Shoring Solutions Should Be Top Priority for Contractors

Knowledge of soil classification and proper shoring techniques will help you and your crews remain safe in the trenches

Shoring Solutions Should Be Top Priority for Contractors

If trench boxes can’t be used, shoring is also an option. This crew uses shoring tools to secure the walls of a trench.

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Eight service workers died in trenching accidents last April. Staying safe requires more than following regulations: It takes a thorough understanding of the tools available.

“Not everything these guys are confronted with is in the rulebooks,” says Jim Sullivan, president of Prospan Shoring. “They’re going to get into all different shapes and sizes; what they need to know is what’s in their arsenal when they pull out the door.”

There are three main types of trench collapse prevention methods: shoring, trench boxes and sloping. While there are some variations on these basic categories, the proper understanding and use of these three methods will ensure workers’ safety.

Anyone on an excavation site should be aware by now that any trench greater than 5 feet deep must have some type of collapse prevention. The method used for prevention depends on two factors: trench layout and soil conditions.

Assessing soil

OSHA rules break down soil conditions into several simple categories — A, B and C. Class A soil is so-called virgin soil, the most cohesive soil; on the other end of the spectrum, Class C is soil with very poor compaction.

“Once you identify what soil you’re in, then you have to look at a couple other influencing factors. If the trench is dug adjacent to a roadway, where most city trenches are and utility company trenches are, that means there’s roadway traffic. And with that comes vibration,” Sullivan says. “So if your ground is subjected to vibration from planes, trains or automobiles, you’re required to downgrade your soil classification by one.”

There are a few instruments and methods to test the soil compaction, but for the purposes of shoring options, everything’s on the table until class C.

“So in other words, all these cities and utility companies declare themselves in class C soil. So you can imagine, if you’re in real good A soil, we let you do more with a product,” Sullivan says. “Most people want the standard to be as simple as possible, so they just say, ‘We’re in class C soil — get on with your day.’”

The problem with sloping

Once you’re into C soil, there is a distinction in the engineering requirements between C-60 and C-80 soil. C-80 essentially means that there’s no cohesion at all, and it’s at that point that shoring is no longer an option, leaving just trench boxes and sloping.

Unfortunately, for most municipalities, sloping, which OSHA treats as essentially the ultimate fallback for collapse prevention, is all but impossible in most urban settings.

“The reason people don’t use sloping is because they’d have to remove the street on every dig, if they followed the OSHA formula,” Sullivan says. “OSHA would say open up that trench. We don’t care: The employee comes first. And I don’t disagree with that. I’m just speaking to you from a reality standpoint: They’re not going to remove the streets, they’re not going to tear up somebody’s front yard to the max, so really, people are out there using trench boxes or they’re using shoring.”

Shoring vs. boxes

So for practical purposes, what the vast majority of contractors and municipalities need to understand is the difference between shoring columns and trench boxes.

“There’s a difference between shoring and trench boxes. Even though it all falls under one heading called ‘shoring,’ there’s a distinct difference. Shoring pressurizes the trench walls so they can’t cave in. They’re designed to pressurize the trench wall and take away its ability to lean in,” Sullivan says. “Trench boxes are designed strong enough to take on the collapsing soil.”

In simpler terms, while a trench box is designed to simply hold back any soil that does collapse, shoring is meant to prevent a collapse in the first place.

Shoring functions via a principle called an “arch effect.” Basically, at the point where the shoring contacts the soil, it compresses outward in an arching pattern. So from point of contact, there is a dome of protection in the arch wall, the size of which depends on soil type.

This means you must know the maximum allowable separation between each column of shoring, which is to say how far apart horizontally two shoring bars may be placed. Manufacturers must provide those distances for each soil type. For example, Prospan dictates a maximum of 6 feet for class C soil.

The manufacturer specifications, or “tabulated data,” also tells users when plywood is required with shoring. While it’s generally not needed due to the arch effect, sometimes it will be required to prevent minor collapses from the arch wall surface soil.

Due to the relatively complex nature of shoring devices, it can be tempting to use only trench boxes, but you could be asking for trouble.

“With trench boxes, you would think, Oh, this is a no-brainer: If they’re strong enough to take the collapse, why wouldn’t you use a trench box every single time?” Sullivan says.

The simple answer is that they can be less convenient and less versatile depending on the situational factors of a given trench. If laterals or other utilities are a factor in digging the trench, it could limit the space for a bulky metal box.

Another factor is that different types of boxes have varying weight limits. Depth is the main factor here — weaker aluminum boxes, while often easier to maneuver, cannot be used past a certain depth because it cannot hold the weight of all the soil that could potentially cave in. Even the sturdiest steel construction boxes have limits, though they are in most cases deeper than any realistic trench.

Boxes are also required to be stacked up to the top of the trench, so multiple boxes may be needed, which is a lot to haul or maneuver to and around a job site.

“You can’t rely on trench boxes all the time, because what if you can’t get the box in the hole?” Sullivan says. “And you can’t use shoring all the time because what if you’re in C-80 soil where shoring is not allowed. So anybody who’s attacking this the right way needs to have both shoring and trench boxes in their arsenal.”

Prepare for anything

The fact of the matter is that preventing trench collapses is not always a simple endeavor. OSHA regulations attempt to simplify it as much as possible, but if it were a piece of cake, workers wouldn’t be dying.

Stocking collapse prevention devices may be a hassle, but supervisors at all levels don’t have the luxury of cutting corners — or fieldworkers will be those who pay, possibly with their lives.

“Depending on the task at hand, sometimes it’s hard to be compliant. As simple as it seems, it isn’t. If they’re putting in a catch basin or a manhole, or they’re doing something where their options are limited with the dimensions of that trench, it’s very difficult for them to stay protected,” Sullivan says. “Sometimes where they start to dig and where they finish can be two different animals altogether; they need to have trench boxes and shoring in their bag of tricks.”


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