads, including excavated soil (spoils) and other materials, at least 2 feet from trench edges
- Know where underground utilities are located before digging
- Test for atmospheric hazards such as low oxygen, hazardous fumes and toxic gases in trenches deeper than 4 feet
- Inspect trenches at the start of each shift
- Inspect trenches following a rainstorm
- Inspect trenches after any occurrence that could have changed conditions in the trench
- Do not work under suspended or raised loads and materials
For additional information on trenching and excavation safety, see OSHA Trenching and Excavation Fact Sheet.