It can happen in an instant. Workers are down in a trench, when all of a sudden, the trench walls collapse, burying the workers under potentially thousands of pounds of soil. These types of incidents are tragic. They are also preventable.
The fatality rate for excavation work is 112 percent higher than the rate in general construction. Furthermore, the number of trench collapse fatalities doubled from 2015 to 2016. These statistics emphasize the need for all involved to take the proper steps to secure trenches so that injuries and fatalities don’t occur.
“Excavation and trenching are some of the most dangerous activities in construction, but the hazards are preventable,” says Dean McKenzie, director of OSHA’s Directorate of Construction.
“Workers are not always fully cognizant of what the hazards are and how dangerous this work can be.”
Cave-ins can occur at excavation sites for a number of reasons, from an unfamiliarity with the hazards, to a lack of training due to worker turnover, to trying to complete the work as quickly and inexpensively as possible. One key, according to McKenzie, is to help people realize that you don’t need to sacrifice safety for efficiency when conducting trenching operations.
“You can be efficient, fast and profitable while still protecting your employees and your operation,” says McKenzie. “Training can help people realize that.”
When cave-ins occur, a cubic yard of soil can weigh up to 3,000 lbs. and suffocation can occur in as few as three minutes. Contractors should ask themselves, “Do I want to have to worry about getting someone out of a trench within three minutes, or do I want to make sure the trench walls don’t collapse in the first place?”
Although cave-ins may be the first hazard that comes to mind when thinking about trenching and excavation, there are many others, including falls, workers being struck by natural elements or equipment, and electrocution. Those involved should familiarize themselves with each hazard and work to mitigate them before excavation work begins.
Understanding Soil Mechanics Improves Safety
Understanding soil mechanics is the first step toward safer trenching and excavation. Soils fall into one of four categories – stable rock, type A (e.g., clay), type B (e.g., angular gravel, silt), or type C (e.g., gravel, sand). Instances of stable rock are rare, so contractors should be prepared to work with type A, type B or type C.
By having a competent person conduct an inspection and test to identify the kind of soil(s) involved and the properties of those soils, contractors can then use that information to determine the best solution for protecting worker safety.
It’s important to keep in mind that soil conditions can and will change throughout a project due to weather changes, movement of materials and similar factors. So, the competent person should conduct daily inspections of the work site and make any necessary changes to see that work is performed in as safe a manner as possible.
Once the soil type is determined, the next step is to determine the best method for securing the trench, considering its depth and width. Depending on these and other factors, there are a several ways to secure the trench.
- Sloping. This involves slanting the soil away from the trench. The degree to which the soil must be sloped will depend on the type of soil.
- Benching. This process involves cutting back soil in a step-like fashion. This method should only be used for cohesive soils and it is not an option for Type C.
- Shoring. This involves using support systems such as hydraulic cylinders to create a barrier between the workers and the trench walls.
- Shielding. This method uses systems such as trench boxes (use plates used to brace the sides of the trench) to protect workers from cave-ins.
Whichever protective system is chosen, workers must understand that they need to stay within that system to keep themselves safe, according to Paul Colangelo, STSC, CHST, CET, CRIS, national director of compliance programs at ClickSafety.
“It’s not just about having the systems in place and having them correctly engineered,” he says. “It’s very important for workers to understand that if they go outside the confines of the systems, they are exposing themselves to hazards.”
Additional considerations include keeping spoil piles and machinery away from the trench to avoid causing movement in the soil that could lead to a cave-in. If rescue is needed, those involved must know how to properly remove someone from the trench in order to avoid further injuries.
In addition, trenches can present an oxygen-deficient atmosphere, so contractors must consider the probability of asphyxiation for anyone entering a trench. Furthermore, cases of cave-ins, depleted oxygen or toxic fumes require contractors to not only ensure that their workers are safe in the trench, but that they also have a way to get out should an incident occur.
By following OSHA CFR 1926 Subpart P - Excavations and other subparts of CFR 1926, as well as the best practices described in industry consensus standards such as ANSI/ASSP A10.12, contractors and workers can have confidence that they can conduct excavation and trenching operations safely.
A10.12 includes requirements for sloping, benching, shoring and shielding protective systems. In addition, it offers further guidance on soil classification and use of particular types of protective systems such as timer shoring and aluminum hydraulic shoring.
“It all comes down to this – effective, proactive measures or emergency rescue,” says Colangelo. “What do you want to put your time and effort into? You make the decision.”
OSHA is conducting a two-year-long special emphasis program to bring awareness to the hazards and risks associated with trenching and excavation. Its goal is to “by Sept. 30, 2019, increase trenching and excavation hazards abated by 10% compared to FY2017 through inspections and compliance assistance at workplaces covered by OSHA.”
The agency also offers free on-site consultation services to small and medium-sized business interested in creating or improving their safety and health management programs. Learn more on the agency's consultation services web page or call (800) 321-OSHA (6742).
OSHA Trenching and Excavation Safety Publication
OSHA Trenching and Excavation Public Service Announcement
Construction Design: Its Role in Incident Prevention
Improving Construction Safety: A Team Effort
Preconstruction Safety: Plan for Safety Excellence