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, structure deterioration, and low scouring velocity. Below the minimum self-cleaning velocity of two (2) feet per second, gas pockets form more readily, and where gas accumulates against the pipe wall, sulfide corrosion gradually deteriorates the pipe and its appurtenances.
DC Water project description
The Anacostia Force Main (AFM) in Washington, D.C. is a 33,500 linear foot, 108-inch prestressed concrete cylinder pipe (PCCP) constructed in phases between 1969 and 1973, conveying approximately 200 million gallons per day. A pipeline failure in 2018, attributed to corrosion from gas pockets forming under reduced average daily flow rates, increased the risk of operating and maintaining this critical asset. The size of the force main and the limited access points make it difficult to inspect. The purpose of this assessment was to detect and locate leaks, gas and air pockets, and magnetic anomalies indicating metallic appurtenances, material changes, pipe joints, external repairs, and other constructed features. The assessment was performed in partnership with Brown and Caldwell and RJN Group, Inc.
Operational description
The Pipers® were inserted through a 4-inch air release valve (ARV) just downstream of a primary pumping facility. Flow rates were known to be below the capacity of the 108-inch force main. The force main is therefore not completely full and is not under pressure at this location, so the ARV was temporarily removed and the Pipers® were inserted directly into the force main without stopping the flow. The Pipers® were captured in two downstream tunnels using metal bar screens lowered into the stop-log grooves, and a steel plate diversion gate was inserted into the outfall structure to divert flow toward the tunnels and facilitate retrieval.
Inspection tool
Pipers®, INGU’s free-floating multi-sensor tools, provide an in-service condition assessment of force mains. In a single inspection, Pipers® identify gas pockets and leaks, determine the hydraulic grade line, and perform a magnetic inspection. The free-floating design allows Pipers® to navigate the force main’s flow without getting stuck in air release valves, tees, or deposits. Pipers® were selected for the initial inspection of the AFM because they are neutrally buoyant, detect gas pockets, and are cost effective.
Project results
No leaks were detected within the sensitivity of the free-floating Pipers®. Nineteen (19) anomalies indicating gas or air in the force main were identified. Twelve (12) carry the spectral signature of localized gas pockets, and the remaining seven (7) show repeating signatures typical of gas pockets along an extended length. Three (3) of the identified gas pockets coincide with locations predicted to have gas pockets by the DC Water hydraulic model. At one of these locations, a pole-mounted camera inspection and a drone inspection confirmed open channel flow and pipe wall deterioration consistent with biogenic corrosion.
Twelve (12) magnetic features were identified. Three (3) correspond with pipe joints and four (4) are near isolation valves or ARVs. The remaining five (5) magnetic features do not align with any known force main feature. One documented feature is a 72-inch gate valve. The Pipers® detected a potential valve approximately 850 feet downstream of the recorded location. The separation is too large to correlate the two directly, and the detection provides a reference point for future investigation.
Recommendations
The project partners have recommended follow-up inspection at the identified gas pocket locations, and at locations where the force main is known or anticipated to flow partially filled, to document biogenic corrosion and the associated wall loss. The resulting data will contribute to the structural condition assessment of the AFM. They also recommended field verification of the magnetic features, including excavation where inline valves are suspected.
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