ROLE OF UNDERGROUND INVESTIGATIONS IN UNDERSTANDING THE GEOLOGY OF THE PROPOSED NUCLEAR WASTE REPOSITORY AT YUCCA MOUNTAIN, NEVADA

The original plan for scientific characterization of Yucca Mountain (YM), Nevada, as a nuclear-waste repository was to investigate primarily from the ground surface using a combination of outcrop mapping, borehole studies, and surface geophysics. Large-scale features such as significant faults and general lithostratigraphy were reasonably well understood from these surface-based investigations. Geologic factors however, such as complexity of fracturing and extent of lithophysal zones in ignimbrites of the Topopah Spring and Tiva Canyon Tuffs, and the contrasts with less fractured and interstratified nonwelded tuffs, made surface-dominant investigations impractical for defining these factors. Characterization of YM for an underground nuclear-waste repository required detailed understanding of the geology and hydrology in the proposed repository area. Because the proposed repository host horizon (RHH) rocks are not exposed at the surface, underground investigations, primarily in a 7.6-m-diameter, 7800-m-long tunnel, and a 5-m-diameter, 2600-m-long tunnel, allowed sampling of the spatial variability of the RHH to further define the heterogeneities of the mountain. Nine significant revelations resulted from these underground investigations: (1) the extremely stratabound nature of fractures with distinct variations in fracture orientation and characteristics between zones and subzones of individual ignimbrites, (2) asymmetry of “damage” across fault zones at depth, (3) narrow width of most faults at depth, (4) a nearly 1,000-m-wide, intensely fractured zone, (5) several faults not exposed at the ground surface, (6) relatively short trace lengths of fractures in nonlithophysal zones, (7) relative lack of post-cooling fracture mineralization, (8) variations in lithophysal characteristics in rocks of the RHH, and (9) lack of water seeping into excavations. These features could not have been characterized by surface-dominant investigations alone – they resulted from investigations enabled by underground excavations.