Influence of Lithophysal Geometry on the Uniaxial Compression of Tuff-Like Rock
A large portion of the rock of the high-level nuclear waste repository at Yucca Mountain contains lithophysae or voids. These voids have a significant detrimental effect on the engineering properties of the rock mass and its performance. The lithophysae were formed at the time of volcanic deposition by pockets of gas trapped within the compressing and cooling pyroclastic flow material. Lithophysae vary by size, shape, and spatial frequency of occurrence. Due to the difficulties of testing actual lithophysal rock, the current mechanical property data set is limited and the numerical models of lithophysal rock are not well validated. The purpose of this task was to experimentally quantify the effect of void geometry in the mechanical compression of cubes of analog lithophysal-like rock. In this research the mechanical properties of the analog rock were systematically studied by examining various patterns of voids based on variables consisting of hole shape, size, and geometrical distribution. Each specified hole pattern was cast into 6 by 6 by 6-in. Hydro-StoneTB® specimens (produced in triplicate) and then tested under uniaxial compression. Solid Hydro-StoneTB® specimens exhibited similar mechanical properties to those estimated for rock mass solid specimens of Topopah Spring tuff. The results indicated that the compressive strength and Young’s Modulus values decrease with increasing specimen void porosity. The modulus and strength with void porosity relationships are essentially linear over the 5 to 20 percent void porosity range. When zero void porosity (solid specimen) results are added, exponential functions do not provide a good fit to the data due to a significant sensitivity of strength and modulus to the presence of macro-sized voids. From solid specimens there is roughly a 60 percent drop in strength with about 7 percent void porosity, increasing to an 80 percent drop at about 20 percent void porosity. The percent change in modulus from the solid specimen value is roughly 30 and 45 percent at 7 and 19 percent void porosity, respectively. A bilinear model gives a much better fit to the observed experimental data. Shape of hole appears to be significant for strength, but not for Young’s Modulus. Size of hole (at similar values of porosity) does not effect modulus values, but there may be a correlation with strength (smaller hole specimens are slightly stronger). Overall, the results help to validate the Yucca Mountain numerical model of lithophysal rock, but there are also some differences that should be looked into and explained. Hydro-Stone TB® specimens give mechanical strength results that are about one rock mass category lower than is expected based on their lithophysal porosity.
- Research Organization:
- Nevada System of Higher Education (NSHE), University of Nevada, Las Vegas
- Sponsoring Organization:
- USDOE - Office of Civilian Radioactive Waste Management (RW)
- DOE Contract Number:
- FC28-04RW12232
- OSTI ID:
- 908818
- Report Number(s):
- TR-07-001; ORD-FY04-013; TRN: US0703775
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
42 ENGINEERING
54 ENVIRONMENTAL SCIENCES
COMPRESSION
DEPOSITION
DISTRIBUTION
GEOMETRY
MECHANICAL PROPERTIES
PERFORMANCE
POROSITY
SENSITIVITY
SHAPE
TESTING
TUFF
YUCCA MOUNTAIN
lithophysae
Topopah Spring tuff
Yucca Mountain
Hydro-Stone TB®
mechanical properties
compressive strength
Young's Modulus