Case Study: Repurposing an Abandoned 3.2 Mile 66-Inch RCP Gravity Sewer as a New Force Main

The Orange County Water District (OCWD) and the Orange County Sanitation District (OCSD) are partners in the groundwater replenishment system (GWRS) project, an innovative, award-winning water supply program to maximize local resources. The GWRS project provides an industry-leading example of how utilities can repurpose aging infrastructure into new services, extending its useful life and saving valuable financial and environmental resources. The GWRS final expansion project is currently underway. A critical component of the $292 million expansion is the repurposing of 15,750 linear feet (LF) of abandoned 66-in. reinforced concrete pipe (RCP) gravity sewer as a new force main to convey additional secondary effluent to the GWRS advanced water treatment plant, called the Plant 2 Secondary Effluent Pipeline. The existing abandoned sewer, constructed in 1957, runs parallel to the Santa Ana River and immediately adjacent to the river levee. Two other large diameter critical active sewer lines (120- and 84-in.) lie in close proximity within the same narrow easement. OCSD required the new force main to be capable of reverting back to serve as a gravity sewer during a major storm event. A condition assessment of the existing gravity sewer was performed which included laser scanning and the use of a virtual mandrel to determine the largest diameter pipe that could be inserted, thereby maximizing hydraulic capacity and minimizing annual pumping cost. The rehabilitation technique ultimately selected was to provide a fully structural liner with either fiber-reinforced polymer (FRP) or high-density polyethylene (HDPE). A steel pipe liner section was ultimately selected for the 300 LF siphon due to existing site constraints. This lining system will provide the desired minimum 50-year service life. This paper will provide an example to other utilities of how existing aging infrastructure can be successfully repurposed. Readers will learn how constructability and cost are used to select rehabilitation options and how to optimize performance of the finished pipeline by incorporating key elements into design.