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Subterranean Atlanta Flooded With Activity For Sewer System

Fri August 19, 2005 - Southeast Edition
Lisa Coston

Rocky is back, bigger and more bulked up than ever before, and fighting at approximately 300 ft. (91 m) underneath the city of Atlanta.

But don’t start humming that familiar movie theme just yet.

Rocky is one of the tunnel boring machines (TBM), owned by Japan-based Obayashi Corp. currently cutting and grinding through the bedrock in one of a series of three connecting sewer tunnels, which will serve to hold more than 177 million gal. of Atlanta’s sewage overflow during storm events.

Currently, the outdated sewer system, located in a 19-sq.-mi. area within Atlanta, continually overflows, creating a mixture of raw sewage and storm water. When the Combined Sewer Overflow centers reach their capacity to treat, screen and disinfect the sewage and storm water, the foul mixture is forced into surrounding creeks and the nearby Chattahoochee River.

Under siege from local and national environmental groups, as well as state and federal authorities, a federal court order mandated that Atlanta put the wheels in motion to update the antiquated and crumbling sewer system.

In response, the city’s Department of Watershed Management is in the midst of a 14-year, $3.2-billion sewer and water infrastructure-rebuilding program. Included are the construction of three sewer tunnels, four construction shafts, eight intake structures and a 100-million-gal. per-day pump station.

Parts of the new sewer system tunnels are spread through three counties, and are being constructed in cooperation with the city of Atlanta, Dekalb County and Fulton County.

In July 2002, construction of the first part of the project — the 8.25-mi. long (13.3 km) Nancy Creek Tunnel —began, with the combined efforts of Obayashi Inc., and Atlanta-based CJB Inc. under the name of Nancy Creek Constructors.

Now 90 percent complete, the tunnel consists of a 16-ft. (4.9 m) diameter, 43,700-linear-ft. (13,300 m) tunnel with four access shafts and eight intake structures and 2,200 linear ft. (670 m) of small-diameter tunnel.

Using two 18-ft. (5.5 m) TBMs with a 4-in. (10 cm) diameter, built by the Ohio-based Robbins Company, to drill a perfectly circular tunnel shape, crews mined the tunnel beneath the surface in two drives.

With workers mining 24 hours-a-day, seven days a week, the first drive from the Clayton to Roswell shaft is 26,212 ft. (7,990 m) in length, and the second drive from the Roswell to Johnson Ferry shaft is 16,457 ft. (5,016 m) All that is left to finish is the 1-ft. (.3 m) layer of concrete as a lining around the tunnel, as well as grouting and final clean up.

Upon completion, Obayashi/MassAnna will have removed 300,000 cu. yds. (230,000 cu m) of bedrock, and will have pumped up to 110,000 cu. yds. (84,000 cu m) of concrete for both the main tunnel and the construction shafts.

West Avenue Tunnel

The Atlanta Department of Watershed Management awarded the second part of the sewer tunnel construction contract, again, to Obayashi, which won the job with a low bid of $210 million, partnering this time with Atlanta-based MassAnna Construction LLC.

The West Avenue Combined Sewer Overflow (CSO) Tunnel Program includes the North Avenue Tunnel, the Clear Creek Tunnel, and a new pump station in the downtown Atlanta area.

Both tunnels will serve as an 8.5-mi. (13.7 km) container for more than 150 million gal. (567 million L) of sewage and runoff water, which will be treated by a new dedicated CSO treatment facility to be built along the Chattahoochee River. They will serve as storage only after the pump station is construction.

Enter Rocky into the West Avenue CSO Tunnel ring.

Built by the Herrenknecht company from Germany, the 1,000-ton (907 t) model S-288 has a cutting diameter of 27 ft. (8.2 m).

Rocky’s cutting face of 50, 19-in. (48 cm) disk cutters makes six to seven rotations per minute, and is equipped with nine 469-hp (350 kw) drive motors.

Armed with such impressive torque, all of its bulk and cutting strength will come in handy, as Rocky cuts through layers of bedrock, boring a 4-mi. (6.4 km) long tunnel, 27 ft. (8 m) in diameter, and finished out at 24 ft. in diameter (7 m) from Atlanta’s Marietta Street Boulevard location to the Clear Creek CSO facility to Monroe Drive.

Even with the strength of the machine, for a project this size, Rocky needs his Adrian.

Or, in this case, Rocksanne, which is an exact replica of Rocky, and the second TBM working on the West Avenue CSO Tunnel project.

Herrenknecht/Obayashi’s Rocksanne — model S-289 — will begin boring at the other end of the project, from the R.M. Clayton Water Reclamation Center to the North Avenue CSO for a distance of 4.5 mi. (7.2 km).

North Avenue Tunnel

The North Avenue tunnel and pumping station includes approximately 22,400 ft (6,830 m) of TBM tunnel, 1,772 ft. (540 m) of drill/blast tunnel, one 24-ft. (7.3 m) finished diameter overflow shaft, two 40-ft. (12.2 m) finished diameter construction shafts, one 66-ft. (20 m) finished diameter pumping station shaft, one 10-ft. (3 m) finished diameter vent shaft and the pumping station.

Obayashi’s project manager, Taro Nonaka, believes the most challenging part of this project has been building the TBMs for the West Avenue CSO Tunnels, as well as staying on schedule for the overall project itself.

“Building two 27 feet in diameter TBMs in nine months was super fast,” said Nonaka. “It usually takes 12 months plus to build this size of TBM. The other challenging part is trying to meet the tight schedule of 1,360 calendar days that the city of Atlanta has required of us to complete this project.”

According to Nonaka, more than 125 workers from Obayashi and MassAnna are working to remove more than 1 million cu. yds. (765,000 cu m) of rock from both tunnels, as well as pouring approximately 90,000 cu. yds. (82,000 cu m) of concrete within the tunnels.

Aiding in the removal and finishing, Obayashi is using its own Kobelco CK2500 crawler cranes for general hoisting, Tamrock Comando 300 hydraulic drills for shaft blasting, Redrill 65HEs for tunnel blasting and Wagner ST-6 LHDs for mucking out the tunnels.

The excavated rock that falls off will be scooped up and transferred by horizontal and vertical conveyors to the surface.

Caterpillar 988, 980 and 966 front-end loaders are being used to load the material into trucks, and Grove 650 and 750 RT cranes are used for general hoisting.

According to Atlanta officials, the West Avenue CSO Tunnels should be finished and online by March 2008. They estimate the Nancy Creek Tunnel will be lined and ready by January 2006.

Nonaka and Atlanta officials share a mutual excitement about the conclusion of this project.

“This job is one of the biggest tunnel projects on going in the U.S.,” said Nonaka. “We have very a tight schedule and budget; so far everything is going all right. We are very excited about being part of the team working on the city’s Clean Water Program.”

“This project is a linchpin in the city’s efforts to eliminate CSOs. We have worked with Obayashi on the Nancy Creek SSO Tunnel, and we are very comfortable with the company’s professionalism and skill,” said Department of Watershed Management Commissioner Rob Hunter. “This tunnel will help us make the promise of Clean Water Atlanta a reality.” CEG

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