The Sewer Authority Mid-Coastside (SAM) transports and treats wastewater from several coastal communities in San Mateo County, California. The Princeton Force Main (PFM) conveys wastewater from the Princeton Pump Station to SAM’s treatment plant. It is a 40-year old 8-inch (0.2m) diameter ductile iron pipe. SAM conducted a condition assessment on the PFM to determine if it can provide reliable services for the next 60 years to reach the typical 100-year service life assumed for ductile iron pipes.
SAM’s team performed initially a desktop assessment that included hydraulic modelling of the PFM and researching geologic conditions nearby. The desktop assessment did not find any adverse hydraulic or geological factors that could impact PFM’s service life. The next step was to excavate test pits in three locations along the PFM alignment to assess the physical condition of the pipe. Since the Princeton Pump Station could not be removed from service, non-destructive testing methods were used to determine the remaining wall thickness of the pipe including ultrasonic thickness testing and electromagnetic scanning. Soil samples and groundwater samples were also taken from the test pits to evaluate their corrosivity.
The assessment results showed that the pipeline is in good overall conditions, and its surrounding soil and groundwater are not corrosive. The majority of the wall loss percentage was between 18% and 30%, and the highest wall loss percentage is 49%. All measurements of wall thickness were higher than the minimum required design thickness of 0.04 inches (0.001m) given PFM’s working pressure. Using the lowest measured wall thickness and assuming a linear annual wall loss, it would take over thirty years for the amount of wall loss to reach the minimum required design thickness. Considering the abundance of conservative assumptions, it is realistic to assume the PFM has several decades of remaining service life.