Selecting the Right Trench Shoring System

The two overriding priorities in selecting a trench shielding or shoring system can be summarized in a pair of familiar phrases: “dollars and cents” and “life and death.” The first expression is about doing a job cost-effectively, which is an important consideration. The second is about protecting human life, which is even more important.

“I remember distinctly talking to a contractor back in the 1980s,” recalls Bill Edgerly, Sales Representative for Newburgh, N.Y.-based American Shoring. “He said, ‘I don’t need a trench box. I’m only going to be down there a few minutes.’ My first thought was, ‘You may be down there the rest of your life.’”

Edgerly says that Occupational Safety and Health Administration (OSHA) regulations in the early 1990s, which required a trained “competent person” on a worksite, “really changed the industry. Contractors finally recognized that a trench is dangerous, and they should do something
about it.”

Yet a less than total commitment to trench safety was evident as late as 2003. An OSHA study revealed that 86 percent of excavation fatalities in that year occurred when the officially designated competent person was not on the jobsite. Five years later, OSHA determined that a quarter or more of trench fatalities were attributed to a contractor not employing a shield or shoring system at all or installing it incorrectly.

This record of nonchalant noncompliance may be changing. Mitch Post, Training Manager at Elkridge, Md.-based Mabey Inc., believes the recent recession and post-recession economies winnowed the worst offenders from the ranks of utility contractors. “Most of the surviving ones out there are doing it right. The difference between working safely in trenches and not working safely in trenches is knowledge. Trench safety knowledge has just skyrocketed.”

So how does a conscientious utility contractor go about selecting among numerous types and sizes of excavation shields and shoring protective systems? In January interviews, Edgerly and Post talked about some of the considerations contractors should weigh when choosing a solution.

Cost: “The least expensive method can be sloping the trench sidewalls,” says Edgerly. “You can do a 20-ft bench, a 1½-to-1 slope. But you need to look at the cost of removing all that soil and trucking it away and then trucking it back. You have to weigh the total cost of sloping.” After sloping, the hierarchy of cost for a protective system starts with using plywood and hydraulic supports and from there scales on up through trench boxes and shields, then single-, double- or triple-slide rail systems and, finally, sheet piling.

The choice of renting or buying a shielding system can depend upon how busy and financially strong a utility contractor is. “We are a rental company, of course, but we think it often is more cost-effective to rent than to purchase because the need for a particular type of shielding can change daily,” says Post. “It is difficult for a small contractor to invest in inventory to cover all the bases. If he rents the equipment, he can invest in manpower and excavators instead.”

Renting also lets contractors shift as the prevailing work shifts. “The need for slide rails versus boxes varies from year to year,” Edgerly says. “One year it is all boxes and the next year just the reverse, all slide rails. There’s no rhyme nor reason for it.” In such shifting markets, prudent contractors let rental companies idle their inventory. In the Northeast marketplace, for example, Edgerly says that about 60 percent of customers rent the staple shielding product, which is a trench box.

Fitting the job: “A shoring or shielding solution should be pretty much job-specific,” says Edgerly. “Working along a two-lane roadway, for example, you want to keep the trench a lot tighter, so you might use a slide rail system instead of a box, particularly in unstable soil. You can install it without undermining the nearby roadway.” Other factors to consider in choosing a system include soil stability, the depth of an excavation, ground pressures created by adjacent buildings or passing trucks and groundwater. Of course, anything deeper than 20 ft is required by OSHA to be professionally engineered and that will determine the appropriate system.

Post agrees that a solution should meet the requirements of a specific job — with a contractor and renter both fully understanding those requirements. He tells of a customer (not one of Mabey’s) who asked for a box and carted off one rated for an 11-ft depth. Unfortunately, the box went into a trench nearly twice that deep and was destroyed when the embankment subsequently collapsed. (No one was in the hole at the time.) The contractor was billed for the loss of equipment. That all happened, Post says, simply because the rental company representative didn’t ask, “How will you be using the box?”

Crew comfort: No small consideration is the experience and capability of a contractor’s trenching crew, says Post. “What is a crew most comfortable using? Sheet piling could be the best solution for a contractor if that is what the crew is used to using.” It follows that contractors sometimes choose to work with what they know rather than considering something else that might serve them better. “That’s not necessarily a negative. But when a contractor gets into work he usually doesn’t do — maybe a deeper excavation than he is used to doing — that is when he needs to be willing to lean on someone else’s expertise.”

Another commonsense recommendation for a contractor is that he rent a trench system suitable for his stable of equipment. “Too many times, a contractor will try to use bigger boxes but he doesn’t have a large enough excavator to do so,” says Edgerly. “It is very important to have appropriate sizes of equipment so a contractor can use the trench materials he is renting or buying.”

Whatever trenching system is rented or bought, the next step is to keep it fully functional each workday. It is the responsibility of a project’s designated competent person to recognize when the integrity of a shield or brace has been compromised. While shields and supports are over-engineered so a stated capacity generally is less than what the equipment really can bear, a damaged component of the system makes hash of such declared limits.

“A compromised piece is a huge concern, especially when a load-bearing part of a system is what is damaged,” says Post. He recommends that a competent person inspect equipment at the end of a shift as well as before the next shift begins, adding, “I know that seems kind of counterintuitive, since no work will have been done overnight.” However, an inspector might more readily recall damage inflicted on equipment that day than remember the damage at the start of the next day. As to what causes damage, bowed panels or bent struts typically are the result of an excavator bucket’s wayward swing.

Some companies are having entire crews trained to OSHA “competent” standards and otherwise acting to protect everyone on a jobsite, whether in or out of the trench. To that end, National Trench Safety offers a “Work Zone Safety System.” It incorporates hand-railing along the edges of a trench to prevent workers and visitors falling into the excavation, a ladder access platform for safe ingress and egress by trench workers, and a “davit arm” or topside winch for attaching to a worker harness or for safe retrieval of materials. All are OSHA-certified.

Why all this bother about protective systems? Because they help a contractor complete a project on time by avoiding cave-ins and crew member injuries. According to OSHA, the fatality rate for excavation work is 112 percent greater than for general construction. As Post puts it: “Every hole wants to fill itself in” — even when someone is standing in it. With a cubic yard of soil weighing about 2,700 lbs, the danger of a collapsing trench wall is constant, deadly and best shielded against.

Giles Lambertson is a Freelance Writer for Utility Contractor. Tags: February 2016 Print Issue, National Trench Safety, Trench Shoring