Just inaugurated, the South Cobb Tunnel Lift Station in Austell, GA, is the largest expansion project and the deepest tunnel construction for Cobb County Water Authority. PENETRON ADMIX was used to treat the concrete lining for structural support and to control groundwater flows at extreme depths.
The Cobb County Water System (CCWS) collects and treats wastewater for most of Cobb County, a metropolitan county located northwest of Atlanta, Georgia. The CCWS provides water, sewer, and storm water management services to more than 680,000 people in a 348 square mile area. An evaluation of projected population growth led the CCWS to invest in an upgrade and expansion of its infrastructure.
The South Cobb Tunnel project was carried out by Shea-Traylor J.V. (a joint venture of J.F. Shea Construction and Traylor Brothers); PENETRON waterproofing specialists also provided expertise to address the potential water pressure levels that can result from the unique depth of the tunnel construction.
Bigger Tunnel, Better Tunnel
Two primary interceptor sewers and two wastewater pump stations that formed the South Cobb Basin wastewater collection system were nearing capacity and unable to support future requirements. The South Cobb Water Reclamation Facility (WRF) needed to expand capacity to better equalize influent flow rates, which fluctuate during each 24‑hour period, and also handle precipitation inflows.
With a length of 8.9km (5.5 miles) and an 8.2m (27 ft.) diameter, the South Cobb Tunnel was built through hard rock. At a maximum depth of 122m (400 ft.), it is the deepest tunnel in the Atlanta area. The project also included a 130 MGD lift station to transport wastewater flows from the tunnel to the plant, with six 32.5 MGD vertical non-clog centrifugal pumps in a conventional wet well / dry pit configuration.The height of the lift station was approximately 212 feet.
The new South Cobb tunnel allowed CCWS to eliminate two major pumping stations and 87,000 linear ft. of aging sewer line and force main.
Structural Support and Groundwater Control
A “permeability-reducing admixture for hydrostatic conditions (PRAH),” as defined in the “Report on Chemical Admixtures for Concrete” (ACI 212.3R-10 / January 2011, published by the American Concrete Institute), PENETRON ADMIX was the ideal waterproofing admixture to resist high levels of water pressure, as well as for watertight tanks, foundations and containment structures. While the groundwater inflow rates were relatively low at the considerable depths of the South Cobb Tunnel, concrete lining was used to support areas of fractured rock and groundwater inflow. The design called for partial lining with cast-in-place concrete for structural support and/or groundwater control. PENETRON ADMIX was used to treat over 25,000 cubic yards of concrete.
“Working at extreme depths resulted in a number of difficulties during construction. But the whole South Cobb Tunnel project was a paradigm of seamless cooperation between the owner, contractor, construction manager and engineer,” explains Christopher Chen, Director of The PENETRON Group. “The cooperative atmosphere resulted in quick resolution of issues, in cost savings and schedule improvements. The six-year project was completed a year ahead of schedule!”