West Area CSO Tunnels and Pump Stations

8.5 miles of CSO tunnel

One of the largest intake shafts ever constructed

Challenges

The City of Atlanta launched an aggressive ten-year capital improvement program to upgrade the city’s wastewater and drinking-water systems. It included eliminating combined sewer overflows (CSOs) to the environmentally sensitive Chattahoochee River and its tributaries.
The tunnel alignment had to traverse the highly variable metamorphic rocks of Piedmont province. Gneiss, schist, granite, and mylonite are typically strong and found in areas that dip gently toward the south.
The northern reaches of the alignment traverse the Brevard zone—an ancient fault, shear, or thrust zone that crosses Alabama, Georgia, and the Carolinas. Up to one-hundred feet of weathered saprolite overlie the rock mass and create a highly variable profile of the soil-rock contact.
The hydraulic performance of the system during operations was also of major importance due to the extreme flows through the CSO tunnel. These flows exert tremendous forces on the structures due to the rapid rate of filling and transient pressures.

This CSO tunnel system, situated to the west and north of Atlanta, is sized to provide 150 million gallons of CSO storage with total system peak inflows exceeding 10,000 cubic feet per second.
Hatch was part of a joint venture retained by the City of Atlanta. The task was to design and provide CM services for 8.5 miles of CSO tunnel and associated intake drop structures, construction shafts, approach structures, access shafts, odor control facilities, a 100 MGD pump station, and other appurtenant structures.
Conceptual studies were also performed regarding the future feasibility of adding over three miles of tunnels to control stormwater-related flooding in the Butler Highlands and Mineral Springs areas of Atlanta.
Terminal shafts for the West Area CSO facilities were designed to accommodate future construction and connections of these flood-relief tunnels while maintaining operations of the CSO tunnels.

Large-diameter TBM and drill-and-blast tunneling through faults and shears
Unlined tunnel in areas of competent rock
Fast-tracked design to meet federal court-ordered Consent Decree project milestones