Framework for Analyzing Cast Iron Water Main Fractures due to Moisture-Induced Soil Expansion

Abstract

Cast iron pipe break rates in North American water distribution systems have increased significantly in the last six years (a 43% increase with respect to 2012). Of the numerous mechanisms that are responsible for these breaks, pipe fracture data from the City of Sacramento indicates that corrosion induced damage followed by pipe flexure due to moisture-induced soil expansion is one of the dominant mechanisms. This mechanism results in full-circle breaks in which fracture occurs transverse to the pipe axis. This paper presents an analytical model to predict such fractures, given a range of parameters that describe pipe configuration, soil conditions, and triggering factors, such as soil saturation, that leads to expansion. The model is based on (1) classical solutions for beams on elastic foundations that are enriched to reflect material nonlinearities in the soil medium, and (2) a corrosion equation to estimate pitting damage in the pipe wall. The model development and validation are supported by a suite of continuum finite-element simulations that simulate detailed interactions between the pipe and soil. The prospective use of the model is outlined in the context of decision-support frameworks to identify pipe segments at a high-risk of fracture. The limitations of the approach are discussed.