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South Cobb Tunnel – Secant Pile Supported Shaft

Cobb County, GA

Project At-a-Glance

  • Project included a 120-ft diameter 200-ft deep pump station shaft
  • Design of dewatering and blast cover systems
  • Secant Pile support of excavation design for the pump station shaft

Project Narrative

Cobb County determined that its 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.(now Keller), 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 a 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.

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