Failure modes and mechanisms in cast iron pipe

Failures in cast iron water mains are more complex and diverse than is widely understood in the industry. This paper discusses the modes and causes of pipe failures that have been encountered during a three year investigation by the National Research Council Canada. In addition to corrosion, manufacturing defects, human error and unexpected levels of pipe loading all play a role in the large number of pipe failures that occur each year.

Gray cast iron is the most common material used in North American water systems, representing about 50% of the total length of installed water mains (Kirmeyer, Richards and Smith, 1994). It is also the material that is most prone to failure. A large water utility may experience 300 or more water main breaks a year. Each such main failure causes disruptions to water supply and may require emergency repairs. If a large diameter main fails, millions of dollars of damage may result. Even without a large main failure, the total cost of small diameter water main breaks represents a significant portion of the annual operating budget of most water utilities.

Despite the economic and social significance of water main breaks, little work has been done to analyse the failures and understand the mechanisms that cause them. Past research has concentrated on soil and corrosion behaviour (Romanoff, 1964; Zamanzadeh, 1990; Fitzgerald, 1968). The work that has been done to investigate the failures themselves
has largely been for legal purposes to determine liability for a particular single water main failure.

This has led to a widely held belief that the failure process is a simple one, where a pipe corrodes to the point at which it can no longer withstand the applied internal and external forces, resulting in a main break. 

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