Digging Deep: South Cobb Tunnel
The Atlanta area is home to some of the biggest sewer and water projects in the four-state region, and the South Cobb Tunnel is one of the largest. This $305-million contract for the Cobb County, Ga., Water System represents the largest capital improvement project in the water system’s history. Shea –Traylor J.V., a joint venture of J.F. Shea Construction of Camarillo, Calif., and Traylor Brothers of Evansville, Ind., started work on the contract in March 2008. Construction began four months later, with completion expected by 2014.
Pasadena, Calif.-based Parsons and its subcontractor, Jacobs Associates, are acting as construction managers.
According to the Cobb County Water System, this project will provide needed long-term sewer conveyance capacity and equalization for the Sweetwater Creek and Nickajack Creek basins in western and southern Cobb County. The new facility will enable the county to eliminate two major pumping stations and about 87,000 lin ft of aging sewer line and force main.
The project entails constructing a 6-mi, 27-ft excavated diameter tunnel located in hard rock, with depths ranging from 150 ft to 400 ft. Shea-Traylor is using a 27-ft tunnel boring machine to excavate most of the main tunnel. The contract also includes several 6-ft diameter tunnels and a 10-ft diameter tunnel, with lengths ranging from 100 lin ft to 3,200 lin ft.
Shea-Traylor also will build a deep lift station with a capacity of 130 million gallons per day at the South Cobb Water Reclamation Facility to convey waste water flows from the tunnel to the plant for treatment.
David Rendini with Parsons says that Shea-Traylor has completed approximately 6,800 lin ft of the eventual 30,000 lin ft of main tunnel excavation from the Sweetwater Construction Shaft, or approximately 23%. He added that the project is on schedule and within budget.