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Chattahoochee Tunnel

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The Chattahoochee Tunnel project, the Cobb County Water System’s first deep tunnel, exemplifies Cobb County’s commitment to be good environmental stewards by utilizing proactive planning to keep pace with continued growth. The goal was to provide sufficient build-out sewer conveyance capacity in east Cobb by relieving the Rottenwood Creek, Sope Creek, and Chattahoochee Interceptors, which were near capacity. Options evaluated included conventional open-cutting, a deep tunnel, and a combination of both methods. The tunnel, with the least amount of environmental and community impacts, was ultimately selected.

The Chattahoochee Tunnel is nearly 9.5 miles long and ranges in depth from 100 to 375 feet below ground surface. The excavated diameter is 18 feet, and approximately 75 percent of the tunnel was lined with concrete to a finished diameter of 16 feet. Construction began in 2000, and it took four and a half years to complete. The project included excavation of the 100-foot diameter, 170 foot deep Pump Station Shaft at the R.L. Sutton Water Reclamation Facility and two 32-foot diameter construction shafts, which were used for staging tunnel boring machine (TBM) construction. The Chattahoochee Tunnel was constructed in three sections: one 1,300-foot drill and blast section (between the Pump Station Shaft and the Elizabeth Lane Shaft) and two drilled sections that were excavated using large tunnel boring machines. The South TBM Drive was staged from the 180-foot deep Elizabeth Lane Construction Shaft. The North Drive was staged from the 230-foot deep Circle 75 Parkway Construction Shaft. At the peak of production, approximately 220 people, including construction workers, engineers, geologists, and support staff, worked on the project. The final construction cost was $110.8 million, which was almost $3 million below the original contract amount.

At the northern end of the Chattahoochee Tunnel, sewage from the Sope Creek and Sewell Mill Creek Interceptors drop into the tunnel through the Indian Hills Intake Structure. Two other intakes are located along the tunnel alignment, one at the Rottenwood Creek Interceptor and another at the Little Nancy Creek Interceptor. A fourth intake diverts sewage from the Chattahoochee Interceptor to a 900-foot long, 8-foot finished diameter tunnel, which joins the Chattahoochee Tunnel at its southern end. Each intake consists of a subsurface drop structure, a small drop shaft, and a receiving chamber at tunnel level. The tunnel conveys the wastewater to the R.L. Sutton Water Reclamation Facility where it is treated and discharges to the Chattahoochee River. To view a map of the project, click here.

Jordan, Jones & Goulding (now Jacobs) designed the project and provided full-time construction engineering services. Parsons Engineering Science (now Parsons) and Jacobs Associates were the construction managers for the project. Gilbert/Healy L.P. (a partnership between Gilbert Southern and S.A. Healy) was the contractor.

To view a photographical history of the project, click here.