Composite Sewer Pipe North America's Largest
By Staff | August 01, 2006
A 500-ft/152m corrosion-damaged box culvert installed in 1977 to convey wastewater from primary to secondary treatment facilities at the Denver Metro Wastewater Reclamation District (Denver, Colo.) was recently reclaimed with a composite pipe. The pipe, with an inside diameter of 11 ft, 2 inches (3.4m), was filament wound by Belco Manufacturing Co. (Belton, Texas), using corrosion-resistant Vipel F010 bisphenol A epoxy vinyl ester from AOC (Collierville, Tenn.). It is believed to be the largest diameter composite sewer pipe ever made in North America.
Source: AOC
The original 12-ft wide by 9-ft high (3.7m by 2.7m) culvert was designed to be filled to capacity, but shortly after it was put in service, the operational depth of the conduit was lowered. As a result, naturally decomposing wastewater components released hydrogen sulfide gas into the space between the water line and culvert ceiling. Bacteria ingested the gas, then secreted sulfuric acid that, in turn, attacked the ceiling and unsubmerged sidewall surfaces. Consequently, 35 percent of the 14-inch/36-cm thick roof was destroyed, jeopardizing its structural integrity.
Since construction space was very limited, District engineers sought a solution that would preserve the undamaged lower portion of the concrete culvert, which conveys about 50 million gal (189,270 kiloliters) of wastewater per day. The roof and the upper 4 inches/10 cm of the sidewalls would be removed, leaving a U-shaped concrete channel.
To maintain wastewater flow through the treatment plant during installation of the replacement pipe, engineers designed a bypass system, using pumps and 3.8 miles/6 km of smaller diameter, flexible thermoplastic pipe. To reduce costs and the potential for problems, engineers specified that the bypass be in service for as little time as possible.
Belco designed the pipe to fit precisely into the U-shaped channel, notes Belco's Guy Gentry. Pipe sections were connected with internally applied composite butt-and-strap joints. Because installation took place late in 2005, a plastic tent was erected over joint sites to prevent cooler temperatures from retarding cure. Grout was injected between the trench wall and pipe.
Ted Knutzen, engineer for URS Corp., the project's engineering design firm, says that his firm initially "considered topping the remaining concrete with a new precast concrete 'cover' that would be PVC-lined to protect the concrete from corrosive attack." Since the composite pipe could be made in such long lengths, it could be made and installed in a shorter time, he explains. Knutzen credits Belco for recommending the vinyl ester composite, based on its inherent resistance to the corrosive effects of moisture and sewer gases. "Many of us in civil engineering are more familiar with concrete. It is easier for us to recognize quality in concrete manufacturing and installation," he admits. "To help us better understand composites ... the people at Belco ... provided information, data and photographs."
General contractor Stanek Constructors (Golden, Colo.) completed the composite pipe installation within the 52 days specified for the bypass. Belco manufactured the composite pipe in accordance with the standards of AWWA M-45 and ASME RTP-1.
