By Greg Northcutt

When American Backhoe Co., of Omaha, NE, won the contract to excavate and install a diesel fuel tank at a new hospital addition in Winnebago, NE, it faced a job that challenged the company's typical approach to this type of work.

The environmental contracting firm specializes in hazardous-waste removal and containment, tank installation, and removal and utility installations. Still, it had never faced a project quite like this. Normally, installing a fuel tank is a pretty straightforward job for the company: Put in a sheet-piling system to prevent cave-ins. Dig the pit. Set the tank in place. Then, remove the shielding and backfill the excavation to finish the project.

In this case, the job called for installing a 15,000-gallon fiberglass tank to power backup generators as part of a US Army Corps of Engineers project to expand a health care facility for the Native American tribes of northeast Nebraska, northwest Iowa, and southeast South Dakota. However, several factors complicated this particular installation:

A tight fit. Installing the 8½-foot-diameter tank, which measured 36 feet long, required a trench shielding system that would provide and maintain an unobstructed opening at least 46 feet long, 16 feet wide, and 16 feet deep. The required excavation would leave just 8 feet of clearance between the edge of the pit and three structures—a retaining wall on one side and a concrete slab for HVAC equipment and a utility building on another.

Nearby utilities. The work also meant protecting a water main. Located 6 feet deep and parallel to the excavation, it ran between the edge of the pit and the concrete slab and utility building. Other adjacent utilities plus railroad tracks and a street restricted access to the site.

Structural concerns. To prevent structural damage to the existing hospital building while protecting such sensitive equipment as computers, imaging devices, and other high-tech hospital systems, no heavy vibrations could be generated while doing the work.

The second of two liner posts is installed for the second bay of the shielding system.

The third and final bay of the shielding system is being excavated to a depth of 16 feet.

Challenging soil conditions. The soil was C-60 weak clay down to 8 feet. Also, as it turned out, the water table was just 9 feet below the ground.

Choosing a Shielding System

Facing these conditions, Wally Kanne, American Backhoe's project superintendent, considered his options for protecting workers from cave-in:

The threat of structural damage from vibrations during installation eliminated the use of a sheet-piling system that would have required a diesel or vibratory hammer to drive the sheets and piles. Even if this approach had been allowed, it would have required a site-specific engineering plan stamped by a Nebraska professional engineer. A beam-and-plate system also would have required an engineering plan for the project plus the added cost of drilling equipment and a crew to install the soldier piles. Also, both approaches are typically most cost-effective for shielding projects that involve both longer and shorter trenchers and longer duration. However, for this job, the shielding system would be in place for only three days.

Kanne turned to Todd Hayes, with United Rentals–Trench Safety Division in Council Bluffs, IA, for advice. The two had worked together on variety of trench- and pit-shoring systems in the past.

Hayes recommended Efficiency Production's Slide Rail System for this particular application. His company has rented this system to contractors for many other successful projects. Also, because United Rentals stocks the system, it would be readily available.

Kanne had never used a slide-rail system but he liked the advantages of this approach. It not only meets the shielding requirements of a steel sheeting or beam-and-plate system, but installation is easier and faster. It can be installed using onsite equipment and a smaller crew. The modular system is also more versatile. It can be adapted easily to fit a variety of excavation sizes and configurations. As a result, rental and installation costs of the slide-rail system can be as much as half the cost of a steel sheeting system, Hayes notes.

Like all trench-protective systems, this slide-rail system has an OSHA-required manufacturer's tabulated data sheet of tables, charts, and other information, which is approved by a registered professional engineer and used to design and construct the system.

"Also, it was important to work with a distributor, such as United Rentals—Trench Safety, who has experience installing slide rails on many projects with various applications and who can provide the equipment within several hours of when we need it," Kanne adds. "We were on a tight schedule and couldn't afford to wait for equipment from a dealer or manufacturers hundreds of miles away."

Installation Procedures

Kanne and his crew installed a three-bay linear slide-rail system, consisting of panels, linear posts, and corner posts. With two 16-foot bays on either end and one 14-foot bay in the middle, it measured 46 feet long and 16 feet wide.

Typical excavation site equipment—a large excavator and a front-end loader—were used to install the system.

Unlike tongue-and-groove posts, unique open-face corner posts of this system provide a margin of error in terms of keeping the system plumb and level during installation. This design minimizes post and panel binding when installing the system down to grade, allowing panels to be inserted easily, even if the system is not truly square.

Unlike sheet piling, in which the sheets are driven to the desired depth before the pit is excavated, slide rails offer a dig-and-push approach in which the pit is excavated as the system is installed. After digging a pilot trench several feet deep, the first 8-foot-high panel is placed in the cut and the ends are inserted into the outer guides of corner or linear posts. Then, one after another, additional panels and posts are installed and connected to enclose the perimeter. Once the system is in the desired location and the posts are perpendicular, the panels are backfilled on the outside to secure the system in place.

Then, excavating and pushing the panels and posts down in 1-foot increments using an excavator bucket and alternating from one end to the other, the system is installed to the bottom of the excavation. When the excavation reaches the 8-foot depth, another panel is inserted into the interior guide of the posts and the dig-and-push process continues until the edge of the bottom panel reaches the desired depth of the excavation. Using either 4- or 8-foot-high panels, this system can be built to depths of 24 feet or more.

The system is removed much the same as it was installed—in increments. After backfilling and compacting in a 2- to 4-foot-high lift, the panels are pulled up and the process is repeated until the entire pit is backfilled and compacted.

A safety officer and Corps of Engineers representatives were on-site overseeing this installation.

A Smooth Process

The completed slide-rail system prior to removal of the spreader assemblies and tank installation.

A slide-rail system was built to install this 15,000-gallon fiberglass fuel tank.

"The panels went in really smooth," Kanne says. "We were able to keep things nice and level and had no problems with binding."

Efficiency's ClearSpan tie-back waler system was used so that the parallel beam spreaders could be pulled out for a continuous unobstructed opening once the entire system was set.

"The wide-flange beams bolt together and pin to the side panels," Kanne explains. "This holds everything together when you remove the spreaders to get a continuous unobstructed opening after the system is set up."

Wide flange beams were also attached to the bottom of the linear posts to prevent the posts from toeing in. These beams were abandoned in place, since the tank installation prevented their removal.

"Other slide-rail systems require a welder on-site and a tremendous amount of time to weld the linear posts to the beams used to prevent the system from cantilevering," Kanne says. "This Efficiency tie-back design with a C-clamp allows multiple slide-rail bays to be built in both directions while working within OSHA regulations."

A Wet Surprise

Not long after starting excavation work, the crew hit ground water at a depth of 9 feet. This water pushed the weak clay soils up from under the shoring system and threatened to undermine the water main and adjacent structures. Three pumps, running 24 hours a day, were able to remove the water to prevent buildup, and sand was filled in under the water main to protect it.

The knife edge on the bottom of the slide-rail system allowed Kanne to push the panels several feet below the excavation level, creating a dam to stop water from flowing into the pit. Once the water was under control, a single pump at the bottom of the excavation was sufficient to prevent water from accumulating. However, dealing with the water and its related problems threw the project behind schedule.

The pressure from the water also affected removal of the panels, which was done with a crane and excavator. "The initial lift was hard," Kanne says. "But, once the hydropressure was off, the panels slid right out."

Assessing the Results

With no prior slide-rail experience, Kanne was hesitant to try the approach at first. But, the results quickly changed his view.

"I was really pleased with it," he says. "This system installs a lot faster and much easier than a sheet-piling or beam-and-plate system. We were able to dig down to grade in just two days. It would have taken six to 10 days to reach grade with a beam-and-plate system. The slide-rail system saved us a lot of time. In fact, using this equipment and working with good people helped get us back on schedule to complete the project on time."

Author Greg Northcutt writes frequently on construction and business issues.

GEC - September/October 2004