A five-gallon plastic bucket filled with washed stone and a pair of gallon-sized water jugs helped city engineers in Greenville, SC, decide what kind of drainage system to install on a fast-growing hospital campus.
The project engineers and contractors claimed that 40 percent of the space in the voids between the stone – specifically, a foot of No. 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 a retention pond unnecessary, allowing for safe, maximum use of the property.
When the team proved it by pouring more than two gallons of water into the stone-filled bucket, the decision was made, and the system has been working successfully ever since.
“That demonstration convinced the hospital and the city that this system was the one to use,” says Robert Norris, project manager with Greenville-based Strange Bros. Contractors.
The Patewood Medical Campus of Greenville Hospital System was growing rapidly and in need of more parking. With approximately 1,000 physicians, more than 7,500 employees and 1,146 licensed beds spread among five campuses, it was easy to understand why the owner wanted to use the valuable space for parking 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 Advanced Drainage Systems (ADS) N-12® HDPE pipe with 1 foot of stone above and below it. 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,” says Norris.
In a heavy rain event, the perforations in the pipe allow the collected storm water 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.
HDPE pipe is preferred in this application because it will maintain its structural integrity and its surface will not be damaged when the perforation holes are drilled. Steel pipe has a coating that can be compromised when drilled, leading to a more rapid corrosion of the pipe, and it is not practical to drill concrete pipe.
“There are solutions to underground storage systems that can only be accomplished due to the versatility of HDPE pipe and the creativity of those in our industry,” says Tony Radoszewski, executive director of the Plastics Pipe Institute (PPI).
“The Patewood project is an example of the creative solution driven by both engineering and economics,” Radoszewski states. “And environmental reasons can be added to that list.”
Perforated pipe is typically used in retention systems so that the stored storm water 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,” says R. Michael Batie, P.E., CFM, technical and engineering manager for the PPI.
According to Batie, this innovative 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 storm water system that could be installed,” Batie explains. “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 emphasizes.
The bulk of the work was completed in just a three-week span, according to Jay Clark, Strange Bros. project superintendent.
“We were able to do this job with minimal labor and minimal equipment,” says 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 says. “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. 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 says. “In this area, it's still the exception, not the rule. However, 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.”
Editor's note: For additional information about the Plastics Pipe Institute, Inc. visit www.plasticpipe.org or call 469-499-1044.