802 Trench and Excavation Safety

Hazardous Atmospheres In Excavations

Common hazardous atmospheres might be due to methane, hydrogen sulfide, and carbon monoxide. Precautions include providing workers with proper respiratory protection or forced air ventilation. Do not assume that natural ventilation (natural air movement across and into the excavation) is sufficient to keep an atmosphere safe in the trench.

Employees must not work in hazardous and/or toxic atmospheres including those with:

• Less than 19.5% or more than 23.5% oxygen;
• A combustible gas concentration greater than 20% of the lower flammable limit; and
• Concentrations of hazardous substances that exceed those specified in the Threshold Limit Values for Airborne Contaminants established by the ACGIH (American Conference of Governmental Industrial Hygienists).

All operations involving hazardous atmospheres must be conducted in accordance with OSHA requirements for occupational health and environmental controls (see 29 CFR Part 1926 Subpart D) for personal protective equipment and for lifesaving equipment (see 29 CFR Part 1926 Subpart E). Engineering controls (e.g., ventilation) and respiratory protection may be required.

A competent person must test an excavation before employees enter an excavation greater than 4 feet (1.22 m) in depth for where hazardous atmospheres exist or could reasonably be expected to exist, such as in excavations in landfill areas or excavations in areas where hazardous substances are stored nearby.

When testing for atmospheric contaminants within excavations or confined spaces in excavations, the following should be considered:

• Testing should be conducted before employees enter the trench and should be done regularly to ensure that the trench remains safe.
• The frequency of testing should be increased if equipment is operating in the trench.
• Testing frequency should also be increased if welding, cutting, or burning is done in the trench.

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