A Guide to Trench Boxes: Here's What Contractors Need to Know

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Updated Aug 16, 2023
orange trench box in trench with silver ladder sticking out
One of the most-often cited violations in trench collapses is failure to use a protective system. Safety experts say a trench box is a contractor's best bet for protecting workers.
National Institute for Occupational Safety and Health (Wikipedia)

For contractors ordering workers into trenches and excavations, trench boxes are most often the best bet for protecting their employees, say safety trainers.

“For pipeline laying, a trench box is hard to beat,” says safety trainer Rick Padgett. “It's only restricted by how much room do you have and how much weight can your machine carry.”

Despite that, some contractors still don’t use them or use them incorrectly, leading to injuries and deaths. In 2023, OSHA has reported that 39 people died in trenches in 2022 – the most fatalities since 2005. One of the most-often cited violations in trench collapses is failure to use a protective system.

In this article, we’ll take a look at what types of trench boxes are on the market, how to choose one, how they stack up against other trench safety methods, and how to use them properly.

Types of Trench Boxes

There are basically four types of trench boxes, says Padgett, who has been training construction employees in trench safety for over 30 years at Padgett Risk Consultants in Roswell, Georgia.

“They're all just basically pieces of steel and pieces of aluminum, and quality has a lot to do with weight.”

The four main types of trench boxes are as follows:

  • Heavy-Duty Steel Boxes are going to provide the best protection and better withstand abuse over the years. The boxes have thicker walls of 6 inches and upward, P1 steel, and better engineering. They are typically used by the rental companies and large contractors doing day-in and day-out excavation because of their ability to handle abuse. “If you take any kind of care of it, it's going to last you 15 or 20 years, even if you use it every day,” says Padgett. “It's a rugged piece of steel.” The heavier boxes require heavy machinery to put them into place. That could be a problem, however, for smaller contractors that have only lighter construction equipment.
  • Lightweight Steel Boxes are easier for contractors with lighter equipment to put in place and for those not performing deep trenching day-in and day-out. They are made of lighter steel and have 4-inch walls or so. “You're going to have more abuse and a lighter depth rating,” Padgett says. “But of course, you're going to gain with the weight advantage that you're going to grab.”
  • Aluminum Boxes are the lightest. “The aluminum boxes are super-handy. They're super-easy to manage, and they have decent depth ratings,” he says. “But they are easier damaged, too.” They’re also more expensive than steel boxes. They can come in handy for those who don’t have a large excavator and are digging shallower trenches for installing small-size pipe or performing utility repairs. “But if I’m laying pipe out in a large field and I’m laying large diameter pipe in deep and rugged ground, I don't want an aluminum box anywhere near,” Padgett says.
  • Modular Boxes are Padgett’s personal favorite for small contractors doing occasional light pipe-laying. “It comes in sections like a Lego set, and you're able to configure it – two-sided, three-sided, four-sided, put the spreaders where you want. You can even make 90-degree angles, when you’re making a turn,” he says. “You kind of build it specifically for jobs.” They’re made of lightweight aluminum, are portable and can be put together by two or three workers by hand. Light equipment can be used to pick up the box after it’s built to place in the trench. “It's not expensive to rent, but it is expensive to own,” he says. “The good news is, if you damage one piece of it, you've only damaged that piece. You haven't damaged the entire frame.”

Trench Boxes vs. Shoring

The U.S. Occupational Safety and Health Administration calls for using one of three types of protection for workers in trenches and excavations that are 5 feet or deeper: shield, shore or slope.

Trench boxes are included in the shield category. Padgett estimates that 90% of companies that lay pipe or dig for underground structures use a trench box. “It's easier to move; faster; you move less dirt – more economical. It's just an easier way to do things.”

Shoring, which involves hydraulic or static jack devices, are called for in certain conditions. Those include large holes and for excavations that will be open for a long time, where the dirt needs to remain in place.

While shoring systems hold the dirt in place by pushing against the earth, trench boxes don’t prevent collapse but shield workers if one occurs.

Trench Boxes vs. Sloping

Sloping is another method allowed by OSHA. Safety experts say it is also the method most misunderstood and misused, leading to dangerous conditions.

Sloping involves widening the trench or excavation by cutting back the trench wall at an angle inclined away from the excavation to prevent a cave-in. Different types of soil require different slope ratios. So contractors need to know what type of soil they’re in.

Padgett notes that in most soil conditions, every foot of excavation requires a foot and a half of slope. For example, he says, “If you're 10 feet deep, you're 30 feet wide at the top. And that's just impractical. So they use trench boxes, to also be more efficient and to save money on material.”

Wendell Wood, owner of Trench Safety Training in Michigan, said that when the OSHA standard on excavation and trenching came out in 1989, most contractors moved away from sloping, which was too complicated and had been leading to over 100 deaths a year because they were rarely sloped wide enough. OSHA added standards for sloping in various soil types, but in most cases, the process is too costly to do correctly.

“A 5-foot-deep trench 3 feet wide would actually be 18 feet across the top,” says Wood, who has been teaching trenching safety for 40 years. “And that's a huge restoration cost, a huge amount of time before you can lay the pipe, great maintenance on your machine, higher fuel costs.”

How to Use a Trench Box

Though a trench box won’t prevent cave-in, it will keep a worker safe, if it is used correctly.

“He has a loud bang, and he’s shook up,” says Wood of when a worker is in a trench box during a collapse. “And then he's grateful that he's inside a box.”

The boxes typically come in wall thickness of 4, 6 and 8 inches, but other sizes and custom engineering of boxes are also options. Wood says that 4-inch boxes will typically be used as deep as 15 to 20 feet; 6-inch boxes for 15 to 25 feet, and 8-inch boxes for 20 to 30 feet deep.

The boxes are typically 16, 20 and 24 feet long. Contractors should choose a box that is 4 feet longer than the pipe being installed. The width of the box is also important. It needs to be about 1 foot wider than the wider of the pipe or the excavator bucket.

But the most important thing to do before using a trench box is to follow the manufacturer’s tablature data on the structure’s specifications.

“One of the things that the contractor needs to be cognizant of – and many are not – is that there is tab data that comes with a box that specifies its use,” Wood says.

OSHA can cite a contractor for not following the manufacturer’s tab data and also for not having the tab data readily available. The data outlines the type of plates to be used, how to restrict lateral movement, allowable surcharge loads and other critical issues.

The box must be able to handle the pressure at the depth it is in, OSHA says, information also provided in the tab data.

OSHA also has the following rules:

  • Boxes cannot be more than 2 feet off the bottom of the trench.
  • Trench boxes can be stacked on top of each other as trenches get deeper.
  • Workers can’t be in the box when it is being installed, removed or moved vertically.
  • A ladder must be installed within 25 lateral feet of all workers and extend at least 3 feet above the top of the trench.

Wood notes that though trench boxes won’t collapse, they can fail.

“Failure is defined by OSHA as a permanent deformation of a structural member,” he explains, such as a bend in the spreaders or the walls.

Renting vs. Buying a Trench Box

For those in need of a trench box, rental can be the best way to go, especially for small contractors.

“I would say small contractors almost exclusively rent,” Padgett says. “… They may be laying 20-foot-long, 12-inch PVC today, and tomorrow, they're laying 4-foot concrete by 8 feet long. And their depths vary.”

Medium-sized contractors that do daily excavating may have a fleet of boxes and rent as needed. Larger contractors tend to buy trench boxes in bulk, but on large projects, they’ll rent and add the rental cost into the bid.

An advantage of renting is that many of the rental shops have designated divisions solely devoted to trench protective devices. They can help contractors match the system to the project, as well as provide instructions and the manufacturer’s tab data for using the box.

“They will usually give him a piece of tab data when they deliver the box. He will sign for it that he understands it,” Wood says.

“And I would encourage him to read the tab data and not simply say that ‘a box is a box.’”