Re-assessment of resistance of cast iron pipes subjected to vertical loads and internal pressure

Abstract

Longitudinal fracture is one predominant observed failure mode in large diameter cast iron pipes installed between 1850 and early 1960s. Some of these failures occur where there is evidence of some corrosion activity. This paper addresses three specific issues, namely, (1) review of expected loads, i.e., internal pressure, earth loads, traffic loads, (2) estimated pipe failure load (pipe resistance), and (3) estimated structural factor of safety. The second and third issues can be equally applicable to non-deteriorated and deteriorated pipes. The procedures described in this paper were motivated by the need to purposefully use the information derived from pipe inspection to assess the margin of safety of deteriorated large diameter cast iron pipes. A mechanistic model that accounts for cast iron material non-linearity is described. The model is validated using data obtained from experimental tests conducted in the mid 1930s on undamaged (new) pipes. The model is subsequently extended to permit the analysis of pipes with corrosion pits or corroded areas. Reliability analysis using Monte Carlo simulations is proposed to account for uncertainties in input data. Illustrative analyses of 16″ and 48″ diameter pipes subjected to vertical loads and internal pressure showed that significantly high margins of safety exist in cast iron pipes with no damage (due to corrosion and or casting voids) while this margin of safety is meaningfully reduced in the presence of corrosion pits or corroded areas. The methodology described should enable engineers to realistically assess the pipe resistance capacity and hence the corresponding margin of safety of undamaged or damaged cast iron pipes encountered in current operational conditions.