South Cobb Tunnel – Circular Concrete Secant Pile Wall

Cobb County, Georgia

Cobb County determined that their existing infrastructure would not support future wastewater flows based on projected population increases in the region. To convey and store future combined sewer flows, a 5.5-mile long, 27-ft diameter tunnel, a 120-ft diameter 200 ft deep pump station shaft, and two, 45-ft diameter construction shafts were planned.

Brierley Associates was retained by Hayward Baker, Inc to design dewatering and blast cover systems, as well as the temporary excavation support  of soil and weathered rock  in which the pump station shaft was constructed.  A circular, concrete secant pile wall, with concrete and steel ring beams were designed to retain the 15-ft to 48-ft thick layer of soil and transition zone material (residual soil and weathered rock) during rock excavation to a depth of over 200-ft for the construction of the permanent pump station structure.