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WHAT RESULTS AFTER A CSO STORAGE TUNNEL IS FLIPPED

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What happens when the concept of a CSO storage tunnel is flipped on its side is, a new innovative large vertical shaft CSO control facility is born. A large shaft is an underground, deep vertical CSO storage tank. The facility has a smaller surface footprint as compared to a conventional CSO control basin, while requiring less concrete than a tunnel, making it a more cost effective CSO control alternative, as compared to a conventional CSO control basin or a tunnel.

The City of Dearborn, Michigan combined sewer tributary area total of about 8,260 acres are handled by three CSO control projects; West Dearborn which has two phases, Phase A and Phase B, and EastDearborn. The City of Dearborn currently has four large shaft CSO control facilities under construction to control CSO flows from six large outfalls that represent West Dearborn Phase A and East Dearborn segments. These two areas constitute half of their combined sewer watershed area, with a total of six large outfalls. These facilities are anticipated to become operational prior to 2008. West Dearborn Phase B of their Long-Term Control Plan anticipates the construction of an additional five large treatment shaft facilities to control CSO flows from the remaining 12 CSO outfalls within the City.

The two large capture shafts for Phase A are being constructed to a depth of approximately 120 feet deep through soft clay, into the hard rock, using sinking caisson construction. The large shaft for outfall CSO 013 is 116 feet in diameter, with the capability to store 7.2 million gallons (MG)of combined sewage. Outfall CSO 014 is the largest size capture shaft, 136-foot diameter structurewith a 10.1 MG storage capacity. Outfall CSO 015 will reuse a tunnel-mining shaft from a previous project and has 0.8 MG storage capacity. In addition, a sodium hypochlorite storage facility is being constructed to provide disinfection at times when the capture shaft is full. Additional treatment features include fine bar screens with 5 mm open spacing to screen the excess CSO flow prior to discharge. Taken together, these facilities were sized to satisfy the Michigan Department of Environmental Quality (MDEQ) requirements to provide full capture of the 1-year, 1-hour design storm event or 30 minutes detention of the 10-year, 1-hour storm event.

This paper will present the design elements and construction of these innovative facilities, with particular attention to the benefits of the treatment shaft CSO control facilities for the City ofDearborn, as compared to other CSO control alternatives. The selection of the large shaft facilities saved the City of Dearborn approximately 70 million dollars, as compared to their originally proposed tunnel alternative. The treatment shafts also significantly reduced the geotechnical risks of constructing a capture tunnel, as compared to constructing the capture shafts using sinking caisson construction. Taken in total, the City of Dearborn has benefited from flipping a CSO storage tunnel on its side.
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Document Type: Research Article

Publication date: 2006-01-01

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