Mechanical degradation of emplacement drifts at Yucca Mountain—A modeling case study—Part I: Nonlithophysal rock

Abstract This paper outlines rock mechanics investigations associated with mechanical degradation of planned emplacement drifts at Yucca Mountain, which is the designated site for the proposed US high-level nuclear waste repository. The welded tuff emplacement horizon consists of two groups of rock with distinct engineering properties: nonlithophysal and lithophysal units, based on the relative proportion of lithophysal cavities. The term ‘lithophysal’ refers to hollow, bubble like cavities in volcanic rock that are surrounded by a porous rim formed by fine-grained alkali feldspar, quartz, and other minerals. Lithophysae are typically a few centimeters to a few decimeters in diameter. Part I of the paper concentrated on the degradation behavior of the generally hard, strong, and fractured nonlithophysal rock. Part II concentrates on the host rock in the lithophysal units. Lithophysal rock is characterized by lithophysal cavities interconnected by fracturing. Fracture sets are not as clearly defined as in the nonlithophysal rock. The rock mass porosity in the lithophysal units has been shown to be the primary physical factor that governs elastic and strength properties. The degradation behavior of the tunnels in the lithophysal rock is controlled by the spalling of the surrounding rock mass. A discontinuum material model has been developed to represent the drift scale rock mass properties in which slip and separation of contacting rock blocks can be estimated. Two-dimensional discontinuum analyses were developed with the consideration of in situ, thermal, and seismic loads. Time-dependent degradation was also considered. In this study, field and laboratory data and numerical analyses are well integrated to provide a solution for the unique problem of modeling drift degradation.