Fracture characterization of the large-block test, Fran Ridge, Yucca Mountain, Nevada

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The US Department of Energy (DOE) is investigating the suitability of Yucca Mountain as a potential site for the nation's first high-level nuclear waste repository. The site is located about 120 km northwest of Las Vegas, Nevada, at the Nevada Test Site. Favorable aspects of Yucca Mountain as a potential repository site include its arid nature and the sorptive properties of the rock materials. The arid environment results in unsaturated conditions at the potential emplacement horizon, which is the Topopah Spring tuff of the Paintbrush Group. The Large Block Test (LBT) was designed to be one of a series of tests at different scales and conditions that assist in defining the physical processes that need to be considered in models of a potential repository in Yucca Mountain. The LBT is a critical test because it is of sufficient size to incorporate a fracture system that is representative of the distribution of fracture dimensions and characteristics--with the exception of major structures, such as faults--that would likely be present in a repository. The LBT location was chosen to include large, through-going fractures as well as small, healed fractures that are of limited extent. The LBT location also includes a variety of fracture sizes, connectivities, and characteristics that fall between the bounds of the large and very small fractures. The LBT allows for boundary controls and monitoring that are somewhat similar to those typical of laboratory studies, and it allows for three-dimensional (3-D) characterization and monitoring. The unique combination of size with boundary controls of the LBT allows processes to be evaluated and models to be tested more completely than in tests of any other scale (Wilder et al. 1997, Section 1).