A 5-gallon plastic bucket filled with washed stone and a pair of gallon-sized water jugs helped city engineers in Greenville, S.C., decide what kind of drainage system to install on a fast-growing hospital campus. The project engineers and contractors claimed that at least 40 percent of the voids between the stone—specifically, 1 foot of #5-grade stone above and below a corrugated, high-density polyethylene (HDPE) drainage pipe—could be used for water retention. The innovative project would make unnecessary a retention pond, allowing for safe, maximum use of the property. When the team proved it by pouring more than 2 gallons of water into the stone-filled bucket, the decision was made, and the system has been working successfully since it was completed in 2004.

"That demonstration convinced the hospital and the city that this system was the one to use," said Robert Norris, the project manager at the time for Greenville-based Strange Bros. Contractors.

The Patewood Medical Campus of Greenville Hospital System was growing rapidly and in need of more parking. At the time, it had approximately 1,000 physicians, more than 7,500 employees, and 1,146 licensed beds spread among five campuses. With that kind of demand for parking, it was easy to understand why the owner wanted to use the valuable space that would have been taken up by a retention pond.

The corrugated HDPE pipe system was the only solution that could provide the nearly 100,000 cubic feet of water storage capacity required in such a small footprint of just 100 feet by 420 feet. The system is made up of 4,900 feet of 48-inch-diameter Advanced Drainage Systems (ADS) N-12 HDPE pipe, with 1 foot of stone above and below it. The HDPE pipe will support H-25 live loads with 12 inches of minimum cover.

The stormwater drainage system for the Patewood Medical Campus consists of ADS N-12 HDPE pipe that was perforated and surrounded by stone to allow water to slowly drain into the collection unit. Installation of nearly 5,000 feet of 48-inch-diameter HDPE pipe was completed in less than two weeks.

The pipe was perforated with 3/8-inch holes all around using a standard pattern and wrapped in 6-ounce, non-woven geotextile fabric, as is the entire system. Wrapping each pipe prevents sediment from lodging in the stone voids. Enclosing the entire system in fabric prevents the migration of fines from the native soil into the select backfill material.

"Using voids in the stone allows you to reduce the footage and/or the diameter of the pipe, with the ultimate goal to achieve more volume for less money," said Norris.

In a heavy rain event, the perforations in the pipe allow the collected stormwater to leach into the stone layers, where it is held before slowly draining back into the corrugated HDPE pipe that makes up the retention unit via smaller drain pipes in the bottom native soil backfill.

Because HDPE pipe will maintain its structural integrity and its surface will not be damaged when the perforation holes are drilled, it is the ideal product. Steel pipe has a coating that would be compromised when drilled, leading to more rapid corrosion of the pipe; it is not practical to drill concrete pipe.

"The Patewood project is an example of the creative solution driven by both engineering and economics," said Tony Radoszewski, executive director of the Plastics Pipe Institute (PPI). "And environmental reasons can be added to that list."

Perforated pipe is typically used in retention systems so that the stored stormwater runoff can recharge groundwater. A discharge outlet is designed into the system to limit the flow rate into the receiving sewers or channels.

"Specifications at the time required the first inch of runoff in any storm to be held and allowed to recharge the groundwater, and this system is designed for that," said R. Michael Batie, P.E., CFM, technical and engineering manager for the PPI.

According to Batie, this solution could become commonplace nationwide as open land becomes scarcer and land use becomes more dense. "A system like this returns more water into the ground than any other stormwater system that could be installed," Batie explained. "That’s not something that people think much about when population growth keeps pushing more and more into our former undeveloped open lands, but it’s very important."

Radoszewski explained that because components for systems like the one installed at Patewood are engineered for retention, they easily fit together. The pipe and components are lightweight to help speed handling and placement. Joints are easy-to-assemble standard designs, making installation a rapid process. Contractors spend their time completing the system, not making field modifications to the product.

"The ability to design and manufacture a system with the exact pipe segments and fittings ahead of excavation makes installation a snap," Radoszewski emphasized.

The bulk of the work was completed in just a three-week span, according to Strange Bros. Project Superintendent Jay Clark. "We were able to do this job with minimal labor and minimal equipment," said Clark, who installed the pipe system in less than two weeks and had the entire job completed (excavation, stone bedding, and backfilling) in about a month. "With just myself, two other men, a loader, and a trackhoe, our best day was putting down 720 feet of pipe. On average, we would do between 400 and 600 feet a day.

"For this application and situation, HDPE pipe was the only kind we could have used," Clark said. "The pipe holds water, the stone holds water, and putting it underground saves the valuable property space. Making this choice had a lot to do with downstream capacity capabilities and our need to have a system that would gradually let the water out and still have enough capacity to handle a flash flood situation."

David Elliott of Design Strategies and the lead engineer on the project said there were three main obstacles to overcome and convince the city to approve this system—the holding capacity of the stone, the potential for fines to contaminate those voids, and "of all things, the light pole installation. With the water capacity issue well covered, the light issue, however, was an easy one."

To install large light poles for the parking lots, 8-foot- to 10-foot-deep cylindrical foundations are typically used to anchor the structures. But since that would compromise the geotextile wrap, not to mention the HDPE pipe system itself, Elliott recommended spread footings to support the light poles.

"I would call what was done at Patewood a new use of established technology," Elliott said. "In this area, it’s still the exception, not the rule. But we’re seeing changes in the way reviewing agencies feel about retention ponds. They used to see them as a very good thing, but that’s starting to change. Ponds take up too much valuable land with no other usable purpose. At Patewood, HDPE pipe was the solution."

This article was contributed by the Plastics Pipe Institute, Inc. (PPI), a Texas-based, not-for-profit trade association representing all segments of the plastic piping industry. PPI provides educational sessions, materials, and expert testimonials. Additional information is available online at www.plasticpipe.org.