Thinking Outside the Box

Sept. 1, 2005

Excavation contractors have long used trench boxes to shield employees from cave-ins when working inside trenches. Still, despite their popularity, there are times when a trench box isn’t the best or even a practical choice. For example, you may not have a trailer big enough to haul the box, or a backhoe or excavator strong enough to lift and position the size of box you need. Then again, if you’re digging a large, wide opening in the earth pit, you need a way to shore up the sides of the excavation rather than a shield to protect workers.

Here are ideas for equipment that can provide a cost-effective way to keep workers protected and productive when you need an alternative to the popular trench box. In some cases, it may pay to buy the systems. In others, rental may be a more economical option. Either way, workers require some type of protection from cave-in when they venture into a hole in the ground.

The Need for Safety
“Anytime you cut into the earth, Mother Nature wants to heal the cut by filling it in,” says Matt Vogel, a sales manager with GME, a Union City, MI, manufacturer of trench shielding equipment. “Cave-ins can be caused by a number of factors, including vibrations from nearby traffic or equipment parked near an excavation. Sometimes, walls can collapse from above, other times from below. You can’t predict when a cave-in will occur. There are no warning signs. The wall just goes.”

When that happens, unprotected workers in a ditch or other excavation can suffer serious injuries, even death. That’s why OSHA regulations require protective measures—either sloping and benching, shielding, or shoring and sheeting—whenever workers are inside an excavation that extends 5 feet or more below ground. Some states require such practices at shallower depths.

“There’s a misconception that protection is required only with Class C soils and not the more stable Class A or B soils,” says Bob Checca, sales manager with shoring and shielding distributor Allwest Underground of Pacific, WA. “That’s not true. In fact, more cave-in-related injuries occur in clay-based soils, where a contractor may not anticipate a failure, as opposed to sandy conditions where you might expect and watch for a failure. Also, most failures take place in trenches between 5 and 15 feet deep, where many falsely believe they can quickly escape if a trench wall collapses.”

Sloping and benching prevents cave-ins by cutting back the trench walls to a shallow enough angle to prevent them from collapsing onto workers. But this approach can require a much wider right of way, not to mention the higher costs of extra excavation, backfilling, and compacting compared to a trench or pit with vertical walls. Shielding devices, which protect workers in the event of a cave-in, and shoring equipment, which puts pressure on sidewalls to prevent them from giving way, overcome these limitations.

A Versatile Alternative to Boxes
Utility installation contractor RPD of Sumner, WA, faces two major shoring and shielding challenges—working around existing utility lines and dealing with widely varying soil conditions. The company installs water mains, sewer mains, and storm systems with pipes ranging in size from 4 to 72 inches in the Seattle area. The trenches can range in depth from 4 to about 30 feet. Although most of the jobs involve single-family residential development, the company also handles road improvement and commercial projects.

Most of the time, trench boxes and manhole boxes are used to shield workers from cave-ins when working in the trenches. However, in older, developed areas, an underground maze of pipes of different sizes and buried at various depths over the years in the rights of way calls for a different approach to protecting employees. “Often, it’s impossible to use conventional boxes, because they don’t fit in between the existing pipes,” says Marc Rickabaugh, who co-owns RPD with Ben Pentecost.

Then, there are the unpredictable soils. “We work in some goofy ground,” he adds. “We can run into many different soils—hardpan, rock, sand, or clay—in a distance of just a thousand feet.”

In these situations, where trench and manhole boxes aren’t practical, Rickabaugh uses lightweight, re-usable aluminum hydraulic shores to stabilize trench walls. He rents them from his supplier, Allwest Underground. The company, which offers shoring products manufactured by Efficiency Production Inc. and Pro-Tec Equipment Inc., recommends the size and spacing of the systems to fit the specific job.

Hydraulic shores are today’s much faster and easier-to-use alternative to the timber shores and screw jack systems of years ago. “Hydraulic shoring systems are designed to provide a quick, simple way to secure a safe and productive environment when trenching,” says Checca.

These professionally engineered, low-cost systems are installed and removed from above ground, keeping workers out of the unprotected trench. They include an aluminum hydraulic cylinder or cross brace, which presses against vertical (uprights) or horizontal (walers) rails to preload the trench walls and stabilize the sides. Normally, hydraulic shores can be used to a depth of 20 feet depending on design and application. In more stable soils, they can be installed directly against the trench walls. However, to reduce exposure to raveling and sloughing in less stable soils, shoring-grade plywood (Finform) or steel panels are placed behind the rails.

The manufacturer’s tabulated data are used to match the shoring system and placement to specific job-site conditions such as soil type, trench width and depth, placement and spacing of the shores, and weight of machines operating near the trench.

Two Basic Choices
Vertical shores are especially useful where working room is tight, where spot bracing is needed, or where utilities cross the trench. One person can install them by hand, allowing a backhoe or excavator to continue digging. Waler systems can be used to stabilize trench faces at greater depths and to protect a larger working area than vertical shores. Placing them in the trench, however, may require an excavator or backhoe.

These systems are available in a wide range of sizes. American Shoring Inc., for example, makes vertical hydraulic shoring equipment with 2-foot rails and a single hydraulic cylinder; 3.5-, 5-, 7-, and 8-foot two-cylinder models; and 9- and 12-foot rails with three cylinders for use in trenches 17 to 88 inches wide, depending on the model. The weight of these systems ranges from 25 pounds for the smallest to 130 pounds for the largest. Extension systems are also available for each model. The company’s waler systems include dual-cylinder 8- and 12-foot units and 16-foot rails with three cylinders.

Installing hydraulic vertical shoring is a fast, simple, one-person job. After connecting a hydraulic hose line to the shore, the assembly is lowered into the trench to the desired height and a hand pump is used to pressurize the system and expand the shore. For added safety, bleed-off ports prevent over-extension of the cylinders. Removal involves releasing the hydraulic fluid and pulling the assembly out of the trench. The entire unit folds flat for easy transport.

Rickabaugh uses hydraulic vertical shores with either Finform sheets or 1-inch steel plates, usually in trenches no deeper than about 12 feet. He installs them where a crossing communications line, gas main, or other utility prevents use of a trench box. “We use the hydraulic spreaders with the wood or steel sheets to piece around the existing utility,” he says.

Depending on the depth of the trench, he may place the panels horizontally, either a single panel or two panels (one above the other) or vertically side by side, and installing a hydraulic shore at each end of a panel.

“Hydraulic shoring offers us a quick way to shield our workers in a trench without a lot of labor,” Rickabaugh observes.

Protecting Wide-Open Spaces
Up until about six years ago, Excavation Supervisor Bill Bogan used steel beams and sheeting to shore up the sides of large pits for installing underground fuel storage tanks. Then, he switched to a slide-rail shoring system. He likes this approach a whole lot better.

In fact, since then, he’s used beam-and-plate shoring only once, because of a high water table.

“I enjoy using slide-rail systems, and I’ll continue using them whenever I can,” he says.

Bogan works with Oscar W. Larson Co. in Clarkston, MI, which specializes in the installation of steel or fiberglass storage tanks for retail fuel stations. Typically, these are 20,000-gallon tanks that measure 10 feet in diameter and about 38 feet long. Installation requires 15- to 16-foot-deep, 42-foot-long excavated pits ranging in width from 14 feet for a single tank to 42 feet for a triple-tank installation.

The slide-rail system is a vertical sheeting and trench support design that provides a cost-effective alternative to an engineered sheet piling system for short-term projects, like these, which require a large clear span excavation. While there is no steadfast rule, the experts point out, an onsite engineered beam-and-plate system may be a more cost-effective approach for longer-term projects.

The pre-engineered slide-rail system is installed from the ground level down. The modular design features steel panels that are inserted between vertical rails to form a square or rectangular structure to shore up the walls of the pit. Spreaders placed across the width of the system lock in place along the rails to support the sidewalls during installation. Once the system is in the final position, these rolling braces are removed, leaving a large, open space into which a fuel tank or other object can be lowered without any supporting beams or braces to block the way. Vertical clearances can be adjusted if removal is not required.

This system can be used in such applications as pump stations, bore pits, pre-cast structures, and pipelines. One manufacturer’s slide-rail system, for example, can protect trench walls as deep as 40 feet, while allowing an adjustable vertical clearance over the bottom to install very large-diameter pipe. In one cast-in-place concrete box culvert project, a slide-rail system was used to stabilize the walls of an excavation measuring 500 feet long, 20 feet wide, and up to 28 feet deep in a linear application.

“It’s a dig and push system,” says John Powell, slide-rail specialist with Pro-Tec Equipment Inc. in Charlotte, MI, which manufactures and distributes slide-rail shoring systems. “You dig from inside of it and push it down with an excavator as you dig. For a short-term project, it’s much less expensive and much faster and easier to install and remove than sheet piling.”

Using the excavator to push them into the ground, the interlocking posts and panels are installed after completing the initial shallow excavation to a depth of no more then 4 feet. Alternately digging and pushing the system in a few feet at a time, the system is kept level and square until reaching the desired depth. Unlike steel sheeting, which is typically driven into place with a hydraulic hammer, the dig-and-push approach means much less vibration. That can be an important consideration when working close to buildings or walls. Once in place, this system provides a large open space for installing a fuel tank, pipeline, or other structure. After work inside the pit is completed, the slide-rail system is removed by extracting the panels and rails as the pit is backfilled and compacted.

Profitable Savings
A high-water table can be a drawback with slide-rail shoring, Bogan notes. “If a small amount of water is coming into the pit, you might be able to dewater a slide-rail system by installing well points around the outside of it,” he says. “Otherwise, the sheet piling is usually a better choice in these cases.”

Other than that, he says, the slide-rail system holds a clear advantage on his tank installation projects. They include:

Less Space
A slide-rail system fits tight spaces by eliminating the need for sloping to prevent a cave-in. Sloping requires a fair amount of horizontal area all around the pit, which Bogan seldom has on his job sites. Even if he did, Bogan points out, sloping would require removing a lot more soil, adding to excavation and soil disposal costs. Plus, he would have the added expense of bringing in a crane to set the tank in place, since his excavator would lack the reach to do that.

Less Time
It takes longer, of course, to install a slide-rail system than simply to excavate a similar-size hole. But it’s still about twice as fast as installing sheet piling, Bogan notes. He describes one of his recent slide-rail projects involving a 42- by 42-foot pit to accommodate three fuel tanks. “It took us about five days to install the system and set the tanks, and two days to pull it all back up and load the components on a truck,” he says. “I’m very satisfied that we could do all that in that amount of time.”

Less Work
Bogan rents slide-rail systems for his projects from Pro-Tec.

“With all the different configurations we need, it wouldn’t be cost-effective to own the various systems we’d need,” he explains.

He provides Pro-Tec with the dimensions of the excavation project, and Pro-Tec provides him with installation drawings and a technician who lends onsite expertise for safe and proper installation. “The technician tells you where to push on the panels and posts when pressing them down and how to keep everything level and square while you’re building the system,” Bogan says. “Installation doesn’t require any special training on our part.”

Less Equipment
Installation doesn’t require any extra equipment, either. Depending on the size of the system, he can position all the components and dismantle the system using his Caterpillar 330 or 345 excavator. That contrasts to sheet piling, which requires a crane and boom plus the services of welders to build the internal framing.

More Profits
The do-it-yourself slide-rail system eliminates the need for a subcontractor to install the sheet piling. That pays off for the Oscar W. Larson Co. and the client, Bogan reports. “It can save our customers a boatload of money; plus it gives us more work to do to increase our profits on the project,” he says.

Solving Tight Trenching Problems
GME’s line of trenching shielding and shoring products includes Inner City Linear Shoring, a slide-rail system developed by Emunds + Staudinger, for tight-space applications. Features include a rigid roller spreader. Unlike permanently hinged spreaders, the roller units keep the vertical rails and shoring panels a fixed distance apart in the trench to maintain the same desired trench width from the top to the bottom of the trench throughout the installation process.

“With hinged spreaders, the trench can become a few centimeters narrower or wider as the rails are lowered or lifted, leaving a small cavity between the earth and the panels,” says Matt Vogel, a sales manager with GME of Union City, MI. “The roller-spreader ensures that everything is aligned linearly, always at exactly the same distance from the opposite side during all phases of installation. This allows you to work faster, more efficiently, and more accurately while cutting costs.”

The panels of this system can also be swung into position between the rails from the side at ground level instead of sliding them in from above, he notes. “This can be an advantage in inner-city applications where overhead cables can be an obstacle when lifting the panels in place with an excavator,” Vogel says. “This side placement also makes it easier to use longer rails for deeper applications.”

Build Your Own Box
Maybe you don’t have equipment big enough to handle the size of a conventional trench box required for a job. Or, perhaps, the box you have may not be appropriate for a particular project. Then, a modular aluminum trench shield may make sense. This strong, lightweight, engineered shielding features spreaders and panels of different sizes, which can be assembled onsite in modules to build a trench box. Depending on the manufacturer, they can be configured with two, three, or four sides. “They’re popular with municipalities and utility companies for line installation and repair work,” says Scott Sessler, sales and rental manager for American Shoring Inc. in Newburgh, NY.

For example, his company makes the two-sided Mighty Lite modular soil support system. It offers a choice of three spreader systems—manually adjustable screw jack, pinned-in-place pipe spreaders, or telescopic steel square box spreaders. The panels, either 2.5 or 3.5 inches thick, are available in 2-, 4-, 6-, or 8-foot heights in nine standard lengths from 2 to 12 feet. Individual modules can be stacked one above another using a tongue-and-groove alignment to achieve the required depth. “The entire system can be hauled in a pickup truck,” Sessler says. “Two people can assemble it onsite by hand in minutes.”

For easy movement of the modular box in a trench, a wheel kit assembly can be added to most models in this line. “The wheel assembly rides on the surface, holding the shoring slightly above the bottom of the trench so that one person can move it by hand in the trench,” Sessler says. “This frees up your equipment for other tasks while workers are in the trench.”