Expressions of interest have been called to build the South Cobb wastewater tunnel and associated works in the state of Georgia, USA, with bids and general construction start due in the third quarter of the year.
Most of the 8.92km long tunnel is to be driven by hard rock TBM, the remainder of the 8.3m wide excavation being opened by drill and blast to link with five diversion/drop shaft structures.
Geology along the route of the alignment consists of medium-grade metamorphic rocks with some granitic intrusions. Laying in the Piedmont region in the greater Atlanta area of Georgia, a key characteristic of the regional geology is the 4m to more than 30m thick mantle of residual soil and partially weather rock.
The depth of the tunnel, running from the 12.3m wide launch shaft to the 9.2m wide reception shaft, is from about 34m in fresh bedrock below the mantle up to 138m. Finished diameter of the tunnel will be almost 7.4m with three-quarters of the sewer to have 400mm thick cast in-situ concrete lining.
Main tunnelling works are due to start in March 2009 and the US$200M project is scheduled for completion by mid-2013. The deadline for expressions of interest was 12 April, invites for tenders are to be issued in June and the deadline for tenders is expected to be July.
The large diameter, deep rock tunnel project also includes construction of a 58.5m deep by 35m wide pumping station shaft, and a separate electrical and equipment housing structure.
Designer on the project is Jordan, Jones & Goulding. The client, Cobb County Water System (CCWS), has hired Parsons along with its subcontractor Jacobs Associates (JA) to manage construction of the sewerage project. Parsons and JA handled construction management from 1999 on the Chattahoochee Tunnel project, which completed under budget in December 2004.
