On the residual strength of aging cast iron trunk mains: Physically-based models for asset failure

Worldwide, a significant proportion of the large diameter (trunk) mains within water networks are still made of aging cast iron material. With corrosion seeming to be the most significant cause of deterioration in cast iron trunk mains, the traditional structural view of the residual strength of the pipe has been based on the strength of the remaining wall thickness, i.e. a loss-of-section approach. In some situations this may lead to an over-estimate of the residual strength and better predictions can be made using an approach based on fracture mechanics. The present research has shown how loss-of-section models of residual strength can be used alongside fracture mechanics models in a twin approach to provide boundaries to the failure envelope for a “ring element” subjected to combined bending and direct (tensile or compressive) forces. When the application of such a failure envelope to a ring from a pipe under combined vertical loading and internal pressure is considered, it was found that in addition to its size, the angular position of a corrosion defect can have a significant effect on the residual strength of the pipe.