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Boring Hard, Abrasive Gneiss with a Main Beam TBM at the Atlanta Water Supply Program

Atlanta, Georgia’s water supply program is a priority project involving a 5.0-mile long tunnel connecting up with the Chattahoochee River, which will establish an emergency water supply for the city. A 12.5 ft diameter Main Beam TBM is boring the area’s deepest tunnel through hard, abrasive Gneiss rock at rates of up to 100 ft per day. This paper examines the project specifics and design, as well as the performance of the TBM. It also draws conclusions as to the optimal TBM design for excavation in the area’s exceedingly hard geology based on this project and past projects in the area.


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