Its Big, Critical and Has Vacuum Issues, Can It Be Rehabilitated? Seattle DN1200 Sewer Forcemain Rehabilitation Story

Abstract

This paper explores the unique performance parameters of a pressure sewer rehabilitation that experiences significant vacuum conditions, the current available alternatives, their limitations and what the rehabilitation manufacturers offer versus what the industry has installed. 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, in Seattle USA, was constructed in 1966-67 and consists of 840m of dual DN1200 reinforced concrete pipes (RCP) between the Interbay Pump Station and the Discharge Structure, with an overall rated capacity of 5.8m3/s. The station collects sewage from the majority of Seattle and a failure would result in significant overflows into the Puget Sound. Inspections of the force mains indicate a remaining service life of 10- 20 years, leading to the need for rehabilitation to meet an extended service life of 50 years. The system pressures include an operating head of 8.5m, shut of head of 27m with a negative surge pressure of - 6.3m. The site poses significant challenges including a short summer construction period, tidal groundwater, historical landfill, adjacent railroad, adjacent armory, works underneath a freeway bridge and the need to keep a single main online at-all-times. The alternatives considered: • Geopolymer • Nonstructural and Structural CIPP Hot Water Cure • Structural CIPP UV Cure • Sliplining • Tight-fit Sliplining • Spiral Wound Pipe Alternatives were evaluated on a range of technical performance requirements, focusing on hydraulic ability to meet the required minimum internal diameter and maximum and minimum pressures. The refined CIPP alternatives were further compared against available resin alternatives considering their performance capabilities, environmental impacts, historical cure performance and the current manufacturer endorsements versus proven installed track record. 1 No-Dig Down Under 2023 Seattle DN1200 Sewer Forcemain Rehabilitation Story Matthew Tooley, Stephen Lindsey