Large Diameter Sewer Pipe

Part of concrete structure remains

Due to very limited construction space, District engineers sought a solution that would keep the undamaged lower portion of the concrete culvert in place. The roof and about 4 inches (10 centimeters) of the upper sidewalls would be removed, leaving a U-shaped concrete channel.

The sewer line in need of repair conveys about 50 million gallons (189,270 kiloliters) of wastewater on an average day. To keep a 24/7 flow moving through the treatment plant, engineers designed a by-pass system using pumps and 3.8 miles (6 kilometers) of flexible thermoplastic pipe. To reduce costs and the potential for problems, engineers specified that the temporary by-pass be in service for as little time as possible.

Composite pipe designed by Belco specifically for the project offered the best solution for speed of installation. The custom pipe fit precisely into the U-shaped channel which served as a trench. A concrete grout was injected between the trench inner wall and pipe outer wall. Cables held the pipe in place to keep the pipe from getting buoyant in the wet grout before it cured.

The pipe was joined together using butt-and-strap composite joints that were applied internally. Installation during October through December required the use of a plastic tent around joint sites to shelter the newly laminated joints against cooler temperatures which could retard cure. The Vipel resin formulated by AOC technical service performed well under these circumstances.

Longer lengths and lighter weight

“We considered topping the remaining concrete with a new precast concrete ‘cover’ that would be PVC-lined to protect the concrete from corrosive attack,” said Ted Knutzen, engineer for URS Corporation, the engineering design firm for the project. “But because composite pipe could be made in longer lengths, the composite provided the opportunity to reduce installation time.”

New precast concrete would have been made in 8-foot (2.4-meter) long sections. In contrast, the composite pipe was manufactured in 28-foot (8.5-meter) sections. “With one composite length for every three to four concrete lengths, composite piping reduced the number of joints, resulting in less installation time,” said Guy Gentry, Engineering Manager for pipe manufacturer Belco Manufacturing. The lighter weight of the composite made it easier to handle and allowed for the use of less expensive equipment, added Gentry. Composite density is about 120 pounds/cubic foot (1,922 kilograms/cubic meter), and the nominal pipe wall thicknesses are only 1.375 inches (3.5 centimeters) thick. Concrete density is about 400 pounds/cubic foot (6,400 kilograms/cubic meter), and wall thicknesses are typically 8 to 10 inches (20 to 25 centimeters) thick.

General contractor Stanek Constructors, Golden, Colorado, completed the composite pipe installation within the 52 days specified for the by-pass. The pipe was supplied through Process Equipment Corporation, a Belco Manufacturing representative located in Boulder, Colorado.