Client:
Cobb County Water System

Construction Value:
$113.6 million

Project Duration:
September 1999–December 2004

Project Manager:
Darby Clay

Construction Manager:
Dave Rendini

Parsons Services:
Preconstruction constructability and biddability reviews, construction management,
dispute and claims management, quality assurance and testing, public outreach, periodic survey checks, full-time observation and inspection, review of
the contractor’s
safety plan, environmental monitoring, and additional services
as required

Like many growing metropolitan areas, Cobb County, Georgia must increase the capacity of its wastewater conveyance system in order to meet future capacity requirements. The Little Nancy Creek, Sewell Mill Creek, Sope Creek, Rottenwood Creek, and Chattahoochee sewer interceptors (ranging from 24 to 72 inches in diameter) serve the eastern portion of Cobb County, gathering wastewater flows and conveying them to the R.L. Sutton Water Reclamation Facility. The Chattahoochee interceptor, which is the largest interceptor, has a capacity of approximately 100 million gallons per day and runs near capacity at peak flows. Additional conveyance infrastructure is needed to accommodate future peak hour flows, which are forecasted to be 300 million gallons by the year 2040.

After evaluating several alternatives, the Cobb County Water System chose to construct the Chattahoochee Tunnel. A deep tunnel causes significantly less community and environmental disturbance than traditional, open-cut sewer lines. The Chattahoochee Tunnel takes a 9.5-mile-long direct path, an option logistically more attractive than the conventional method, which would involve installing over 15 miles of sewer lines parallel to the existing lines. The tunnel is designed to meet east Cobb’s long-term wastewater capacity needs and to provide flow equalization to the county’s R.L. Sutton Water Reclamation Facility.

Chattahoochee Tunnel profile.

Cobb County chose Parsons and its subcontractor, Jacobs Associates (JA), to provide construction management on the largest project the Cobb County Water System had ever undertaken at the time. Parsons began work on the Chattahoochee Tunnel project in September 1999, and construction began in June 2000.

Parsons’ services include preconstruction constructability and biddability reviews of the 70% contract documents, construction management, dispute and claims management, quality assurance and testing, public outreach, periodic survey checks, full-time observation and inspection, review of the contractor’s safety plan for compliance with the project safety plan (which was developed under an Owner Controlled Insurance Program), environmental monitoring, and any additional services the client requests.

Left: Top view of the 100-foot-diameter pump station shaft under construction. Right: Inside view of 16-foot-diameter steel forms used to cast concrete tunnel liner in place.

The Chattahoochee Tunnel’s principal elements are:

• Approximately 49,600 linear feet of hard rock tunnel, most of which—48,300 linear feet—was excavated using two high-power tunnel boring machines (TBMs). The rest of the tunnel was excavated using drill and blast techniques. Each TBM used 39, 19-inch-diameter cutter disks with 70,000 pounds of thrust exerted on each cutter. It took seven 422-hp electric motors to drive the 18-foot 4-inch diameter cutter head on each TBM. The tunnel slopes at a 0.1% grade and varies in depth with the surface contours from 110 to 350 feet below the surface.

• Over 1 million tons of cuttings were removed from the tunnel via horizontal and vertical conveyor belts. The tunnel is partially lined with approximately 35,000 linear feet of 1-foot-thick concrete liner, which is currently being installed.

• A 100-foot-diameter pump station shaft and two 34-foot-diameter construction shafts were sunk to depths ranging from 180 to 230 feet using concrete diaphragm walls socketed into rock to support the overburden section. Pre-excavation grouting of the transition zones and rock was performed to limit groundwater inflows. The rock sections were then excavated using drill and blast techniques and supported by rock bolts, shotcrete, concrete, and similar methods.

• The four tunnel intakes constructed include surface structures to intercept flows from existing sewers and drop the flows through shafts to underground chambers connected to the tunnel. Three drop shafts and three vent shafts were excavated using raise bore methods. Ductile iron pipe was installed in these shafts and the annular space was backfilled with concrete. The drop shafts include tangential vortex inducers to maximize flow capacity.

Left: One of two Robbins tunnel boring machines assembled at the factory. Right: View from top of construction shaft looking down on tunnel boring machines being assembled.

Accomplishments:

• As part of its public outreach and community involvement program, Parsons personnel visited each resident immediately adjacent to the tunnel alignment to discuss what to expect as the TBM passed. This one-on-one attention was well received and succeeded in minimizing complaints.

  After driving 22,000 feet, the tunnel boring machine breaks through into the Circle 75 shaft. Left to right: Ted DePooter (JA), Fred Estep (Parsons), Denver Chandler (Parsons), Mark Tilly (JA), David Rendini (Parsons), John Geyer (Parsons), Dwayne Easterling (JA).

• Parsons performed seismograph monitoring of vibrations along the length of the alignment while the tunnel was being excavated. Backed up by pre-excavation surveys of structures, the seismic records provide a means to evaluate any damage claims by residents.

• Numerous comments during the preconstruction constructability and biddability reviews helped refine the project documents to obtain the best pricing among competitive bids.

• Parsons worked with the design engineer, owner, and contractor to revise rock bolt requirements, thus saving $500,000. Parsons is working with these same people to develop a secondary grouting program that will minimize impacts from groundwater inflows in the most cost-efficient manner.

While a project’s success is typically measured at its completion, the ability to achieve that success is present each working day. Parsons used its considerable experience in construction management of large underground projects to identify problems at the earliest stage, resolve problems in a time effective manner at the lowest level, and mitigate impacts to this immense project. The project is scheduled to be completed in December of 2004 within the original project budget.

For more information, visit the project website: www.chattahoocheetunnel.com

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