Crossing the creek
To keep construction time and effort to a minimum and maintain alignment, the tunnel would have to “daylight” at Willoughby Creek or cross beneath the creek as a siphon. Maintenance issues prompted the district to choose the former: a 700-foot bridge.
“The only other option was an inverted siphon below grade,” says Hatch Mott MacDonald Senior Vice President Michael Vitale, PE. “But siphons can fill and plug. A bridge also allows the entire 7-mile sewer to flow by gravity with no pumps or siphons.”
Located downstream of Shaft 5, the steel bridge was designed with 102-inch CCFRPM pipe selected primarily for its lower weight.
“The pipe allowed the bridge to be built more economically by reducing the load requirements, which in turn, allowed for less foundation and support structure,” says Christopher Novak, SD1 deputy executive director of operations. “It also provided inherent corrosion protection, which was very important to us.” The material is also UV-resistant.
Accommodating thermal expansion
One challenge was the differential thermal expansion between the steel bridge and the pipe material. The solution was to anchor each 20-foot pipe joint to the bridge with steel cradles and install a special expansion section at one end to take up any movement.
Hobas provided two flanged ends to connect to the expansion section designed by the project engineering team.
Pipe thermal change was accommodated at every joint along the bridge’s 700-foot length. A 100 degree F temperature change would result in 3/8-inch growth or shrinkage in a 20-foot section. Since each joint can accommodate 1 inch of movement, the Hobas FWC couplings served as expansion joints while providing a leak-free connection.
Installing and testing the pipe took two weeks. “Each of the 35 joints was air-tested at 5 psi and held for roughly one minute,” says McNally Kiewit Project Manager Gary Bulla.
“It’s performed extremely well since it was put in service,” says Novak. “The installation went flawlessly. There’s no question in my mind that using CCFRPM pipe was the right decision.”