The South Cobb County Tunnel project is the biggest project in the history of Atlanta, the capital city of the US State of Georgia. The South Cobb water reclamation facility, near the Chattahoochee River, collects and treats sewage from Cobb County. 26 kilometers of old sewer pipe was to be replaced by the South Cobb Tunnel to cope with demographic changes.
A Gripper TBM (Herrenknecht S-503) started driving the 8.5-kilometer bore for this project in September 2009. Tunnelling through gneiss and granite, the S-503 achieved peak performances of up to 50 meters daily. After just 18 months – eleven months ahead of schedule – the machine broke through into the target shaft on March 16, 2011.