Effects of Corrosion Pits on Wall Stresses in Cast-Iron Water Mains

Abstract

Circumferential cracking or ring fracture has been identified as the most common type of failure mode for cast-iron water mains. However, the cause of circumferential cracking in water mains is not well understood. A number of different postulates exist in the literature on the causes of longitudinal stresses leading to circumferential cracking. The postulates are, however, not proven. This paper presents a finite element analysis to investigate the effect of corrosion pits on the failure mechanism of cast-iron water mains. The study shows that corrosion pits, voids in the bedding, and high relative stiffness of the pipe (with respect to the surrounding soil) can contribute to longitudinal stress development in the pipe wall. Longitudinal stress is generally higher than circumferential stress for a pipe with high relative stiffness. For a particular pipe, longitudinal stress is higher for smaller corrosion pits. Localized forces on the pipe wall from the surrounding ground can increase the longitudinal stress significantly. A localized support equivalent to a spring with a spring constant of 400 N/mm can result in circumferential cracking of pipes. Based on the findings, a failure hypothesis has been provided for cast-iron water mains.