Lining Charlotte's Main 78 Sewer: 64 Feet Deep, ½-Mile Between Manholes, Bridges Over Roads, Busy Shopping Mall, and 57 MGD

Charlotte Water is the largest sewer utility in the Carolinas with approximately 4,500 miles of gravity sewer. Charlotte has been implementing a substantial and aggressive long-term sanitary sewer rehabilitation program to reduce sewer system overflows (SSOs), correct historic maintenance problems, and reduce infiltration and inflow (I/I) into aging sewers. Charlotte's rehabilitation program has a current annual budget of about $25.5 million, including an annual budget of approximately $10 million for small diameter sewer rehabilitation and approximately $6 million for large diameter sewer rehabilitation. Charlotte has been systematically rehabilitating their large diameter sewers over the last 10 years. Charlotte's main trunk sewer entering their largest treatment plant (McAlpine Creek Wastewater Management Facilities) is a 78' concrete sewer installed in the 1960s. The sewer has deteriorated over the years, including significant structural defects/cracks, heavy infiltration into the sewer, and severe hydrogen sulfide corrosion. Failure of this large sewer would cause a major sewer system overflow and be extremely difficult to repair. Charlotte prioritized this 78-inch sewer as their most critical sewer asset in need of rehabilitation. This presentation will discuss the first phase of the 78-inch sewer rehabilitation work that was completed in 2022 for $6.5 million. The 78' sewer is routed through the Carolina Place Mall and big-box store parking lots and crosses two major 4-lane roads. The sewer is 64 feet deep at its midpoint and 32 feet deep at each manhole. The total length of 78' sewer rehabilitated was 2,700 feet, with one section of sewer being 2,200 feet long with no intermediate manholes. Cured-in-place pipe lining (CIPP) was installed in the 78-inch sewer via an 'over-the-hole' wetout installation, meaning the CIPP was wetout (manufactured) right on site and immediately inverted into the 78' sewer. Special CIPP thickness design was required due to the 64-foot depth. The bypass pumping system had a capacity of 57 mgd and included ten 12' pumps manifolded into three 24' force mains over 5,000 feet long each. The bypass piping was routed across two major roads utilizing portable steel truss bridges over the roads -- the first time this was ever approved for such use by NCDOT. This was a major project with many difficult obstacles and challenges. The design issues, construction techniques, and bypass pumping system were very unique and definitely not an everyday type of rehabilitation project. The complex design issues that were encountered that will be presented to the attendees include determining the accurate inside pipe diameter using specialized LIDAR CCTV inspections to profile the pipe (this pipe was severely corroded so the inside diameter was larger than the original pipe diameter), modifying the CIPP thickness design parameters to achieve a design that could be successfully installed (including the soil modulus, short-term flexural modulus and safety factor), utilizing a fiberglass reinforced liner to reduce the CIPP thickness and allow the 2,200-foot section of CIPP to be transported to the jobsite and installed in a single installation (including shipment via a special 94-foot-long trailer with 44 wheels that could be steered independently), and designing a temporary steel truss bridge over a 5-lane and 3-lane road to allow the bypass force main piping to cross the roads (the first time NCDOT ever approved a bridge installation for this purpose). The primary purpose of this presentation will be to thoroughly describe the unique challenges with this project and transfer our lessons learned and unique solutions to the attendees. Large diameter sewers are the most critical sewers in the sewer system as they convey large amounts of flow. Failures can lead to major sanitary sewer overflows (SSOs) and significant environmental impacts. Rehabilitating large diameter sewers is extremely complex and difficult, and the design challenges often seem insurmountable, just like was the case for this project when Charlotte prioritized this sewer for rehabilitation. Sharing our lessons learned, out-of-the box thinking, and unique solutions to difficult design issues will be invaluable to the attendees as they return home to tackle their own difficult, large diameter rehabilitation projects. In addition, several of our design and construction solutions will impact the rehabilitation industry as a whole, in particular in the Southeast where this project was performed. We have demonstrated that substantial CIPP material can be shipped to the jobsite using a special 94-foot-long trailer, whereas shipments were previously limited to a much smaller quantity of CIPP on a much smaller trailer. Understanding that a trailer this large is available will impact how other parts of the country start planning their CIPP designs and installations. Further, we have convinced NCDOT to allow temporary truss bridges over their roads for bypass pumping which will significantly change how rehabilitation projects can be performed. We now have another method to address the difficulty of how to cross DOT roads with bypass piping, and we are already seeing that solution being utilized in other parts of North Carolina. Sharing this solution at a national conference may have wide-ranging impacts across the country.