Assessing PCCP Transmission Mains

The Washington Suburban Sanitation Commission (WSSC) has over 400 miles of prestressed concrete cylinder pipe (PCCP) in its transmission network for potable water, 150 miles of which is larger than 36 inches in diameter. WSSC has recently undertaken a condition assessment of several of their large diameter PCCP transmission mains, including the 96-inch Potomac main and the 60-inch North Adelphi Main. The assessments relied on the application of electromagnetic inspections combined with visual and sounding inspections and acoustic monitoring for the North Adelphi Main. Following the inspections, pipes deemed to be in a state of incipient failure were excavated to confirm the findings and repaired as needed. The focus of the inspections was to assess the condition of the prestressing wire for each pipe section, the structural component that provides the strength to PCCP. Electromagnetic inspections were used to test the condition of the wire. This type of inspection requires entering the pipe and traversing its full length with equipment that is capable of electromagnetically testing the prestressing wire wraps. The technology detects breaks in the prestressing wire and estimates the total number of breaks for each pipe section. Visual and sounding inspections are a reliable method of detecting pipes in an advanced state of distress. The visual portion of these inspections looks for indications of distress resulting from a lack of prestressing. The sounding portion of these inspections rely on impacting the interior surface of the pipe wall with a tapping rod and listening for hollow sounds indicative of a lack of prestressing. The inspection of the 96-inch Potomac main identified two pipe sections that required immediate repair. Excavations of these pipe sections revealed that the prestressing wire for these pipes was heavily corroded with numerous wire breaks. These identification and replacement of these pipe sections most likely avoided a catastrophic failure. For the North Adelphi main, following the inspection, a fiber optic sensor was installed in the main to permit continuous acoustic monitoring. The main was returned to service and is being continuously monitored to detect breaks as they occur in the prestressing wire. These wire breaks will be added to the wire break estimates from the electromagnetic inspection to update the wire break estimates for each pipe section. Using this data, the remaining useful life for each pipe section can be estimated.