Replacement of the Outfall Trunk Sewer.
Hobas Pipe USA Replacement of the Outfall Trunk Sewer.

Sioux Falls is the largest city in South Dakota. With population increasing, the community needs infrastructure to grow as well. Since the first sewer was installed in the early 1890s, the Sioux Falls Water Reclamation Division has focused on reconstructing the main sanitary sewer system, replacing an outfall trunk sewer, and adding a pump station parallel force main.

Built in the late 1970s, the outfall trunk sewer consists of 2 miles of 66-inch pipe and is made of reinforced concrete pipe (RCP) with an asphalt/bituminous liner for corrosion protection. After conveying 90% of the city's wastewater for decades, it had become structurally weakened from hydrogen sulfide (H2S). Division managers and employees in the Sioux Falls office of HDR Engineering considered their options, factoring in lifecycle costs, constructability, operation and maintenance, compatibility with third-party products, and longevity.

Relining with cured in place pipe (CIPP) wasn't feasible due to the pipe's condition, extensive bypass pumping required, and limited capacity for future development. Instead, HDR recommended replacement with 72-inch fiberglass reinforced pipe (FRP). In addition to increasing overall system's capacity by 21 million gallons per day (mgd), reducing the risk of backups and sewer overflows, replacement enabled the division to move the line away from a major source of inflow and infiltration.

“The old line is next to the Big Sioux River,” says Public Works Director Mark Cotter, PE. "We moved the new line as far from the river as we logistically could, which also gives us better access to the line in the future.”

Individual joint air tests were conducted to an air pressure of 3.5 pounds per square inch.
Hobas Pipe USA Individual joint air tests were conducted to an air pressure of 3.5 pounds per square inch.

The 9,700-foot project, which included installing an additional siphon pipe under the river, was the last phase of a larger sewer system upgrade encompassing four segments of the central main and three segments of an interceptor. Hobas Pipe USA supplied 8,500 feet of 72-inch centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe installed by open cut. More than 500 feet of 72-inch CCFRPM jacking pipe was used on two crossings -- beneath Cliff Avenue and Interstate 229 -- with South Dakota DOT approving the installation method and pipe materials. The remaining length was smaller-diameter pipe connecting to the city’s equalization basins.

After installation, a mandrel test was performed through the entire line to test for out-of-roundness or deflection after backfilling was complete. Then, a closed-circuit video inspection through the entire line was done as a final inspection for leaks or defects and to document pre-operation conditions. When the video revealed a leaking joint, Hobas repaired the joint by injecting a grout through the pipe wall on each side. Once the joint was sealed on the exterior, the interior side of the joint was filled with resin putty and sanded. Finally, fiberglass lamination sheets were layered over the joint and injection holes to completely seal the joint. Re-inspection found the joint to be water tight.

Construction took approximately 16 months. The contractor was H&W Contracting, which has completed numerous water main, sanitary sewer, and storm sewer projects in the surrounding municipalities.

The division is working to complete a Water Reclamation and Collection System Master Plan to help the city for the next 20 years to 100 years.

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