Large Diameter Watermain Condition Assessment and Evaluation

Abstract

The City of Salem, Oregon became concerned about the condition and reliability of their 75-year old, 36-inch diameter water supply main following high water losses and the reoccurring need to repair leaking pipe joints. The pipeline of concern is over seven miles long, consists of both steel and reinforced concrete pressure pipe sections, and is one of two large parallel water mains which supply drinking water to the City of Salem (City). This paper discusses methods used and challenges encountered during the inspection and evaluation of this large diameter pipeline and rationale developed for recommendations to extend the service life of this infrastructure. A comprehensive external and internal inspection of the buried pipeline was conducted by Murray, Smith & Associates (MSA) in the fall of 2009. Several test pits were excavated to expose portions of the pipe for external inspection and assessment. From these excavations, test leads and test stations were connected to the steel pipe to conduct pipeline continuity testing, soil resistivity testing and an over-the-line pipe-to-soil electrical potential survey as part of a corrosion evaluation. Internal pipeline inspection included the use of video equipment to observe pipe joint condition, access port locations, areas of corrosion and other anomalies. The pipeline was inspected for leakage using flow data and sonic leak detection methods. These assessment techniques have efficiently provided the City with a thorough understanding of their supply main's condition, materials, and as-built components. Criteria were developed to prioritize the pipe rehabilitation work, focusing City resources on the highest priority issues. These criteria included the risk to human life and infrastructure due to pipe failure, the cost of water loss, the type of retrofit required, and the accessibility to the pipeline. Several trenchless pipe rehabilitation options were determined to be feasible, as was pipe replacement. By applying these criteria to the evaluation findings, a cost-effective pipe rehabilitation and replacement program may be implemented to lessen the risk of pipeline failure and extend the service life of this valuable City infrastructure. The techniques employed for the City of Salem's pipeline assessment were also used recently on a similar project for the City of Everett, Washington and builds upon prior work completed for the Joint Water Commission, Hillsboro, Oregon; the City of Beaverton, Oregon; and others.