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The modified ring test was used to determine the mode I fracture toughness of bedrock cores from the Department of Energy Oak Ridge Reservation in east Tennessee. Low porosity sandstones, limestones, and dolostones from the lower part of the Paleozoic section in the Copper Creek and Whiteoak Mountain thrust sheets were sampled. In general, the average mode I fracture toughness decreases from sandstone, dolostone, and limestone. The fracture toughness of the limestones varies between rock units, which is related to different sedimentologic characteristics. The quality of the results was evaluated by testing cores of Berea Sandstone and Indiana Limestone, which produced results that are similar to published results.
Hazardous, radioactive, and mixed wastes have been detected to depths of over a thousand feet within the fractured bedrock groundwater system on the U. S. Department of Energy's Oak Ridge Reservation. This has lead to the site being the focus of numerous studies aimed at delineating the geology and the processes that control groundwater flow and contaminant transport (e.g., Hatcher and others 1992; Solomon and others 1992). One topic that has received little attention considers the mechanical properties of the bedrock based on either laboratory or in-situ tests (Lemiszki and others 1995). The data are needed for geomechanical studies related to fracture development in the area and for hydromechanical models that couple rock mass deformation associated with fluid injection and withdrawal. We have been testing bedrock core to determine the mode I fracture toughness of the Paleozoic stratigraphie section in or near proposed environmental restoration sites in the Oak Ridge area. Here we will present mode I fracture toughness measurements based on unconfined tests on competent lithologies within cored parts of the Cambrian and Ordovician section. To our knowledge fracture toughness tests on these rock units have either not been conducted or reported. Also we will discuss our experience with using the modified ring test to measure the mode I fracture toughness of low porosity sedimentary rocks (Thiercelin and Roegiers 1986).
Encompassing an approximately 1000 km² area, the Oak Ridge Reservation is located in the western part of the Appalachian Valley and Ridge province in east Tennessee (Rodgers 1953). The stratigraphie units range in age from the Early Cambrian to the Early Mississippian (Hatcher and others 1992; Figure IA). The lithologies consist of primarily shale, siltstone and sandstone (Rome Formation and Conasauga Group) and primarily limestone and dolostone (Maynardville Formation of the Conasauga Group, Knox Group and Chickamauga Group). These units record the Paleozoic depositional history of the southern part of the Appalachian basin and have undergone complex diagenesis during burial, deformation, and uplift. As a result, primary porosity in these rock units is commonly less than 5% and groundwater uses secondary porosity features such as, fractures in the noncarbonates, and fractures, bedding planes, and solution conduits in the carbonates. Because of their regularity, connectivity, and abundance, the systematic fracture sets are considered the major factor controlling groundwater flow and karst conduit development.
