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Excavation work can be deadly. It takes just a few seconds for a construction worker in a trench to be buried under thousands of pounds of soil.
Trenches are one of the danger zones on a construction site, so to protect yourself and your team, follow the trench safety guidance of the Occupational Safety and Health Administration (OSHA). Here’s a refresher course on how to avoid cave-ins.
What is an OSHA Competent Person?
If you’re digging a trench at least 5 feet deep, you’re required by OSHA to designate a competent person to inspect the trench. “Competent” is defined as someone capable of classifying soil, inspecting protective systems, designing structural ramps, monitoring water-removal equipment and conducting site inspections. The competent person must have the authority to stop work to correct any safety hazard.
Soil classification is an important first step because the density of the ground greatly influences the stability of the trench. Soil is classified as Type A, B or C based on its ability to withstand pressure. At the extremes, clay is an example of Type A and sand is an example of Type C. The soil can be tested in a lab or determined in the field.
The competent person must inspect the trench each day to make sure it’s safe and free of standing water or other hazards before work begins and then inspect again after rainstorms or any other event that could change conditions in the trench.
What are the Three Main Protection Methods Against Cave-Ins?
To prevent cave-ins, OSHA excavation standards require a trench to be protected with a slope, shore or shield.
- Slope: Cutting the trench in a V shape gives it better stability. The looser the soil, the wider the top of the trench needs to be in proportion to the bottom. Type A soil requires cutting at least 9 inches out for every 1 foot deep, Type B requires a 1-to-1 ratio and Type C requires a 1½-to-1 ratio. The exception to the rule is a trench cut into stable rock, which can have vertical walls.
- Shore: Hydraulic shoring uses pistons to push steel plates or heavy plywood outward against the trench walls. Another option is beam-and-plate shoring, in which steel beams are driven into the floor of the trench and steel plates are slid behind them to form a temporary retaining wall. A similar technique called soldier boarding uses wooden planks instead of steel plates.
- Shield: A trench shield — also known as a trench box — is a two-, three- or four-sided metal box that fits into the trench. The sidewalls of the box are connected with horizontal spreaders that can be adjusted to fit the width of the trench.
Use Ladders, Steps or Ramps
For trenches at least 4 feet deep, OSHA requires ladders, steps or ramps to help workers get in and out. These devices — which OSHA calls “means of access and egress” — must be located so workers never have to travel more than 25 feet in the trench to reach them.
Lastly, it’s important to keep heavy objects away from the lip of the trench, where they could cause the soil to collapse. Spoil piles should be at least 2 feet from the trench edge, and heavy machinery should not operate or park near the trench. Contractors whose equipment is outfitted with the T3 operating system can set jobsite geofences to keep heavy machinery far away from the trench.
