The Chattahoochee Wastewater Tunnel

$114 Million Project Scheduled for Completion in December

By Scott Judy

Several major tunnel projects are under construction in the greater Atlanta area, and one of the bigger ones, the Chattahoochee Wastewater Tunnel in Cobb County, is heading toward completion.

Lyman File, project manager for Gilbert/Healy, called it "just a big pipe in the ground," but the project that started more than four years ago is slightly more than that. At the time the contract was awarded, it was the largest project Cobb County Water System had ever undertaken, and it includes excavating a 49,600-ft.-long rock tunnel 100 to 350 ft. deep.

Approximately 47,500 ft. of 18-ft., 4-in.-diameter tunnel was excavated using two tunnel-boring machines, or TBMs, and 2,100 ft. was excavated by drill-and-blast methods.

Gilbert/Healy, a joint venture of Kiewit-subsidiary Gilbert Southern Corp. of Atlanta and S.A. Healy Co. of Lombard, Ill., is serving as the general contractor for the $114 million project, which is being built to add capacity to Cobb County's wastewater treatment system. (Gilbert Southern actually bought out Healy's stake in the venture at its request about two years ago but is continuing under the contractual name of Gilbert/Healy.)

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Jordan Jones & Goulding of Norcross, Ga., was the design engineer. Parsons Engineering Science and Jacobs Associates, both of Atlanta, are serving as construction managers.

The Project

Gilbert/Healy began work in June 2000. The original completion date was April 2004 but has since been moved to December. That delay was actually the result of finding different-than-expected rock conditions underground and a consequential increase in the amount of concrete tunnel lining required.

"The rock was of a significantly higher quality, with fewer bad features, than we anticipated," said Steve McCullers, engineering manager for Cobb County Water System. "We had anticipated running into seams of unstable rock. We ran into very much less of that."

Encountering less stable rock would require Gilbert/Healy to set up "steel sets" to reinforce the tunnel, a process that could slow down the team's progress.

Still, while encountering a greater amount of this harder rock did enable the mining teams to maintain a more consistent work effort, it also had a negative impact.

"It hindered the mining because it was harder and more massive than originally anticipated," File said, adding that the mining process took longer than originally expected.

Gilbert/Healy used two custom-built TBMs launched at two separate sites. Both were supplied by The Robbins Co. of Solon, Ohio. One started from the southern end of the tunnel, at the Elizabeth Lane shaft, and proceeded north approximately 4 mi. The other launched from the Circle 75 shaft, an access point at roughly the tunnel's midpoint, and headed north for the remaining 5 mi.

As the TBMs progressed, the excavated material would then be hauled out of the tunnel by a conveyor system that would eventually go up and out of the shaft, where a stacking conveyor would then stockpile the material.

Crews worked two 10-hour shifts per day, with daily four-hour maintenance periods required to change the TBM's lubricants, cutting heads and other items. Major maintenance took place each Saturday and Sunday.

David Hamilton, project engineer for Gilbert/Healy, said the Circle 75 shaft TBM mined an average of about 500 ft. per week, versus the Elizabeth Lane TBM's 350-ft.-per-week rate. Rock conditions made the difference.

"There were no steel sets," Hamilton said of the Circle 75 shaft. "We didn't have any soft ground. We didn't have to stand steel. That slows down the progress quite a bit."

Hamilton was referring to the temporary ground support systems that are put into place to enable the contractors to reinforce and brace the newly formed rock tunnel.

The Elizabeth Lane TBM broke through at the Circle 75 site in October 2002 to complete the connection. The machine that started at the Circle 75 site wrapped up in December 2002.

Water Extends Schedule

As the project progressed, the amount of underground water leakage was found to be more pervasive than anticipated, and consequently the amount of tunnel lining was increased significantly.

"We had anticipated lining about 55 percent of the tunnel to keep water from leaking in - thinking that 45 percent of the tunnel would be basically dry," McCullers said. "We thought it would be reasonably solid."

The more extensive water leakage caused the owner to increase the lining to about 75 percent, roughly equal to an additional 10,000 ft. of tunnel.

Lining the tunnel would be its own trick. The concrete mix would first have to be delivered 100 to 350 ft. down the shaft and then conveyed as much as several miles before being placed along the tunnel walls.

"The biggest issue for us was the longevity of the mix," File said. "We needed a mix that had a lot of shelf life." Gilbert/Healy worked with local concrete supplier Thomas Concrete and admixture manufacturer Master Builders to design the mix.

"We used a retarding admixture as well as one of the new generation, high-performance plasticizers to control the slump of the mix," File said. "We worked very closely with Thomas to develop it. It was a beautiful mix."

He added that though specifications called for a 4,500-psi concrete, the mix is delivering compressive strengths of more than 6,000 psi.

Geotechnical

Both owner and contractor said an extensive geotechnical report kept surprises to a minimum and contributed to a more successful project.

"Tunneling is a very risky business," File said. "And the more owners invest in geotechnical exploration up front, the more comfortable contractors are in being able to define the risks on a project.

"We think the owner had a very good geotechnical baseline report, and it was fairly interpreted. A lot of times (owners) have a lot of data but they have very conservative conclusions. We do not stray far from what they present as the baseline."

McCullers said the agency thought it was a prudent investment.

"We did a tremendous amount of advanced geotechnical work - much more than the norm for a tunnel of this nature," he said. "It really reflected in the bids. We felt like it'd be better to spend an extra $1 million up front and get the bids reduced by hopefully several times that, and have a much more stable project with no surprises.

"It's been a pretty smooth project," McCullers continued. "Everybody's going to make some money. We're going to get a project under the money, and we're going to get more project than we bid."

And Hamilton added: "It's a pretty successful project for us. We're delivering a project on schedule and we're meeting our budget.

Useful Sources:

For more information about this project, readers may visit www.chattahoocheetunnel.com

Project Team:

Owner: Cobb County Water System
General Contractor: Gilbert/Healy, a joint venture of Gilbert Southern, Atlanta, and S.A. Healy Co., Lombard, Ill.
Design Engineer: Jordan Jones & Goulding, Norcross, Ga.
Construction Manager: Parsons Engineering Science, Atlanta
Construction Manager: Jacobs Associates, Atlanta