Cobb County in Georgia awarded a US$305 million contract to build the 5.5-mile (8.8km) long South Cobb wastewater tunnel to a joint venture comprising JF Shea and Traylor Bros on March 11.
The project estimate has risen by over US$100 million since late 2006.
The 27ft (8.2m) diameter tunnel will handle 40% of the county’s flows. Traylor Bros said the hard rock tunnel would be mined with a main beam TBM. Five additional tunnels totalling more than a mile (1.6km) are to be excavated by drill and blast. In addition, the scope includes two 46ft (14m) diameter shafts and a larger shaft and pump station structure.
The tunnel alignment runs at an average depth of 250ft (76m) through the same geologic formations as the south drive of the Chattahoochee tunnel. The bedrock is expected to vary from fractured to solid and could consist of medium grade metamorphic rocks with granitic intrusions. Approximately 75% of the tunnel is expected to be lined with cast-in-place concrete lining.
The tunnel was designed by Jordan, Jones & Goulding (JJG), a regional consulting firm with past experience on the Chattahoochee Tunnel, Nancy Creek Tunnel and West Combined Sewer Overflow Tunnel in the area.
At the same time as the construction contract award, JJG was awarded, a US$7.5 million capped body of additional work to provide construction engineering services such as reviewing submittals, geological mapping and responding to requests for information from the contractor. The Cobb County Water System (CCWS) has previously awarded Parsons and its subcontractor Jacobs Associates the construction management contract for the project.
The South Cobb scheme will allow the county to avoid replacing 16 miles (26km) of aging sewer line and two pump stations. Work is expected to start this summer.
