Trenching incidents are responsible for approximately 25 fatalities each year, according to the Bureau of Labor Statistics. From underground utilities to potential cave-ins to being struck by objects, workers face a range of hazards in these environments.
“When we talk about trenching and excavation, soil is by far our biggest hazard, but any underground utility that we come in contact with, whether we find a gas line we were not aware of, or high-voltage electric that is buried underground, is a hazard. There are so many hazards that have to be accounted for and it's constantly changing,” says Eric Voight, vice president and assistant director at Conner Strong & Buckelew and member of the ANSI/ASSP A10 Committee.
Proper planning, understanding and monitoring the work environment and training can help workers safely complete a trenching and excavation project. Before you begin your next trenching and excavation project, keep these three points in mind.
Understand the Job Site
The first step toward a safe trenching or excavation project is knowing the soil type(s) and hazards workers will encounter on the job site. This requires assessing the soil(s), identifying hazards present and determining the steps needed to protect workers from those hazards.
“Before you put any shovel into the ground, before you bring any excavator or heavy piece of equipment, you have to plan for all of the hazards that we could encounter, and that starts even before you get out on the site,” Voight explains. “You have to make sure that that you call 8-1-1, or whatever your number is in your locality, and have them come out and mark for any known hazards that could be in the area.”
In terms of classification, soils are divided into four types:
- Type A: clay, silty clay, sandy clay and clay loam
- Type B: angular gravel, silt, silt loam and soils that are fissured or near sources of vibration
- Type C: granular soils in which particles don’t stick together and cohesive soils with a low unconfined compressive strength
- Stable rock
Voight stresses the need to do multiple tests on soils, using techniques such as a visual test, a ribbon test or a dry strength test to determine the types of soil present. You may also use tools such as a torvane or a pocket penetrometer to assist in the analysis.
“You really have to get your hands dirty,” he says. “It's not something that you can just step back and take a look and say ‘this looks like Type C soil.’” You actually have to put your hands into the dirt because it's something you may have to justify how you came up with your determination.”
Throughout the planning process and with any soil evaluation and testing, it’s critical to partner with the competent person on the project to determine how job site hazards will be addressed and what will be the best solutions for providing a safe work environment.
“One of the biggest things you have to identify is who your competent person is going to be,” Voight says. “That competent person will be the one who will determine, based on the depth, width, soil type and work processes, which protective structure will be best for your workers.”
OSHA defines a competent person as “one who is capable of identifying existing and predictable hazards in the surroundings or working conditions which are unsanitary, hazardous or dangerous to employees, and who has authorization to take prompt corrective measures to eliminate them."
Monitor the Work Environment
The conditions on a trenching and excavation site change constantly. Therefore, you have to continuously monitor the site to determine how those changes may affect worker safety. These could be weather-related factors, or issues related to the makeup of the job site changing over the course of a project.
“An evaluation must be done any time conditions change on the site,” Voight says. “Whether it’s an afternoon thunderstorm, or freezing and thawing, those are critical times because those conditions can create voids in the soil and drastically change how a trench is going to react.”
He adds that soil wants to fill back in due to the natural pressures of any voids that are created, so that must be taken into account when determining which protective measures are best-suited for a particular project.
“Once you understand that a hazard is always going to be there, then you can plan on different ways that you can go through and minimize those risks,” he says. “Whether it's through a protective structure or some sort of sloping or benching, there are many options that you can choose. Choose the one that matches your task.”
In addition to a safety professional monitoring the conditions on the job site, workers must also continuously assess the conditions in a trench to make sure they stay safe throughout the work day.
“The best thing workers can do to protect themselves is make sure there is some barrier between themselves and the soil,” Voight explains. “Whether it is a trench box, a trench shield, timber shoring or removing soil to create an opening. If there is no soil that can entrap them, then there's no soil that can bury them.”
Know the Numbers
Voight emphasizes that the standards developed around trenching and excavation such as the ANSI/ASSP A10.12 standard are data driven, and you need to know certain numbers to protect workers on the site.
Before you begin any trenching and excavation project, keep these numbers in mind, in accordance with the A10.12 standard:
- Provide a means of access and egress in trenches that are 4 ft. or more in depth so as to require no more than 25 ft. of lateral travel for workers.
- Keep spoil piles at least 2 ft. from the edge of a trench.
- Trenches 5 ft. or more in depth require a protective structure.
- Trenches 6 ft. or more in depth require fall protection.
- Trenches greater than 20 ft. in depth require a professional engineer to review the protection of structures you have in place.
“We need to make sure that ultimately we're keeping that soil away from entrapping our workers,” he says. “It takes time, it takes effort, but it’s the only way we’re going to prevent injuries and fatalities.”
Listen to our podcast with Eric Voight of the ANSI/ASSP A10 Committee to learn more about how you can protect workers during trenching and excavation projects.
ANSI/ASSP A10.12, Safety Requirements for Excavation
Improving Construction Safety: A Team Effort
Construction Design: Its Role in Incident Prevention
Prevention Through Design for Hazards in Construction