Avoiding Tunnel Vision in Large Diameter Sewer Rehabilitation

The Virginia Tunnel is a 5,400-foot sewer that is 78 inches in diameter and 80–100 feet deep. Constructed in 1963, this asset is part of the Potomac Interceptor and conveys flow from the Dulles Airport and portions of Loudon, Fairfax, and Montgomery Counties to the DC Water Blue Plains Wastewater Plant. Based on CCTV and sonar inspection data, the tunnel has exhibited corrosion and deterioration. An initial assessment of this inspection data identified the pipeline as a high priority for rehabilitation. Rehabilitation design was initiated, but due to numerous challenges, including site access, maintenance of flow, capacity requirements, and depths, the project was never constructed. A fresh review of the project identified that the perceived risk of failure may have been overestimated. The project team utilized innovative inspection techniques, including CCTV/sonar/laser/3D lidar and pipe penetrating radar to re-evaluate the structural integrity of the sewer in tandem with a geotechnical assessment. This evaluation allowed the team to properly assess the tunnel condition and associated risk. This presentation will present the project background and the pitfalls of standardized defect coding that contributed to the tunnel vision in the identification of rehabilitation alternatives. The paper will provide a case study in how inspection strategies, when properly applied and evaluated, result in higher resolution data from which to effectively assess risk and identify appropriate solutions that may result in significant cost and schedule savings.