Skip to content


What Makes Large Diameter Sewer Condition Assessments Unique?

What Makes Large Diameter Sewer Condition Assessments Unique?

Performing an inspection and condition assessment of a large diameter sewer pipe in a densely populated urban area requires a specific process. It takes a consultant that has historical knowledge of the area and resources to efficiently conduct a thorough investigation to identify critical elements.

Over the last 20 years, I have conducted and/or managed over 850,000 feet of sewer condition assessments ranging from 8 inches to 156 inches in diameter. During that time, I have discovered large diameter sewer condition assessments (LDCAs) to be unique, especially in a big city. Here are three reasons why:


    A large diameter sewer is a city’s primary artery for collection and can't be shut off during a condition assessment. The best solution? If possible, divert the flow to relief tunnels.

    Diverting the existing and normal dry weather sewage flow to relief tunnels makes it easier to inspect in minimal flow and identify critical defects that are under the sewage waterline. Most importantly, it allows the utility to maintain level of service to its customers. LDCAs performed in drier seasons can potentially save cities thousands of dollars in project costs, maintenance, and upkeep. A thorough investigation will:

    • Give a benchmark for the current conditions of the pipeline and an understanding of the structural needs for future rehabilitation.
    • Assess a pipeline's current operation and maintenance (O&M) practices by determining debris volumes and existing conditions for future O&M locations.
    • Prioritize rehabilitation and identify the sections of pipeline in most need of repair.

    Large diameter sewers are typically designed with access points farther apart than small diameter sewers, pipe bends that are not located at structures, and higher force flows that pull on inspection equipment. Inspection equipment must be durable and intended for submerged sewage use.

    To overcome inspection obstacles, it's crucial to know which technology to use. At minimum, use a multi-sensor inspection technique. Examples include:

    • CCTV video records the current condition of a pipeline through remotely controlled cameras and lights.
    • Sonar navigates through the existing sewer flow to determine debris levels and possibly identify pipe defects under water. It can accurately detect the amount of debris resting on the pipeline's floor and provide an anticipated quantity of material needed to clean out the sewer prior to rehabilitation.
    • Laser Profiling determines if the roundness of the pipes have changed due to traffic, railroad, or live loads acting on the sewer. It can also be used to measure varying cross-sections in pipe that were constructed with primary and secondary flow troughs, egg shaped sewers, and square sewers with chamfers in the corners.
    • H2S Gas Monitoring is typically done in larger diameter sewers where there is a significant draft that moves the air above the sewage surface. If corrosion of facilities is occurring, you could have a H2S gas issue and monitoring could be implemented to determine what the sewer H2S gas levels are.

    Although I recommend using all four methods, CCTV and sonar are the most cost-effective. They allow you to quickly and accurately complete a 360-degree report on all types of conditions, defects, and debris in one pass through a line. And of this can be done without field workers in the pipe, saving municipalities time and money.


    It is not uncommon for pipelines to be overlooked for 30 years at a time. But large diameter sewers should be investigated every 10 years at a minimum to avoid serious, and potentially costly, repercussions. This could include the collapse of a major piece of infrastructure, problematic sanitary and combined sewer overflows, or the replacement of a city's main sewage pipeline.

    Even if an extreme scenario doesn't occur, just cleaning out a large diameter sewer that hasn't been looked at for three decades can be expensive. In this situation, if the pipeline is deteriorating but not failing, rehabilitation should be considered to extend its remaining useful life.

    The key is to have good data collection and evaluation. Make sure you have a field crew that is familiar with modern-day large diameter inspection technologies, the sewer system itself, and is experienced with large diameter sewer environments. It's a proven approach to perform LDCAs correctly and efficiently, and will give cities the confidence to make informed decisions reaching the right solutions.

John Swartzbaugh, PE, Director, Collections and Distribution

John Swartzbaugh, PE 
Director, Collections and Distribution