Awakening the Sleeping Giant: Inspection of the 66-inch/96-inch Anacostia Force Main Gravity Sewer in Washington DC

The Anacostia Force Main Gravity Sewer (AFMGS) is a 66-inch to 96-inch reinforced concrete pipe (RCP) in Washington DC, which spans approximately 6 miles and includes over 45 manholes. The pipeline begins at the WSSC Water-owned Anacostia Wastewater Pumping Station II and ends at the DC Water-owned Barry Road Screen Chamber, intersecting various properties including National Parks Service, District Department of Transportation (DDOT), and Chessie Seaboard Consolidated Transportation (CSX). Construction of the pipeline occurred in the 1950s over the course of five contracts and was decommissioned in the 1990s when the flow was redirected to the Anacostia Force Main (AFM). This left no redundancy for the AFM, which carries one-third of WSSC Water's total flow to the Blue Plains Advanced Wastewater Treatment Plant in Washington DC. In 2021, DC Water inspected the AFMGS with the main objectives to: (1) assess the current condition of the pipeline, and (2) evaluate cost benefit alternatives to reactivate the pipeline to provide redundancy to the AFM. The inspection of the AFMGS was split into two contracts, led by different consultant teams, where inspection schedules for both contracts were synchronized so the pipeline could be inspected under one tool mobilization to save time and costs. Although the AFMGS is currently out of service, significant effort was spent during the planning phase to understand not only which AFMGS access ports to use for pipeline tool insertion, but also the continuity of the pipeline itself. The AFMGS pipeline profile is undulating with portions of the pipeline functioning as a gravity sewer and other portions functioning as a pressurized force main. V-notch weirs and sloped areas along the alignment had to be considered as could not be travelled by the inspection tool. As the DC Metro area continues to grow, so does the DDOT infrastructure projects for the expansion of major roadways and the rebuilding of their bridges. This in tune has resulted over the years in different portions of the AFMGS alignment being either bulkheaded and flowable filled and/or removed from the ground prompting strategic locations for tool insertions. The pipeline was inspected using the RedZone's Responder Unit to collect a combination of closed-circuit television, LiDAR, gas/temperature conditions, and sonar along accessible segments with water depth, in addition to manhole inspections. Eighteen separate inspection runs were conducted with several abandoned surveys due to a combination of capped pipes, impassible debris, V-notch weirs, and surcharged sections along the decommissioned alignment. The Responder unit requires a minimum clearance of 24-inches to be inserted into an access chamber. Hence, ten manholes were identified as insertion points to maximize the total inspection footage where seven of those were retrofitted to accommodate the tool insertion. Manhole retrofitting included developing design drawings, addressing permitting requirements for various agencies along the corridor, securing a contractor, and providing services during construction. In short, 80% of the manholes and 70% of the pipeline alignment were inspected. Mostly, debris and water in the pipeline and paved-over manholes were the primary causes that precluded the teams from fully inspecting the assets. Each pipe defect was assigned a PACP defect code and severity, and each PACP defect code was assigned a condition grade from 1 to 5, where: 5-Most significant defect grade 4 -Significant defect grade 3 -Moderate defect grade 2 -Minor to Moderate defect grade 1 -Minor defect grade The Overall Quick Rating and Structural and O&M Quick Ratings were used to facilitate recommendations for prioritization, cleaning, rehabilitation and repair. The Quick Rating methodology based on NASSCO provides a summary of the number of occurrences of the most severe structural and O&M defects. Two Quick Rating scores were generated: one for structural and one for O&M defects. Various types of internal corrosion were identified throughout the inspection, which would suggest that future use of the AFMGS for conveyance would require significant rehabilitation or replacement. For pipe reaches where the camera became submerged or was inaccessible due to access constraints, it is recommended to dewater and clean the pipe prior to re-inspection. Most of the manholes had structural and/or O&M issues that will require rehabilitation, reconstruction or cleaning. At the time of this writing, DC Water is currently evaluating the potential alternatives to either re-inspect and rehabilitate the assets, abandon the existing AFMGS and build a parallel pipeline, or a combination of the two. It is anticipated that a plan will be selected in the next few months and will be presented at the Collection System conference in Missouri. In short, this presentation will include an overview of the planning and execution efforts to inspect the AFMGS, condition assessment results, applicable rehabilitation recommendations, and lessons learned during the inspection planning and execution phases. --- Purpose: Present the value of a programmatic pipe condition assessment approach for the management and rehabilitation of a critical large sewer.