Large-Diameter RCP Condition Assessment Program — A Progressive Approach

Large diameter sewers are often the backbone of a municipal wastewater conveyance system. Trunk sewers, interceptors, and transfer sewers form the network by which sewage from the customers is collected and transported to the appropriate treatment facility. This method of centralized collection has been the standard for centuries. In the United States, many of these large diameter sewers are constructed of reinforced concrete pipe (RCP) and have been in service well past their design life. As a result, headlines of sinkholes, pipe collapses, and overflows when these sewers fail have become more frequent. Such failures have discharged up to 10 billion gallons of raw sewage into natural waters, cost cities up to $35 million in fines, and have attracted the attention of major public media (USA Today, 5/8/2008). To prevent similar problems from occurring in their systems, many municipalities are implementing condition assessment programs to inspect their large diameter sewers, with the intent of addressing problem areas before they become worse. Condition assessments of very large transfer sewers involves higher flows, elevated levels of H₂S, wider easement requirements, and a sensitivity to the elevated system dependence on these large sewers. Conventional methods of condition assessment that utilize primarily CCTV inspections can be overly expensive and inefficient. A multi-faceted and tiered approach that includes both conventional and higher-level technologies can be more cost-effective and thorough. This progressive approach will also reserve the most detailed, thorough, and expensive inspection methods for the segments that need it most, thus avoiding unnecessary inspection and evaluation costs. This paper will present a explanation of the goals, methods, and results of the progressive approach and is based on experience from several progressive large-diameter RCP assessment programs representing more than 35,000 feet, ranging in size from 36-inch to 84-inch diameters.