One Liner to Rule Them All: King County Interbay 48-Inch Sewer Force Main Rehabilitation Story

This paper explores the unique performance parameters of a pressure sewer rehabilitation (including significant vacuum conditions), the currently available rehabilitation alternatives and their limitations, and industry claims versus installation history. Pipeline rehabilitation is a common industry practice for gravity systems and small diameter pressure applications. What makes this project unique is the size, design pressures parameters, and internal diameter constraints for a critical asset, leading to the need for a rehabilitation alternative with a proven track record. King County’s Interbay force main sewer system was constructed in 1966–67 and consists of dual 36-inch steel pipes and dual 48-inch reinforced concrete pipes (RCP) between the Interbay Pump Station and the force main discharge structure. The overall rated capacity of the station is 133 million gallons per day (mgd) with both force mains on-line, but limited to 130 mgd using station controls. With only one force main online, the maximum flow is 78 mgd. Inspections of the 48-inch RCP mains indicate that the RCP has a remaining service life of 10–20 years, leading to the need for rehabilitation to extend the design life by 50 years. Rehabilitation alternatives were evaluated on a range of technical performance requirements, focusing on the hydraulic capacity and maximum and minimum pressures. Selection of cured-in-place-pipe (CIPP) lining alternatives were further compared against available resin alternatives (such as “green”, vinyl ester, and epoxy resins). The finding was that only structural CIPP hot water cure with epoxy resin has the proven installation track record for this specific size and operating condition.