ACROSS THE BREVARD FAULT ZONE: THE CHATTAHOOCHEE INTERCEPTOR TUNNEL, COBB COUNTY, GEORGIA

The Chattahoochee Interceptor Tunnel (CIT), a 4.9-m (16-foot) diameter tunnel that is approximately 16.1-km (10-miles) long, is currently under construction in Cobb County, Georgia. The CIT crosses the Brevard Fault Zone along most of its entire length, except for the northern-most segment. The alignment crosses beneath numerous surface drainages and is locally close to the Chattahoochee River. Geological and hydrogeological considerations related to the tunnel, such as the aerial distribution and subsurface position of lithologic units, relative rock strength of units, and structural characteristics of units, in addition to the depth of weathering, and potential for groundwater inflow during construction and operation were an early part of the geotechnical baseline investigation and influenced the final design of the tunnel.

Detailed geologic mapping within the alignment corridor identified sixteen different lithologic/geotechnical units. Each lithologic/geotechnical unit was evaluated on the basis of mineralogy, texture, weathering characteristics, structure, and the presence, nature and abundance of discontinuities, including lithologic contacts. This data relates to rock strength, breakage characteristics, type and relative depth of weathering, rock mass classification, and groundwater infiltration potential. Evaluation of this data allowed preliminary estimation of tunneling conditions and served as a guide for the location of vertical and angled exploratory core holes; as well as the location of 6-inch air-rotary drill holes to address depth of weathering and areas with the potential of large-volume groundwater movement that might be encountered during tunnel construction.

Lithologic/geotechnical units identified during surface mapping were correlated with subsurface data from rock-core and air-rotary drilling. Additionally, the surface units are currently being correlated with subsurface exposures during shaft excavation (access and pumping) and initial drill and blast portion of the tunnel; and will continue during the tunnel boring operations.