Identifying Factors that Influence the Factor of Safety and Probability of Failure of Large-diameter, Cast Iron Water Mains with a Mechanistic, Stochastic Model: A Case Study in the City of Hamilton

Abstract

Cast iron water mains comprise approximately 28%, by length, of all water mains in Canada and the United States. Many of these cast-iron water mains are reaching the end of their expected service lives and their rate of failure is increasing. Of particular concern are large-diameter water mains, for which the consequences of failure can be severe. The aim of this paper is to use a physical, stochastic model previously developed by the authors to identify the most important factors that influence the probability of failure of large-diameter water mains in the City of Hamilton, Ontario, Canada distribution system. Failure is assumed to occur due to tensile hoop stress and a reduction in strength due to pitting corrosion. The stochastic analysis is implemented using Monte Carlo simulations. A case study of the City of Hamilton water distribution system is presented, in which the model is applied to 20 large-diameter, cast iron water mains. The results suggest that the diameter and burial depth have a moderate to strong influence on the probability of failure. With some exceptions, small-diameter pipes with shallow burial depths were associated with high probabilities of failure and large-diameter pipes deep burial depths were associated with low probabilities of failure. Soil corrosivity and pipe age were found to have a weak effect on probability of failure. There were insufficient samples of pit-cast pipes in the study to draw conclusions on the effect of the casting technique.