Excavation is defined as any man-made cut, cavity, or depression in the earth’s surface formed by soil removal. A trench is a specific type of excavation that is deeper than it is wide and no more than 15 feet across at the bottom. These activities are recognized as among the most hazardous in the construction industry. Trenching and excavation activities lead to dozens of fatalities every year, demanding strict adherence to safety protocols to protect workers operating below ground level.
Why Excavation Work Is Exceptionally Dangerous
Excavation and trenching pose a high risk because they create a confined space within an unpredictable, shifting natural environment. The Bureau of Labor Statistics reported 39 worker deaths in trenching and excavation work in 2022, more than double the number from the previous year. This fatality rate is significantly higher than that for general construction. Accidents in trenches happen with extreme speed, often giving workers no time to react or escape, as a sudden collapse immediately blocks the escape route.
Trench Collapse: The Leading Cause of Death
Trench collapse, or a cave-in, is the deadliest hazard, accounting for approximately three out of every four excavation fatalities. A single cubic yard of soil can weigh as much as 3,000 pounds, the approximate weight of a compact car. When a collapse occurs, the soil instantly crushes and suffocates a worker, making successful rescue attempts rare.
The stability of a trench wall is determined by the soil’s cohesive strength. OSHA classifies soil into three main types: Type A (most stable, such as cohesive clay) and Type C (least stable, including granular soils like sand and gravel). Type A soil can be downgraded to the unstable Type C classification if it is fissured, previously disturbed, or has water seeping through it. A seemingly safe trench can become hazardous due to a change in moisture content or nearby vibration.
Collapse results from gravity overcoming the soil’s ability to support its weight after lateral support is removed during digging. Signs of impending failure include tension cracks developing on the ground surface parallel to the trench and bulging near the bottom of the trench wall. Another factor is the surcharge load, which is the weight of excavated soil or heavy equipment placed too close to the edge. Regulations require excavated material to be placed at least two feet back from the trench edge. This reduces pressure on the wall and prevents the material from falling back into the excavation.
Struck-By Accidents
Struck-by incidents involve heavy equipment and falling materials. Workers inside a trench are vulnerable to equipment operating too close to the edge, which can destabilize the wall or cause the equipment to tip into the excavation. A worker can also be struck and crushed by the swinging superstructure of a backhoe or excavator if they enter the machine’s swing radius.
Workers can be struck by loads or materials that fall from above, including tools, equipment, or excavated soil improperly stored near the trench lip. They are also at risk of being struck by or caught between shoring components or trench boxes that shift or fail. The movement of any load overhead, such as a pipe being lowered into the trench, poses a threat if the load or the lifting equipment fails.
Hazards Related to Utilities and Atmospheric Conditions
Underground Utility Strikes (Electrocution and Explosion)
The presence of unmarked or unlocated underground utilities presents a threat of electrocution, fire, and explosion. Excavators are required to call 811, the national “Call Before You Dig” hotline, at least two to three business days before starting any digging project. This alerts utility owners, who then mark the approximate location of their electrical, gas, water, and communication lines.
A strike on a gas line can result in an explosion or fire. Contact with an electrical line can lead to immediate electrocution for the worker operating the equipment or for anyone in contact with the machine or the soil. Even after lines are marked, heavy machinery is prohibited within the 18 to 24-inch tolerance zone, requiring hand tools or vacuum excavation to safely uncover the utility.
Lack of Oxygen and Toxic Gases
The confined space of a trench can rapidly develop a hazardous atmosphere. Oxygen deficiency is a primary concern; normal air contains 20.8% oxygen, but levels below 19.5% are unsafe and can lead to unconsciousness, brain damage, and death. Heavy gases produced by decaying organic matter, such as methane, hydrogen sulfide, and carbon monoxide from internal combustion engines, can sink and accumulate at the bottom of the trench.
These gases displace the breathable air, creating an asphyxiating environment, and can also be flammable or toxic. Atmospheric testing is required for excavations four feet or deeper where a hazardous atmosphere could be expected, such as near landfills or sewer lines. A Competent Person must conduct this testing before workers enter and ensure adequate ventilation is provided if unsafe conditions are detected.
Water Ingress and Flooding Risks
Water accumulation poses a dual threat: accelerating the likelihood of a cave-in and presenting a drowning risk. Water rapidly decreases the cohesive strength of the soil, effectively turning stable Type A or B soil into the unstable Type C classification. This sudden destabilization can trigger a collapse even in trenches previously deemed safe.
Accumulating water can trap and drown a worker, especially in a flash flood scenario caused by heavy rain or a burst pipe. Regulations prohibit workers from entering an excavation where water has accumulated unless a protective system is in place. This system may include specialized dewatering equipment or the use of a safety harness and lifeline. A Competent Person must inspect the trench after any rainstorm or water intrusion to re-evaluate the soil condition.
Essential Safety Requirements and Prevention
The Occupational Safety and Health Administration (OSHA) standard 29 CFR 1926 Subpart P outlines mandatory safety measures for excavation and trenching operations. Trenches five feet deep or greater must be protected from cave-ins using one of three primary protective systems. Sloping involves cutting the trench wall back at an angle to the stable limit of the soil, while benching creates a series of horizontal steps in the wall.
Shoring uses a support system of aluminum hydraulic cylinders and vertical rails to brace the trench sides and prevent soil movement. Shielding does not support the wall but provides a safe space for workers in the event of a collapse. A Competent Person is required on-site to inspect the excavation and its protective systems before the start of work and after any change in conditions, such as a rain event or increased vibration.