Preventing a Trench Disaster: Trench shielding and shoring safety

By Jeff Griffin | Feb 15, 2022




Working in trenches and excavations is one of the most dangerous jobs in construction. Prospects for rescuing workers buried alive when a trench wall collapses are not good. It takes a long time to hand-dig them out, and it’s often too late. However, this doesn’t have to happen.

Trench shielding and shoring, when properly installed and maintained, affords protection for workers and maintenance of sites. OSHA has standards on how protective equipment must be used. Such equipment is expensive, so most of it is rented and installed by experienced contractors.

TJ Bryson, United Rentals’ midcentral district manager for trench safety, said OSHA standard (29 CFR 1926.650-652, Subpart P) requires protective systems for any excavation that is 5 feet or deeper, unless the excavation occurs in stable rock. All trenches are considered excavations.

United Rentals, Stamford, Conn., is one of the nation’s largest suppliers of trench shielding and shoring.

“The OSHA standard requires employers to have a designated competent person on-site trained in excavation safety to plan and prevent hazards associated with trench excavations. The competent person is responsible for analyzing the conditions of an excavation and selecting adequate protective systems to ensure worker safety,” he said. “The most commonly used protective systems include sloping or benching, shoring or shielding.”

He added that United Rentals offers the Excavation Safety Training for Competent Persons class to help prepare individuals to meet OSHA standard 1926 Subpart P requirements for excavation oversight. It is offered in English and Spanish. It is available in a classroom, online and virtual learning settings.

OSHA requires that a competent person be on-site and familiar with the shielding and shoring equipment. Photo courtesy of United Rentals

There are many protective system options.

“The protective system selected must be appropriate for the soil conditions at the job site,” Bryson said. “The type of system selected must allow for constructability, the necessary workspace and other clearances needed.”

Bryson explained the types of protective equipment that are available.

Shoring is an active system designed to prevent cave-ins by applying positive pressure against excavation walls. It is essential that positive shoring be placed properly and according to the manufacturer’s tabulated data to prevent the walls of soil from collapsing. Examples of shoring equipment include vertical hydraulic shores, manhole braces, slide rail systems and sheeting and bracing, such as a hydraulic mega-brace.

Shielding is a static system; it doesn’t prevent cave-ins like shoring does. Shielding is designed to withstand a cave-in and protect workers, provided the shield is properly installed. Examples of shielding equipment include steel, aluminum and modular aluminum trench shields. Shielding systems are available with two-, three- and four-sided protection.

“The competent person must be thoroughly familiar with, or often review, the tabulated data that comes with all manufactured shielding and shoring equipment,” Bryson said. “This data details proper equipment use and limitations, covering areas such as assembly instructions, placement, soil types for which the system was designed and maximum depth rating. If the scope of work is outside what the equipment is rated for per the tabulated data, either another protective system needs to be considered or an engineer needs to get involved to create a site-specific plan.”

Soil conditions play an integral part in selecting the shoring or shielding system, Bryson said. Properly identifying the soil type will help the competent person select the appropriate protective system. The depth ratings of shields and shoring change from one soil type to the next. Not all protective systems are designed to work in every soil type.

Some commonly used shielding/shoring equipment may be owned by contractors, but typically this equipment is rented.

“Safety compliance is often a major factor in a contractor’s decision to rent,” Bryson said. “United Rentals has the largest, most diverse fleet in the industry. We work with customers to help ensure the trench-protective systems they rented are the correct applications with the appropriate documentation, including manufacturer’s tabulated data or a site-specific design, that meet OSHA regulations.

“Also, the costs associated with storing, transporting, site loading and unloading, and maintenance of trench-protective systems help make renting equipment a cost-effective alternative to owning,” he said.

United Rentals subject matter experts provide either manufacturers’ tabulated data or site-specific, design stamped plans prepared by its engineering team.

“We do provide on-site consultations related to safety and best practices to contractors,” Bryson said. “For complex engineered systems, United Rentals also has installation advisers available.”

Knowing the soil type is key to choosing the correct shielding or shoring system. Photo courtesy of United Rentals

The fundamental objectives of trench-protection systems are to advance worker safety and job-site productivity. A well-made, OSHA-compliant and
all­-inclusive trench-protection system tackles both objectives on two levels: equipment and personnel. With these needs addressed, contractors can maintain safety on the job site and produce quality work without risk to budgets, deadlines or lives.

Another element in work requiring excavation, Bryson said, is remembering that existing utilities are everywhere, and hitting them while digging is extremely hazardous.

Damage prevention is key, he said. Make sure to call 811, which is the national call-before-you-dig phone number. When existing lines have been marked, use care when digging around any utility so as to not disturb or damage the pipe or cable. Fines for digging without calling for the appropriate locates are significant even if nothing is hit. Know the law and refer to OSHA Standard 1926 Subpart P (calling before digging).

“Injuries and deaths related to excavation collapses and hazards are preventable,” Bryson said. “At United Rentals Trench Safety, we are passionate about safety and care deeply about our customers’ well beings. Our mission is to deploy the best professionals in the industry to support every aspect of a customer’s safety program.

“Specific to underground work, we stand ready to provide them with access to a comprehensive, world-class trench safety experience. Our dynamic subject matter experts provide professional, competent-person excavation and confined space training, on-site planning and consultation, and site-specific engineering, which are all backed by the largest shoring and shielding fleet in the world. We consistently provide our customers with customized, cost-effective trench safety and excavation rental solutions,” he said.

Critical excavation safety tips

Don’t count on a trench box to prevent a cave-in. A trench box is a protective system designed to keep workers safe, not an active shoring meant to keep trench walls from collapsing. If a trench work site needs worker protection and active shoring, contractors should use a system that applies a load to the trench wall greater than the earthen load coming from the trench wall. This may include hydraulic-bracing systems.

Follow manufacturers’ data. It is essential to use a trench box within its tabulated data. A common mistake is to use a trench box at a depth greater than its rated maximum depth. Another is neglecting to build the box properly. The manufacturer’s tabulated data will help instruct the contractor on the allowable configuration, assembly and trench placement.

Watch the top. If a contractor is using sloping in combination with a trench box, typically the trench box top must be at least 18 inches above the point where the soil intersects the shield. That 18 inches serves as roll-off protection, providing added guarding from falling or sliding materials or small equipment.

Watch the bottom. OSHA regulations allow a box to be placed a maximum of 2 feet off the trench’s bottom. If the box is more than 2 feet off the bottom, there is a risk of a cave-in from below. Also, make sure the trench box is rated for the full depth.

Replace any damaged spreader pipe or trench box panels. A competent person’s responsibilities include inspecting all safety and shoring equipment being used, such as the spreader pipe, which can often get bent or damaged, especially if an excavator lifts or moves the trench box using the spreader pipe instead of the actual lift points. The competent person must also review the panel deflection. If the shield exhibits deflection on one of the trench box panels, it should be taken out for further inspection and possibly removed from service. In cases where there is potential or actual structural member damage to the shield, consult the manufacturer or engineer.

Install panels or plates in the right place. If a contractor is using trench shield panels as end panels, always follow the engineering guidelines that address end protection. Do not place the panels directly against the spreader pipes. If there is a cave-in, the panels are intended to push against the end posts of the trench box, not against the spreader pipes, which could deflect, putting the protective system at risk. Per OSHA regulations, a shield system (trench box) must be deployed according to the manufacturer’s or registered professional engineer’s tabulated data.

Stack with care. When stacking trench boxes, make sure the stacking pockets are lined up and the boxes are pinned together so they can be properly secured vertically. If the trench boxes are different thicknesses, the thinner boxes usually go on top of the thicker ones, but always refer to the manufacturer’s or professional engineer’s tabulated data.

Backfill the gap. By definition, a trench will always be wider than a trench shield placed within it. To eliminate the gap, backfill the void to stabilize the shield. Remember that the shield must always be situated to restrict lateral or other hazardous movement.     

—United Rentals/J.G.

About The Author

GRIFFIN, a construction journalist from Oklahoma City, can be reached at [email protected].





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