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A lesson in reservoir risk management

A lesson in reservoir risk management

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    More than 14,600 feet of solid-wall and perforated, dual-wall corrugated high-density polyethylene (HDPE) pipe provides a French drain-like system that keeps water from exerting pressure on a reservoir's geotextile liner. North Carolina's Franklin Water Treatment Plant Reservoir Expansion is a Plastics Pipe Institute 2011 Project of the Year. Photo: Plastics Pipe Institute

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    The new reservoir on the right is one of $200 million in water and sewer improvements Charlotte-Mecklenburg Utilities makes every year. The map at right shows some of the projects completed since 2000. Photo: Steve Montgomery, Skyshots.net

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By Steve Cooper

PROJECT OVERVIEW:

OWNER:8Charlotte-Mecklenburg Utilities; Charlotte, N.C.
PROJECT:8Franklin Water Treatment Plant Reservoir Expansion
DESIGN:8Hazen and Sawyer P.C.'s Charlotte and Raleigh, N.C., offices
CONSTRUCTION MANAGER:8Thalle Construction Co. Inc.; Hillsborough, N.C.
CONSTRUCTION: January 2010 through May 2011
DESIGN AND CONSTRUCTION: $18.7 million

Charlotte-Mecklenburg Utilities wanted to save money, so managers decided to do something that's becoming increasingly popular: pump water when electricity rates are lowest. Water from Mountain Island Lake is pumped from the Catawba River Pump Station to reservoirs at Franklin Water Treatment Plant in north Charlotte, N.C. The raw water is then gravity fed to both the Franklin facility and to the Vest Water Treatment Plant.

The Franklin plant's two reservoirs were built in the early 1900s and were combined into a single, 250-million-gallon reservoir. Utility managers also found another way to save money on the project: use high-density polyethylene (HDPE) corrugated and solid-wall pipe instead of vitrified clay to protect the geotextile liner that covers the new reservoir's entire bed or concrete pipe to convey collected water. A relatively new application for corrugated HDPE, which is perforated, the pipe collects water seepage to keep the liner from building up enough pressure underneath that would cause it to float. More than 14,640 linear feet comprise the system, which sits under 35 feet of water and 4 feet of fill.

“It basically operates like a French drain,” says George Eller, PE, an engineer for Hazen and Sawyer P.C., which designed the new reservoir.

The system covers the circumference of the reservoir along with a grid pattern of pipe on the inside. The pipe's American Association of State Highway and Transportation Officials (AASHTO) Class II perforation pattern enables water to enter the pipe from any direction and be conveyed to a central collection point and then into twin lines of solid-wall HDPE pipe that run under the dam and to a manhole.

“The underdrain does a couple of things,” says Hazen and Sawyer Project Manager Tony Greiner, PE. “There's a 30-mil impermeable membrane liner, so it shouldn't leak. But if it did, we wouldn't water to build up under there. Plus, we don't want the groundwater underneath building up, so it has to drain off. When the reservoir water level is lower or the reservoir is emptied for periodic maintenance, the system will keep hydrostatic pressure from the groundwater from building up under the liner.”

The pipe's bell-and-spigot configuration provides a secure joint between each 20-foot section and can be quickly installed with minimum labor. The pipe's weight and ability to ‘nest' smaller diameters into larger ones lower shipping costs and make for easy handling.

“We've worked with it many times before in other types of projects, and it's easy to work with,” says Charles da Silva, project engineer for construction manager Thalle Construction Co. Inc. of Hillsborough, N.C. “The installation here went pretty smoothly and the pipe went in well.”

Industry standards and deep burial concerns

For decades, fusible solid-wall HDPE pipe has been specified to provide leak-free joints while corrugated HDPE pipe was used to provide the strength for underground burial. This is one of the first times the two pipe profiles have been used in concert to protect the integrity of a very large reservoir.

The corrugated pipe sits in cut-and-covered trenches and is enveloped in a geotextile fabric that lets water enter the perforations while keeping sand and other fine particles out of the system. Some 3,500 feet of 6-inch and 5,400 feet of 8-inch pipe meeting AAS-HTO M-252 and 1,780 feet of 12-inch and 3,900 feet of 18-inch pipe meeting AASHTO M-294 were used.

“Corrugated HDPE pipe's ability to support and distribute live and dead loads enables it to meet most installation conditions,” says Tony Radoszewski, executive director of the Plastics Pipe Institute (PPI), the largest trade association serving all segments of the industry. “But while there's a significant amount of research and many actual deep burial installations, cover depths of more than 20 feet aren't typical and usually require consultation with the manufacturer. The amount of cover in this 35-foot installation will not cause the pipe to exceed any strength or service limits provided the appropriate backfill material selection and compaction levels are attained.”

The pipe's manufacturer, institute member Lane Enterprises Inc. of Camp Hill, Pa., recommended a well-graded crushed stone backfill compacted to a minimum 95% density per ASTM D698 or AASHTO T99. The installation also conformed to the requirements of ASTM D2321.

Solid-wall pipe was used for the runs under the dam from the collection point to the catch basin. “Whenever we run pipe for a reservoir project we're very concerned about the dam and any undermining,” says Greiner. “It's an embankment dam, so corrosion would be a problem. With HDPE we don't have to worry.”

Over the last 15 years, solid-wall HDPE for directional drilling has become increasingly popular.

“This is especially true in environmentally sensitive areas,” Greiner says. “There was a time when you'd use ductile iron pipe — assemble the line, float it across, sink it, and when it hits bottom that would create a terrible disturbance to the ecostructure. With HDPE you start on one side, directionally bore to the other side, hook on to it, pull it back through your bore hole, and your only disturbance is a receiving pit and a bore pipe on each side of the waterway. There's no damage to the ecosystem.”

Installed using cut and cover, the solid-wall pipe also is the outlet pipe from the corrugated pipe and goes through the dam. It's encased in bentonite-cement grout to provide a seal underneath the bottom of the earthen dam that has a structural height of about 45 feet from the reservoir bed to its crest.

“You can't have anything porous down there that would allow water to migrate, or a product that could compromise the dam,” da Silva says.

“There's another reason this system is good for the water plant,” Greiner says. “If there's some sort of spill out in the river where they draw the raw water, this gives them some holding capacity and a supply of water until the contamination has passed.”

According to the PPI's Radoszewski, customers can expect a service life of 100 years in many typical drainage applications in part because of HDPE's inherent resistance to abrasion and chemicals.

—Cooper (steve@scacommunications.com) is a freelance writer based in New York City.