South Cobb Tunnel

Seated in the northwest part of metro Atlanta, Georgia, Cobb County encompasses 340 square miles and more than 680,000 people. The county’s population has doubled since 1980 and with the Atlanta Regional Commission’s projected growth for the area, additional infrastructure and conveyance capacity will be necessary. After careful study, and without a mandate by the EPD, the Cobb County Water System (CCWS) has opted to build a long-term solution. At 27 feet in excavated diameter and approximately 29,000 feet in length, the South Cobb Tunnel will handle the projected 2040 wastewater flows for this basin, and provide 80 million gallons of storage capacity for flow equalization. This deep rock tunnel will carry sewer flows from major wastewater interceptors to a new 130 million-gallons-per-day (MGD) pump station. This project also involves building multiple shafts and inlet structures, as well as small connector tunnels.

At 100 to 400 feet (30 to 122 m) below the surface, the main tunnel passes through ground with fractured to solid medium grade metamorphic rocks with granitic intrusions. It is being excavated by tunnel boring machine (TBM). The five connector tunnels to the intakes are 100 to 3,200 feet long (30 to 975 m) and 6 to 10 feet in finished diameter (1.8 to 3 m) for a total of about one mile (1.6 km). Most of the longest drive will be driven with a 8.5’ diameter TBM, and finished with drill–and-blast. The remaining connectors will be excavated by drill-and-blast. About 75% of the main tunnel and all the connector tunnels will require a cast-in-place concrete lining or fiberglass pressure pipe (Hobas).

Jacobs Associates, in a team with Parsons, is providing construction management (CM) and support services for the South Cobb Tunnel. As part of these services, Jacobs Associates is supporting CCWS staff in dealing with sensitive community issues such as noise associated with blasting and ground vibration induced by the tunnel boring machine.

Construction began in July 2008, and on September 1, 2009, the TBM was launched.  The TBM is progressing very well, and has excavated 25,222 lf of tunnel as of December 31, 2010. Completion is scheduled for mid 2014.