About 400 workers die every year in the U.S. and about 6500 are seriously injured in trenching and excavation related accidents. If you are in a trench when a wall collapses you may get buried in soil very quickly. One cubic yard of dirt could weigh approximately 2,800 pounds-about the same as a small pickup truck! In addition to the obvious danger of getting buried by soil, there are other potential dangers when working in trenches, such as hazardous gas inhalation, falling objects, drowning, dangerous equipment and power sources like electricity. OSHA requires trench protection in all excavations and trenches deeper than 5 feet. However, even in excavation and trenchers less than 5 feet deep, trench protection may be required depending on the soil type and presence of water.
Classification of Soil
OSHA classifies soil types into 3 categories "A," "B" and "C," "A" being the most stable and "C" being the least stable. The competent person in charge of the job (person who has proper training to identify soil types and other excavation and trenching hazards and has the authority to take prompt action, including stopping of the job) has to identify the soil type and select the correct protective system for the trench or excavation.
Sloping, benching, shoring and shielding are the main protective systems used. The type of the soil determines the sloping angle. Shoring is designed to prevent walls from collapsing. Shoring can be made out of timber or hydraulically or mechanically-operated metal jacks. Shielding or trench boxers are used to protect workers from collapsing walls. The tabulated data on the shoring and shielding provided by the manufacturer or the design professional engineer must be followed regardless of the soil classifications.
General Safety in Excavations and Trenching
- Trenches over 4 feet deep must have exists within 25 feet of every worker by way of ladder or ramp.
- Underground utilities should be marked by the "Blue Stake Organization" before the beginning of the excavation.
- Any excavation under a base or a foundation or a wall requires a support system designed by a registered professional engineer.
- Excavation spoils must be placed at least 2 feet away from the trench opening.
- Hard hats are required in every trenching and excavation work zone and traffic vests are required when working in street areas.
- Before beginning of an excavation, look out for overhead power lines and make sure that there is enough clearance to work under the power lines.
- No one should be allowed under the loads handled by the digging or loading equipment.
- Barricade tape, barricades and fences should be used to prevent unauthorized persons from getting inside the work area.
- Heavy vehicles which create vibrations should not be allowed close to excavations.
- The "Competent Person" must examine the excavation site every morning and after rain or changing conditions, such as water in the excavation, for signs of earth movement and movement of shoring.
Hazardous Atmosphere in Trenches or Excavations
A hazardous atmosphere can be present or develop in a trench or an excavation specially when digging close to a landfill, toxic site or a sewer. It can also develop if chemicals are used inside or close to the trench or excavation. Oxygen deficiency, presence of toxic or highly flammable gases (such as carbon monoxide, hydrogen sulfide, methane and solvent vapors) are the main contributors to hazardous atmospheres. Testing for such hazardous atmospheres should be done and if such problem exist it should addressed by providing fresh air ventilation or using personal protective equipment.
Locking Out and Tagging Out Energy Sources
Any type of energy source found in a trench or an excavation should be properly locked out and tagged out.