South Cobb Tunnel

Infrastructure for a Growing Population

Georgia’s Cobb County Water System (CCWS) provides water, sewer, and stormwater management services to more than 680,000 people in a 348‑mile² service area. Historic and projected population growth led CCWS to evaluate its infrastructure and undertake substantial improvements.

The two primary interceptor sewers and two wastewater pump stations that form the foundation of the South Cobb Basin wastewater collection system were identified as nearing capacity and unable to support future requirements. CCWS also needed to increase storage capacity for the South Cobb Water Reclamation Facility (WRF) in order to meet stringent performance requirements. Increasing storage capacity will allow the WRF to equalize influent flow rates, which fluctuate during each 24‑hr period as well as from precipitation.

Parsons is now managing construction of the South Cobb Tunnel—the largest capital improvement project undertaken by CCWS. This project will meet the wastewater transmission and flow equalization needs of the southern and western portions of Georgia’s Cobb County for the next 100 years—cost-effectively and with minimal impact on its citizens and the environment. The project will generate annual operational savings of hundreds of thousands of dollars, and it will be more reliable than other alternatives.

When completed, the South Cobb Tunnel project will provide long-term sewer conveyance capacity and equalization for the Sweetwater Creek and Nickajack Creek basins, and it will allow CCWS to eliminate two major pumping stations and 87,000 linear ft of aging sewer line and force main.

Major Project Components

The project requires the following major construction:

• A 5.5-mile, 27-ft-dia tunnel through hard rock with depths ranging from 150 to 400 ft.
• Several connector tunnels with lengths ranging up to 3,200 ft.
• A 130-mgd deep lift station to convey wastewater flows from the tunnel to the WRF for treatment.
• A 114-ft-dia shaft for the new deep lift station.
• Two 44-ft-dia construction shafts at depths to 285 ft.
• Five inlet structures to divert flows to the tunnel from the existing surface interceptors.

The 29,000-ft-long main tunnel is being excavated by a 27‑ft‑dia main beam-type tunnel boring machine (TBM) through medium-grade metamorphic rock consisting primarily of gneiss and schists. Four connector tunnels are being excavated by drill-and-blast techniques. A fifth connector tunnel with a length of 3,200 ft is being excavated by an 8.5‑ft‑dia TBM. About 70% of the main tunnel will require cast-in-place concrete lining with an inside diameter of 24 ft; unlined areas will receive a concrete invert. Connector tunnels are 100% lined with either cast-in-place concrete or fiberglass pressure pipe.