Graduation date: 1999

Yucca Mountain was designated as the proposed high-level radioactive waste repository by the U.S. Government in 1987. The proposed repository design requires high safety for a long maintenance period of 10,000 years. To satisfy this requirement, evaluation of the influence of earthquakes on the repository is necessary. Prediction of earthquake-induced ground motions around the repository requires knowledge of the dynamic properties of the geologic materials around the repository. The main geologic materials in the vicinity of Yucca Mountain are tuffs (ignimbrites) which are formed by the deposition of volcanic ash mixed with erupted volcanic gas, water vapor and pyroclastic material. Two types of dynamic tests, (1) the free-free, unconfined, resonant column and direct arrival test (freefree URC test) and (2) the fixed-free resonant column and torsional shear test (fixed-free RCTS test), were used to measure the dynamic properties of tuffs. The emphasis in this dynamic testing was evaluation of shear modulus (G) and material damping ratio (D) of the tuffs in the small-strain (linear) and mildly nonlinear (to strains of about 0.02 %) ranges. To evaluate the influence of various parameters on G and D of tuffs, correlations with other features such as total unit weight, porosity and stratigraphic unit were performed and general relationships between them are proposed. In addition, an unconfined, slow-cyclic torsional shear (CTS) device was developed and used to measure the cyclic shear properties of the tuffs from Yucca Mountain at larger strain amplitudes than possible in the fixed-free RCTS tests. Additionally, the CTS device was also used to determine the shear failure strength of the tuffs. By combining the cyclic shear properties of the tuffs from the CTS tests and the dynamic properties of the tuffs from the fixed-free RCTS tests, complete dynamic property curves from small-strain to failure strain were evaluated. / text

Ash-flow tuff (ignimbrite) is a general term indicating consolidated deposits of volcanic ash flow; a flow of a mixture of gas and pyroclastic materials as products of explosive volcano eruptions (Smith, 1960). Two different ash-flow tuffs are studied in this research: 1. Topopah Spring Tuff at Yucca Mountain, Nevada and 2. the Bandelier Tuff at Pajarito Plateau, New Mexico. Various dynamic test parameters (e.g. confining pressure, shearing strain, etc) were studied with two existing devices: (1) the combined resonant column and torsional shear (RCTS) device, and (2) the free-free, unconfined, resonant column (URC) device. The effects of these parameters are evaluated for two different types of ash-flow tuffs. In addition, a Large Resonant Column (LgRC) device was developed and used to test the some tuffs from Yucca Mountain at larger strain amplitudes than possible with the RCTS and URC devices. Relationships between the linear and nonlinear dynamic properties and lithostratigraphic features were further investigated. Finally, potential problems related to sample disturbance and specimen size are considered based on comparisons of small-strain shear wave velocity (VS) values measured in the laboratory and in the field. / text

Yucca Mountain was designated as the proposed high-level radioactive waste repository by the U.S. Government in 1987. The proposed repository design requires high safety for a long maintenance period of 10,000 years. To satisfy this requirement, evaluation of the influence of earthquakes on the repository is necessary. Prediction of earthquake-induced ground motions around the repository requires knowledge of the dynamic properties of the geologic materials around the repository. The main geologic materials in the vicinity of Yucca Mountain are tuffs (ignimbrites) which are formed by the deposition of volcanic ash mixed with erupted volcanic gas, water vapor and pyroclastic material. Two types of dynamic tests, (1) the free-free, unconfined, resonant column and direct arrival test (freefree URC test) and (2) the fixed-free resonant column and torsional shear test (fixed-free RCTS test), were used to measure the dynamic properties of tuffs. The emphasis in this dynamic testing was evaluation of shear modulus (G) and material damping ratio (D) of the tuffs in the small-strain (linear) and mildly nonlinear (to strains of about 0.02 %) ranges. To evaluate the influence of various parameters on G and D of tuffs, correlations with other features such as total unit weight, porosity and stratigraphic unit were performed and general relationships between them are proposed. In addition, an unconfined, slow-cyclic torsional shear (CTS) device was developed and used to measure the cyclic shear properties of the tuffs from Yucca Mountain at larger strain amplitudes than possible in the fixed-free RCTS tests. Additionally, the CTS device was also used to determine the shear failure strength of the tuffs. By combining the cyclic shear properties of the tuffs from the CTS tests and the dynamic properties of the tuffs from the fixed-free RCTS tests, complete dynamic property curves from small-strain to failure strain were evaluated.

The study of the subsurface flow and distribution of water is critical to the evaluation of the unsaturated zone for a potential geologic high-level radioactive waste repository. This site is located at Yucca Mountain, Nevada in the northern Mojave Desert. and was chosen on the basis of its low precipitation, deep unsaturated zone, and layered volcanic rocks providing the potential for natural hydraulic barriers to reduce the downward percolation of water through the waste storage area. The detailed characterization of hydrologic properties is necessary to evaluate the mechanisms responsible for the distribution and flow of water in the unsaturated zone. Analyses in this study have provided detailed hydrogeologic units with unique hydrologic properties and hydraulic parameters. Porosity was determined to be a useful physical property for predicting hydraulic parameters, as it relates to the largescale deterministic processes that created the volcanic rocks. The detailed property dataset, along with field measurements of moisture status, temperature, and chemistry, were used to evaluate the potential for lateral diversion in the rocks above the potential repository. It was determined that lateral diversion is a small-scale process in this natural system. On the basis of analyses performed in this study, it is suggested that large-scale diversion is not likely to occur at this site. This mechanism should not, therefore, be relied upon to perform as a natural hydraulic barrier to flow reducing percolation through the unsaturated zone. / Graduation date: 2002