Wastewater Pipe Condition and Assessment

Wastewater Pipe Condition and Assessment

Assessment of pressure pipe such as water lines or sewage force mains often require special techniques to determine condition or the “probability of failure.” There are several technologies available for performing these assessments – technologies based on pipe size, pipe material, and whether the pipe can be exposed or not.

It is often necessary to perform the condition assessment of a force main through non-destructive testing methods to avoid costly bypass pumping. Recently, Wright-Pierce worked with the Town of South Windsor, CT to perform an internal and external inspection of the Clark Street pump station force main using various techniques, after a collapse occurred just downstream of the force main discharge location due to hydrogen sulfide attack. The force main was a 40-year old, 16-inch, pre-stressed concrete cylinder pipe.

The first step in evaluating the structural integrity of the force main included an internal, non-destructive inspection utilizing the “SmartBall” technology by Pure Technologies. The SmartBall is an acoustic-based technology that detects acoustics associated with leaks or pockets of trapped gas in pressurized pipes. It consists of an outer foam shell and aluminum core that contains electronic components, instrumentation, and a power source. The SmartBall is tracked using a receiver to locate the exact location and position of identified defects in the pipeline.

The SmartBall has many advantages including: the ability to accurately detect leaks in pipes greater than 12 inches in diameter, on a variety of pipe materials, and with high sensitivities; the ability to record acoustic data for long periods of time requiring only a single deployment of the SmartBall; and the ability to be conducted in “live” flow avoiding the need for long periods of shut-down or bypass pumping.

For the Clark Street pump station force main, the results by Pure Technologies concluded that there were no acoustic parameters that resembled leaks or pockets of trapped gas.

The external inspection consisted of soil resistivity testing at varying depths along the 5,600 feet of force main and non- evasive direct assessment of the pipe to identify possible corrosion points. Techniques consisted of pH testing, mortar sounding and Schmidt hardness testing, impact echo thickness testing, and laboratory testing of pipe samples. The results of the external pipe inspection indicated that the soils were non-corrosive and no significant external corrosion was observed at the exposed pipe locations. The concrete compression strength values taken of the pipe material indicated some softening of the concrete matrix.

Based on the inspection, the Town was able to replace the last 80 feet of pipe before failure, and hold off on any additional replacement or rehabilitation of the remaining force main for 10 years. They will, however, continue to monitor its condition periodically.

Given the criticality of wastewater force mains, utilities need to consider a strategy for performing a condition assessment on each of their existing force mains.

Wright-Pierce has the capability and experience to evaluate and use unique technologies to assess wastewater force mains.