Cobb County, just northwest of Atlanta, relies on the Cobb County Waster System (CCWS) to collect and treat most of its wastewater—a demand that is growing. The county saw steady population increases each decade since the 1960s, and added more than 127,000 between 1998 and 2008. At the time of the 2010 census it had 688,078 residents.

The 5.5-mile (9.7km) South Cobb tunnel is part of an infrastructure improvement program that will meet the county’s wastewater demands and eliminate 87,000ft (26.518m) of aging sewer lines.

Notice to proceed had been given to a joint-venture contractor of J.F. Shea Construction and Traylor Brothers in July 2008 and the project’s completion was expected for six years later. As of November, the project was approximately one year ahead of schedule.

David Rendini, construction manager with Parsons, which is providing construction management, along with Jacobs Associates, reports the geology encountered along the tunnel’s alignment was as expected. However, in the three years since Shea-Traylor mobilized on site, the same can’t be said for the weather.

Design details
The South Cobb Tunnel comprises a 28,500ft (8,687m) tunnel drive in hard rock, with a 24ft (7.3m) inside diameter. Depths range from 150ft to 400ft (46m to 122m) between the Sweetwater construction shaft in the west to the South Cobb construction shaft in the east. There are three intakes along the tunnel alignment (see Figure 1, page 12) and the project also includes two smaller tunnels:

• The Sweetwater Connector Tunnel: A 10ft (3m) diameter horseshoe-shaped tunnel of 1,400ft (427m) connecting the Sweetwater intake to the main tunnel;

• The Nickajack Tunnel: An 8.5ft (2.6m) diameter tunnel of 3,200ft (975m) that connects the South Cobb construction shaft to the existing Nickajack pump station.

Engineering firm Jordon, Jones and Goulding (now Jacobs Engineering) have been involved with the project’s design since 2005. In addition to the tunneling work, the project includes constructing a deep lift station with a 130Mg/day capacity to convey wastewater flows from the tunnel to the South Cobb Water Reclamation Facility. The 116ft diameter pump station is being excavated by Shea-Traylor and the pump station is being built by subcontractor Archer Western. The contract also includes various inlet structures, drop and vent shafts, chamber structures, construction shafts, an electrical building and odor control facilities.

Once completed, the project will bring sewer flows from the Sweetwater interceptor to the facility, collecting flows from the Silver Creek, Carroll Creek, I-20, and South Cobb areas along the way.

Contractor Shea-Traylor chose a refurbished Herrenknecht TBM with a 27ft (8.2m) diameter, which was launched from the Sweetwater shaft in September 2009 (for more about shaft construction on the South Cobb Tunnel project see T&TNA, December 2009, page 9). Approximately 70 per cent of the tunnel will receive a cast in-place concrete liner.

Ground expectations
The Atlanta area geology is part of the Piedmont region of the southeast and comprises medium-grade metamorphic rocks that have been intruded by granitic rocks in some places. A key characteristic of the Piedmont region is the thick mantle of residual soil and partially weathered rock that overlies fresh bedrock. This mantle can range from 10ft (3m) to more than 100ft thick.

“We observed conditions were very consistent with the geotechnical work done upfront with the GBR,” Rendini says. “Contacts between units were where we expected to find them. Water inflows were as predicted. So there was really nothing unexpected in the geology.”

The ground along the alignment had been divided into an eastern and western region. The Eastern region, which includes the Nickajack tunnel, dealt with shallower excavation and therefore a more weathered and fractured rock with wet, blocky and seamy conditions. On the western side, the tunnel is deeper and the ground less weathered.

Initially three types of support were designed for the ground following TBM excavation. Mostly, the project has used rock bolts and welded wire fabric for support.

“We did actually allow the contractor to go to a single sheet of wire up in the arch between the center bolts, which are the ones that are just off center,” Rendini explains. “Then they added additional wire as the ground conditions dictated. There were some areas we added some mine straps and spot bolts. We had a short section where we actually put in steel sets. I think it was about 80ft (24.4m) in the last mile—practically nothing.”

There was also the potential for flushing groundwater flows of up to 1,200g/min (4,542.5l/min). “We never really encountered any big flush flows,” he says. “It was as predicted with approximately half the inflows in the first part of the drive, and half the inflows in the last part, and that’s pretty much what we saw.”

TBM talents
Rendini attributes the success of the drive and its early completion to cooperative geology, the machine advance rates being better than the baseline and machine availability. “They did a great job of manning the machine and doing maintenance,” he says.

Dan Martz, project manager for Shea, explains, “We were able to get ahead of schedule by good planning and favorable ground conditions.”

There were roadblocks along the way. The region saw a 500-year-flood event in September 2009 that shut down the project when flooded roads prevented access to the site for several days. Last winter the region saw snow—something out-of-character for the Atlanta area—leaving roads unplowed and icy, and limiting people’s ability to drive. None of the weather-related impacts have been critical to the project. And Hurricane Irene, which threatened to cripple the entire east coast early this fall, had no effect on the project.

The TBM made its breakthrough at the South Cobb construction shaft in March 14 and is currently sitting in pieces at the shaft site before going into storage until it’s needed on another project.

Final lining for the tunnel is underway and currently 40 per cent complete. The liner is comprised of sections with full circle unreinforced concrete, full circle reinforced concrete and invert only concrete.

Subcontractor W.L. Hailey started the 3,200ft Nickajack tunnel drive on March 3, 2010 and completed on July 29 of the same year, using a refurbished Jarva. Rendini admits there were doubts a TBM could be used for the full drive past 2,500ft, however the subcontractor successfully completed the full drive using an attached probe drill. Blasting for the chamber at the end of the Nickajack tunnel completed in September 2010 and the final tunnel lining, consisting of Hobas pipe and cement grout, is in place.

Intake progress
Carter Concrete Structures, a subcontractor to Shea-Traylor, and is constructing the intake structures and recently completed the Nickajack intake.

Drill and blast for the Sweetwater Connector Tunnel started in January 2010 with completion the following August. The intake structure is not done yet, and the contractor is still working on the drop shaft and will do a raised bore before the end of the year before the structure construction can begin.

There are three more intakes that are complete, or nearly complete. A 500ft (152m) tunnel that connects the I-20 intake structure with the South Cobb Tunnel was blasted 300ft (91m) below the Interstate-20 highway between May and December 2010. The 10ft diameter tunnel consists of Hobas pipe and cement grout. The lining was completed in October.

Excavation for the Silver Creek connector tunnel started June 2010 and blasting work finished in August. This tunnel is only 100ft long and has a 12m diameter. It was driven about 230ft (70.1m) below Silver Creek and is lined with Hobas pipe and cement grout.

Rendini reports there is minimal pipework and clean up left for this intake site. The third intake, at Carroll Creek, is close to completion. Blasting took place during August 2010 for the Carroll Creek connector tunnel, roughly 13ft (4m) in diameter and 70ft (21.3m) long with a depth of 220ft.

Pump Station Progress
With excavations completed by Shea-Traylor, subcontractor Archer Western’s construction of the 130 MGD Pump Station is currently 85 per cent complete.


‘Mighty Miner’ fog accumulates every evening in the drill and blast adits on the south Cobb Tunnel Project in Georgia Dignitaries gathered for the TBM hole through in March Figure 1, the South Cobb Tunnel and intakes Figure 2, Tunnel alignment showing intake shafts The 17th lift of the wet well (out of 22) at the Influent Pump Station