Atlanta’s explosive population growth is most obvious in its notorious traffic.
But underneath the bustling city there’s a less-visible congestion problem that threatens residents’ health and the environment. The aging infrastructure that carries sewage to treatment facilities is growth-stressed, resulting in backups into backyards and basements as well as flows into creeks during heavy rains.
To solve the problem, workers are burrowing deep beneath Atlanta to create a new sewer tunnel –– the Nancy Creek Tunnel –– to handle overflow from the existing sewer system.
Tokyo-based Obayashi Corporation entered into a joint venture with CJB Inc. in Atlanta to form Nancy Creek Constructors, which came in with the low bid on the $131 million contract, according to project manager Taro Nonaka.
The project, undertaken in conjunction with DeKalb and Fulton counties, is part of the City of Atlanta Department of Watershed Management’s Clean Water Atlanta program, according to the Clean Water Atlanta Web site.
Work began on the 44,000-ft. (13,400 m) — or roughly 8.3-mi. (13.4 km) — tunnel in July 2002 and is expected to be completed in January 2006, Nonaka said .
Stretching from west to east across the northern part of Atlanta and into Fulton and DeKalb counties, the Nancy Creek Tunnel is being constructed 150 to 300 ft. (45.7 to 91.4 m) underneath the surface with an 18-ft. 4-in. (5.6 m) diameter, he said.
As of early December, everything was on schedule.
The company has had approximately 120 workers on the job, working in three eight-hour shifts.
The crews have been using a variety of heavy equipment, including two 200-ton (181.4 t) Kobelco hydraulic cranes, purchased for the job from Moody Machinery in Atlanta, as well as three 75-ton (68 t) Grove cranes, one Caterpillar 988 and two Caterpillar 966 loaders and Cat 330 and 416 backhoes Obayashi Corporation already owned, Nonaka said.
He praised Moody Machinery, noting that the dealer was able to get his company what it wanted and that the purchase was “a good experience.”
For the job, the company also purchased two pieces of very specialized equipment –– tunnel boring machines or TBM., for short –– needed to bore through the metamorphic rock, myronite, found at the excavation depth, Nonaka said.
“Since this is a deep tunnel, it’s rock — very, very hard rock,” said Nonaka, noting the myronite is 30,000 to 50,000 p.s.i. (207,000 to 345,000 kPa), 10 times harder than concrete.
The Robbins TBM models 299 and 279 were purchased directly from the Ohio manufacturer, he said. Both are being operated 24 hours a day, six days a week.
As of early December, the Robbins 299 had bored in approximately 8,000 of 27,000 ft. (2,400 of 8,200 m) and the Robbins 279 had bored in nearly 4,000 of 17,000 ft. (1,220 of 5,200 m), he said.
The excavated rock is being brought to the surface by a conveyor belt system then taken out in dump trucks by subcontractor M.C. Trucking of Atlanta, Nonaka said. A total of 45,000 cu. yds. (34,400 cu m) of earth will be removed during the excavation phase of the project.
Once the excavation is completed, 100,000 cu. yds. (76,500 cu m) of concrete will be used for the 1-ft. (0.3 m) thick tunnel lining, which will result in a finished 16-ft. (4.9 m) diameter tunnel, he said.
From time to time, other subcontractors have been used on the job. Morris-Shea Bridge Co. Inc., out of Irondale, AL, installed piles for the support system, and Nassana, out of Marietta, GA, did site grading and installed a wall system to minimize noise from the work.
Although large in scale, work on the project isn’t affecting the surrounding community very much, Nonaka said.
“We’re not closing any roads; no detours,” he said. “I would say the impact is minimal.”
The impact of the finished product will be very noticeable, however.
Spanning the northern part of Atlanta, and including parts of northeast Fulton County and a portion of DeKalb County, the Nancy Creek basin has had ongoing sewer backup problems since the 1980s, according to the Clean Water Atlanta Web site.
The problem is that when there are heavy rains, the existing sewer system does not have enough capacity to cary both the raw sewage and additional storm water that enters the lines through manholes and cracks in the pipes, according to the Web site.
Various options for increasing sewer capacity were explored, and the overflow tunnel was deemed most environmentally sound and least disruptive of the options.
The existing Nancy Creek trunk sewers will remain in continuous operation during the construction of the Nancy Creek Tunnel, which will tie in at Atlanta’s R.M. Clayton Water Reclamation Center.
Three construction shafts are being built as part of the project. Eight intakes will be used to divert flow from shallow sewers down to the tunnel, with one intake located at each of the construction shaft sites. Five others will be located at remote sites, tying into the main Nancy Creek Tunnel through smaller connecting tunnels.
The Nancy Creek Tunnel Lift Station will be located at the end of the tunnel, inside the R.M. Clayton plant, and will be used to pump the wastewater from the Nancy Creek Tunnel to the treatment center above, according to the Web site.
The lift station will have a capacity of 100 million gallons per day with a design that will give the city flexibility to regulate flow to the R.M. Clayton plant or storage for later pumping to the plant.
For more information, visit www.cleanwateratlanta.org.