Targeted Inspections Used to Assess Force Main Condition After Failure

The force main serving the Beardsley Pump Station in the Town of Trumbull, Connecticut was installed in the 1970s. A failure affecting a short section and subsequent emergency repair prompted a targeted investigation of the nearly 2 miles of 20-inch Ductile Iron pipe. This presentation will provide background on the project and an overview of the approaches used to analyze the force main, including: desktop review, ultrasonic thickness testing and Broadband Electro-Magnetics. Approximately two-thirds of the Town's wastewater is conveyed through Beardsley Pump Station and its associated 20-inch Ductile Iron force main. In January 2020, a failure of the force main occurred in Bridgeport within Beardsley Park. An emergency repair was completed, and the pipeline was restored to service. Since that time, Arcadis has supported the Town in a review of the force main condition starting with a forensic evaluation of the failure mode from the January 2020 break. In response to concerns regarding the condition of the Beardsley Force Main, evaluation of the pipeline has continued through a targeted and systematic inspection program including desktop analysis, ultrasonic thickness (UT) testing and Broadband Electro-Magnetics (BEM). A desktop analysis was performed to identify locations that are at risk for corrosion damage for further investigation. In addition, UT testing has been completed at the two Air Release Valve manhole locations as well as two test pits in Beardsley Park. A second low portion of the force main exists in the area of Seltsam Road. To assess the condition of the piping in this location, wall thickness testing was performed at two test pits via ultrasonic thickness testing and Broadband Electro-Magnetics. In addition to the UT testing provided by Corrosion Probe, Arcadis employed the use of BEM technology alongside for verification and comparison of the data obtained. UT testing is used to measure the existing/remaining wall thickness of metallic pipes. UT measurements are based on a transmitter-receiver principle; a signal is sent out by a transducer which is in contact with the external surface of the metal structure with the aid of a gel couplant. The signal goes back and forth through the tested material gradually weakening on each return. The gauge, projecting and receiving the signal through the transducer, measures the time between the signal being projected and received. The transit time of the signal is then correlated to the velocity of the material and graphically expressed as a thickness reading on the gauge. As compared with steel, the grainy graphite and iron matrix substructure in ductile iron piping acoustically disperses the UT signal, making the acquisition of accurate data more challenging. Broadband Electro-Magnetics (BEM) is a non-destructive test (NDT) utilized to evaluate the thickness and detect flaws in ferrous infrastructure. This versatile test can detect flaws on both inner and outer surfaces as well as inclusions, graphitization and fractures of ferrous infrastructure which may occur within the wall of the medium under inspection through the placement and moving of sensor antenna to scan the complete surface area of the pipe. The forensic investigation / failure mechanism analysis completed included laboratory testing of a coupon taken from the section of pipe removed during the emergency repair. Based upon the field inspection and laboratory analysis performed, the failure associated with the Force Main was caused by graphitic corrosion of the ductile iron pipe. The acidic environment at the invert of the pipe was the likely result of a build-up of biosolids and under-deposit corrosion associated with sulfate reducing bacteria (SRB). Once the pipe wall was no longer strong enough to resist the operating pressures of the force main, the pipe fractured. This section of the pipeline is scheduled to be replaced and design for the replacement is underway. The approach followed in this case study is an effective way to assess critical infrastructure without taking the pipeline out of service.