Abstract
The removal of wastewater from urban areas is an unglamorous but essential activity for society. Each
day, the London wastewater networks convey over four billion litres of wastewater to treatment works.
The sewers of Bazalgette, built in the late 1850s and still in use today, are primarily brick, but cast iron
is used in situations where the wastewater needs to be conveyed across roads, railways, and rivers, or
if the flow needs to be pumped. As with cast iron used for clean water, wastewater pipes are
susceptible to both internal and external corrosion, and in both cases, local conditions can have a
significant effect on the rate of corrosion.
The network has served London with great success for more than 150 years, but previous research
conducted to understand the deterioration of cast iron pipes in the clean water network has raised
concerns about the remaining longevity. Whilst it is not unnatural to recommend a widespread pipe
replacement as the easiest fix to prevent failure, the network is huge and mostly built as integral parts
of railway and road bridges or underneath buildings. Pipe replacement would be capital intensive and
result in a significant service disruption. Therefore, mechanical performance ranking asset
management strategies are needed, which focus on targeted replacement of deteriorated parts of the
network to help prioritise critical work. This necessitates proactive asset management strategies
capable of predicting the residual mechanical performance of individual sewers in the network.
Prior art regarding graphitic corrosion of cast iron, as a leading cause of in-service deterioration, relates
almost exclusively to clean water networks. Hence, Thames Water, in collaboration with the University
of Surrey, has taken a proactive step through this EngD to transfer knowledge from clean to waste
network, to develop models toward predicting the residual mechanical performance of cast iron assets
found in wastewater network, and to conduct proof of concept studies to select cost-effective nondestructive
evaluation (NDE) techniques.
This work has extracted mechanical data needed to accurately predict remaining mechanical
performance and has revised previously developed loss-of-section and fracture mechanics models to
provide a failure envelope for wastewater pipes under combined vertical loading and internal pressure.
This revision accounts for some important parameters such as the operating environment, loading
regimes, internal and external corrosion, which differ, not unexpectedly, from the clean water scenario.
Finally, this work has conducted conceptual studies to adapt magnetostriction, a direct strain
measurement technique, and digital image correlation (DIC), an optical full-strain approach, as two
complementary NDE techniques for cast iron pipes application. These techniques have been validated
using samples instrumented with strain gauges. Detailed finite element models have also been
analysed. It has shown the magnetostriction and DIC could measure the operating loads in cast iron
pipes without significant surface preparation. These could be used as spot-detection and in situ
monitoring techniques for continuing use of Bazalgette’s sewer pipes.