On the residual strength of ageing cast iron wastewater assets: Models for failure

Abstract

The fitness of ageing cast iron pipe is a problem for both clean water and wastewater networks. Whilst considerable effort has been put into understanding the condition of cast iron mains that carry potable water, wastewater systems have received less attention. Compared with clean water mains, wastewater trunk sewers typically have thinner walls, are often of lesser casting quality, and typically are exposed to a harsher and more complex internal and external environment. It has been established that corrosion is a leading cause of deterioration in cast iron mains. Research has shown that when predicting residual load capacity in clean water main, a loss-of-section approach is not applicable when dealing with corrosion pitting, which is more discrete and notch-like. This research examines the use of both loss-of-section and fracture mechanics models to provide a failure envelope for a ring from a pipe under combined vertical loading and internal pressure. Internal corrosion, defect size, and morphology of this corrosion have a significant effect on the residual strength of the pipe. The failure envelope predicted is rather different to that previously reported for clean water mains, which is not unexpected given the differences in component geometry and operational conditions.