Condition Assessment and Repair of a 40-Year Old Aqueduct

The Providence Water Supply Board (PWSB) Supplemental Tunnel and Aqueduct is about 9 mile long and made with 78 in. and 102 in. diameter prestressed concrete cylinder pipe (PCCP). The pipeline was built between 1966 and 1968 as a part of the Providence water supply system, providing drinking water for Providence, Cranston, Warwick, and other surrounding towns. The pipes are embedded cylinder type (ECP) without shorting straps and are manufactured by Interpace. The 78 in. diameter pipeline has not experienced any failures to date, and the 102 in. diameter pipe failed in 1996. PWSB requested a pipeline condition assessment as a proactive approach to identify and repair high-risk pipes in order to maintain acceptable reliability of the aqueduct. This paper presents the method of approach that included pipeline preparation for internal inspection, internal visual and sounding and electromagnetic inspection, pipe failure risk analysis, external inspection and repairs by post-tensioning and encasement. The results of electromagnetic inspection indicated that about 5% of 102 in. diameter and 7.3% of 78 in. diameter pipes had broken wires, slightly more than in other similar pipelines. Pipe distress was predominantly in the form of corroded and broken prestressing wires and partially dissolved mortar coating, likely as a result of acidic ground water and soil. Failure risk analysis and external inspection identified seven pipes at a high risk of failure that needed to be repaired. The pipes were repaired externally, using post-tensioning or encasement, depending on the internal water pressure and external earth load. At least one pipe was in imminent danger of failure and would have ruptured by now had it not been repaired.