South Cobb sewer tunnel awarded in Atlanta
South Cobb sewer tunnel awarded in Atlanta Jun 2008
Shani Wallis, Editor

The six-mile, 27ft South Cobb sewer tunnel (10km x 8m diameter) is awarded to the Shea-Traylor JV. The bid price of £305 million is lowest of three, the others from the Obayashi/Kenny JV at $347 million and Kiewit Southern at $376.2 million. Confirmation of award is imminent with notice to proceed expected in July.

This is the second major tunnelling project for the Cobb County Water System. Following the 6.5 mile, 18ft diameter (10.5km x 5.5m) Chattahoochee tunnel completed in 2004, the South Cobb project is the largest capital improvement project undertaken by the County. Along with excavation of the tunnel, the contract with Shea-Traylor includes construction of two 46ft (14m) diameter access shafts and an 114ft (35m) diameter x 210ft (64m) deep lift station shaft.

The project is designed 100% by JJG of Atlanta to provide long-term sewer conveyance capacity and allow Cobb to eliminate two large pumping stations and about 16 miles (26.5km) of ageing sewer and force main. The pumping station of the new tunnel has a 130 million-gpd capacity to lift flow to the South Cobb Water Reclamation Facility. JJG has extensive tunnel design experience in the Atlanta region having designed and worked on construction management for the Chattahoochee tunnel for Cobb County and the Nancy Creek and West SCO Tunnel for the City of Atlanta.

At 150-400ft (46-122m) below ground, the tunnel passes through rock similar to the south drive of the Chattahoochee tunnel, comprising fractured to solid medium grade metamorphic rocks with granitic intrusions. Five spur tunnels of 100-3,200ft long and 6ft-10ft in diameter (30m-975m x 1.8-3m) for a total of about one mile (1.6km) will be excavated by drill+blast. About 75% of the main TBM tunnel and all the drill+blast headings will require a cast-in- place concrete lining.

Shea-Traylor intends to use one of the two Herrenknecht hard rock TBMs used by Obayashi Corp on the West CSO tunnels project for the main 10km long drive with a refurbishment by Herrenknecht USA.

JJG will provide engineering services through construction and Parsons with Jacobs Associates are awarded the construction management contract.


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