Failure mechanisms for small diameter cast iron water pipes

Abstract

Water companies have received intense adverse publicity recently due to high levels of water wastage resulting in costs to consumers. Extensive surveys and research studies have shown that the majority of this water loss occurs due to burst water distribution pipes, during the winter months. Failure most commonly occurred in cast iron, 100mm diameter pipes with lead run joints. Several mechanisms for these failures have been proposed. It was believed that frost heave loading was the prime factor responsible for these failures. Much of this research was carried out in climates suffering from severe winters, which does not reflect the UK climate. Traffic loading was another popular theory for the failure of the water pipes. However, none of the research carried out to date has been able to conclusively substantiate a single mechanism to be responsible for pipe failure. Until this study a combination of loading effects and various associated pipe defects had not taken place. It was the aim of this thesis to demonstrate that it is a combination of loading effects and associated pipe defects that is responsible for pipe bursts, rather than a single mechanism as proposed so far.

Four main mechanism were investigated; in-pipe water temperature; internal pressure; traffic loading and frost loading. The strength characteristics of cast iron lead run joints were measured by carrying out axial tensile tests on joints exhumed from the ground. A finite element programme (LUSAS), was then used to model the joints and connecting pipe lengths realistically. 3D, elastic computer models were generated in order to apply the four different loading mechanisms. The effect of each mechanism was then analysed and the maximum principal stress was compared to the ultimate tensile strength of the material. Each loading condition could then be examined to see if it brought the pipe close to failure or not. The different loading conditions and defects were then combined to obtain the overall effect. The results showed that a combination of loading and defects is necessary to bring cast iron pipes to failure.

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