New CSO Tunnel Prevents Flooding After Storm
The First Street Tunnel (FST) is a storm water storage tunnel that was constructed, using a tunnel boring machine (TBM). This portion of DC Water’s Clean Rivers Project extends along First Street NW, beginning at the McMillan Sand Filter site and travelling south along First Street to Rhode Island Ave NW. During a storm event, the First Street Tunnel will store up to 30.3 million liters (8 million gallons) of combined sewer overflow (CSO) in its 853 meters (2,800 feet) of tunnel and four shafts. It will utilize a pump station located at Rhode Island Avenue to pump storm water back to the shallow sewer system, once the storm has subsided.
Keeping the Community Safe
The First Street Tunnel project was accelerated as part of the Mayor’s Task Force to relieve street and basement flooding in the Bloomingdale and LeDroit Park areas. Constructed in a dense urban neighborhood, the primary considerations were to resolve community impact, schedule durations, as well as dust and noise control.
The project involved tunnelling through mixed soil of sands and gravels. In order to minimize the impact on the community, our primary approach for the support of excavation was ground freezing. A central freeze plant was constructed away from residences, and it supplied chilled brine to the three satellite sites. This minimized the requirement for large staging areas and equipment. Further, it resulted in “cleaner” construction, compared to jet grouting.
Construction was completed successfully, thanks to a collaborative effort between the Owner (DC Water), Owner’s Engineers (McMillan Jacobs Associates, Greeley and Hansen), and Design-Builder (Skanska, Jay-Dee, WSP). Currently operational, the project will feed into a larger sewer system serving Washington, DC once the Northeast Boundary Tunnel (NEBT) is completed in 2022.
The use of a centralized freeze plant to construct support of excavations for shafts, adits, and adit to tunnel junctions was a first. A utility trench was constructed along a back alley to provide brine supply to the three satellite sites.
This was one of the first uses of steel fiber reinforced precast tunnel liners on the East Coast of the United States of America.
Our team delivered the following aspects of the project:
- Shaft support of excavation design
- Shaft base slab
- Cast-in-place final lining and internal structures design
- Precast segmental tunnel lining design
- Micro tunnel reinforced concrete jacked pipe design
- Fiber reinforced polymer pipe design
- Secant pile support of excavation design
- Solider pile and lagging support of excavation design
- Geotechnical design
- Structural analysis and design
- Construction impact assessment reports
- Geotechnical instrumentation and monitoring