Forensic Evaluation of PCCP Failures: Green Bay Water Utilities Case Study

The Green Bay Water Utility (GBWU) conducted an electromagnetic inspection in 2010 of three 36 inch diameter water transmission mains (about 25 miles total length) that deliver water between the water treatment plant and the distribution system. The pipelines were installed in 1953, 1965, 1969, and 1974 and were, therefore, manufactured according to different American Water Works Association (AWWA) standards. The results of the electromagnetic inspection in 2010 identified 20 pipe segments that displayed evidence of 5 to 15 broken wires. In December 2012, a pipe failure occurred on the 1974 pipeline at one of the locations identified in the electromagnetic inspection as having broken wires. The GBWU recognized the need to identify the cause of failure and evaluate the potential for additional failures by conducting a forensic evaluation. The first phase of the investigation included inspection of the failed pipe, petrographic analysis and testing of mortar coating, testing of prestressing wires and steel cylinder, soil corrosivity testing, structural and failure risk analysis for pipes with broken wires. In addition, to assess the risk of failure of other pipes, external inspection and half-cell potential testing were conducted on five additional pipes in place and the alignment was evaluated for potential stray currents, soil, and groundwater corrosivity. Interconnects between the two pipelines were identified as the first priority for managing the pipelines, and the construction of new valves and interconnects was completed in 2016. This allowed segments of the pipe to be taken out of service to replace those pipe sections with broken wires and provided flexibility in operations in case of a future failure. Removal of the additional pipe segments with broken wires was completed in February 2017, and these segments were evaluated by removing the mortar coating and inspecting the prestressing wires and cylinder. This investigation identified one pipe segment that was at the stage of imminent failure, other segments with no broken wires, evidence of corrosion on adjacent pipe segments, and wires broken from corrosion and from embrittlement. This paper presents the forensic analysis results that identified the likely causes of failure, an evaluation of pipe segments removed, and the options for effectively managing the pipeline in the future.