A 16-inch PVC pipe (DR 25) failed catastrophically when a contractor was performing a service one-inch tap. A fracture initiated at the tapping hole and it immediately propagated several feet forming a V-shape hole in the new pipe. American Water thoroughly investigated the event to understand the root cause of the failure and avid future failures. The failure was unexpected. The pipe operated within normal working pressures (approximately 70 psi) and the crew made the two taps successfully just prior to this failure. This paper presents the approach and methodology that American Water took in conducting the forensic study of this pipe failure. The project team began this effort with a comprehensive literature and practice review to understand the failure modes and failure factors of PVC pipes water mains. The team then built a three-dimensional (3D) finite element (FE) model to simulate the process of making the three taps on the pipe, following the location of the taps on the pipe. This numerical simulation helped to know the actual stress level in the pipe with three tapping holes placed at differing points along the pipe length and tapping orientation. A series of lab tests were conducted to confirm the mechanical properties of the pipe to confirm the material was not defective. The numerical simulation results were further compared with the site observations to identify the likely factors contributing to the tapping failure. This research outlines a systematic approach for conducting forensic study of PVC pipe failures. With appropriate modifications, the approach can be used for failure investigation of other pipe materials.
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