Introduction
Force mains are a pressurized component of gravity sewer systems that transport wastewater to a discharge point. Unfortunately, they are challenging to maintain, inspect, and repair due to the inability to take them offline for extended periods and the limited access points for inspection or maintenance.
Force main risks
A good understanding of the force main and its high-risk locations is an integral part of force main risk management. The vast majority of force main failures can be attributed to three main factors: internal corrosion, structural deterioration, and low sourcing velocity.
- Internal Corrosion. Over 60% of existing force mains are composed of ferrous materials like steel, cast iron, and ductile iron. The primary cause of failure in these force mains is internal corrosion due to hydrogen sulphide (H2S) gas.
- Structural Deterioration. Force mains have air release valves to prevent gas buildup at high points reducing the risk of sulphide corrosion. If an air release valve fails (or is not present), the resulting sulphide corrosion can gradually deteriorate metallic piping and valves, ultimately causing force main failure.
- Low Sourcing Velocity. Too low flow in force mains can lead to deposit build up which will restrict capacity and can be a risk for abrasion over time.
Pipers® inspection tool
Pipers®, INGU’s free-floating inspection tools, provide a comprehensive assessment of force mains. Pipers® effectively identify leaks, gas pockets, determine the hydraulic grade line, and perform a magnetic inspection in a single inspection. The free-floating design allows Pipers® to navigate the force main’s flow without getting stuck in air release valves, tees, or deposits.
- Leak detection. While moving through a pipeline, Pipers® continuously record the relatively quiet flow noise, creating a baseline for the measured sound intensity. When a pipeline is leaking, the jet of liquid passing through the crack or hole generates a characteristic hissing or rushing sound that significantly deviates from the baseline noise in a localized region around the leak.
- Gas pocket detection. The Pipers® record sounds caused by gas pockets, pumps, and other noise sources in the pipeline. Gas pockets are usually louder than the background noise in the pipeline and can be confirmed by the specific spectral signature at its location.
- Magnetic inspection. Pipers® measure the magnetic flux density inside pipelines. These measurements allow for the identification of material changes, pipeline features (e.g. air release valves and casings), and deterioration of metallic piping.
Washington, D.C.'s Anacostia Force Main project description
The Anacostia Force Main (AFM) in Washington, D.C., is a 33,500 linear feet (LF) 108-inch Prestressed Concrete Cylinder Pipe (PCCP), constructed in multiple phases from 1969 to 1973 conveying approximately 200 million gallons per day. A pipeline failure in 2018, assumed to be from corrosion due to gas pockets resulting from reduced average daily flow rates, increased the risk of operating and maintaining this critical asset.
The size of the pipeline and the limited access points make it very difficult to inspect. Pipers® were selected for an initial inspection given that they are neutrally buoyant, able to detect air pockets, and cost effective.
The purpose of the inspection was to:
- Detect and locate leaks.
- Detect and locate gas/air pockets.
- Detect and locate magnetic anomalies that may indicate the presence of metallic appurtenances such as valves, pipe material changes, pipe joints, external repairs, and other constructed features.
Operational description
The largest Pipers® are 2.8-inches in diameter, which was not a problem to insert through a 4-inch ARV just downstream of a primary pumping facility. Flow rates were monitored and known to be substantially less than the capacity of the 108-inch pipeline. Therefore the force main will not be completely filled and is not under pressure at this location, so it is possible to temporarily remove this ARV and insert the Pipers® directly into the force main without stopping the flow.
The Pipers® ended up being captured in two downstream tunnels. Two metal bar screens were fabricated, with bar spacing less than 2.8-inches, that could be lowered into the stop-log grooves. A large steel plate diversion gate would be fabricated and inserted into the large outfall structure to divert the flow towards the tunnels to facilitate easier extraction of the Pipers®.
Project results
Leak detection
Gas/Air pocket detection
Three of the identified gas/air pockets were also predicted by the DC Water Hydraulic Model and coincide with existing air pocket locations. A visual inspection of the AFM’s interior at one of those ARVs was conducted prior to the Pipers® inspection (July 2024) using a pole-mounted zooming camera. The Figure 2 photo to the left provides visual evidence of the presence of a gas/air pocket with open channel flow in the AFM at/near the ARV, along with apparent pipe wall deterioration. The Pipers® detection of this gas/air pocket and its visual confirmation provides evidence of the validity of Pipers inspection data.
The ARV #1 site was inspected again in December 2024 using a camera-equipped drone, to make further visual inspection to further document pipe wall conditions. The Figure 2 photo to the right provides visual evidence of pipe wall deterioration as recorded by the drone’s camera. The outcome of this effort suggests that pipe wall deterioration in this area is likely due to biogenic corrosion. It is a reasonable conclusion that such corrosion may also be occurring at all Pipers® identified gas/air pocket locations.
Magnetic inspection
Twelve magnetic anomalies were detected during the inspection; these anomalies were cross-referenced with known AFM constructed features based on DCW record documents. Three anomalies appear to correspond with pipe joints and are categorized as anomalous joints, while four are near isolation valves or ARVs. The remaining anomalies do not align with any known features and are therefore recommended for further investigation. One particular interest is the identification of a “known” 72-inch gate valve. While an indicative location is documented, the Pipers® detected a potential valve 850 feet downstream. Although the distance is too far to immediately make the correlation, it provides a valuable reference point for future investigation.
Conclusion
Based on the Pipers® inspection results, additional inspections of the AFM at locations identified to have gas/air pockets, or at locations where the force main pipelines are known or anticipated to experience less than full pipe, gravity flow conditions, have been recommended by the project partners. These follow-up inspections will help document evidence of biogenic corrosion and the associated surface deterioration and wall loss. The resulting data will contribute to the structural condition assessment of the AFM. Additionally, the magnetic anomalies identified by the INGU Pipers® should be field verified, including excavating and exposing the AFM where inline valves are suspected to be located.
Pipeline details
Pipeline Length | 33,500 feet |
Pipeline Diameter | 108 inches |
Pipeline Material | Prestressed Concrete Cylinder Pipe (PCCP) |
Content | Wastewater |
Location | Washington, D.C. (US) |
Further reading
The project was executed in collaboration with Brown & Caldwell and the RJN Group who both presented papers on the project at conferences that can be downloaded below.
Resilience and Innovation Safeguarding DC’s Critical Wastewater Infrastructure – Brown & Caldwell – Presented at WEFTEC 2025
Thinking Outside the Box to Inspect a Critical 108-inch Force Main – RJN Group – Presented at UESI Pipeline 2025
INGU’s Pipers® have inspected over 100 force mains across the United States and Canada. These inspections encompassed a wide range of pipeline lengths, from 0.05 to 70 miles, and diameters, from 3 to 108 inches. The force mains inspected were composed of both metallic materials (such as ductile iron and cast iron) and non-metallic materials (such as PVC and HDPE), and were designed for various pressure ranges.