Case Study: Ductile Iron Forcemain Failures - Inspection & Rehabilitation

Abstract

Ductile iron is a common material for sewage forcemains. However, several forcemain failures on ductile iron pipes occurred in a short period of time in Vancouver. Coincidence or not? Investigations of the breaks showed a similar failure mechanism - internal corrosion on the pipe invert. It is hypothesized that build-up of debris on the pipe invert (exacerbated by low velocities) caused an anoxic environment to form, proliferating sulphate-reducing bacteria that in turn caused the failure of the cement mortar lining and subsequent corrosion of the ductile iron pipe wall. The pipes in question ranged in size from 250 to 900 mm, and in age from 25 to 45 years. Similar failures are rare but have been documented in other jurisdictions. Potential causes, investigation technologies, and trenchless rehabilitation options are discussed. CCTV inspection and in-line Remote Field Testing (RFT) technology were used to assess pipe lining condition and remaining wall thickness. Trenchless rehabilitation methods explored include CIPP, sliplining, swagelining, and pipe splitting.