Influence of Lithophysal Geometry on the Uniaxial Compression of Tuff-Like Rock

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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 ... continued below

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Rigby, Douglas B. June 13, 2007.

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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.

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  • Report No.: TR-07-001
  • Grant Number: FC28-04RW12232
  • DOI: 10.2172/908818 | External Link
  • Office of Scientific & Technical Information Report Number: 908818
  • Archival Resource Key: ark:/67531/metadc884255

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Reports, articles and other documents harvested from the Office of Scientific and Technical Information.

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  • June 13, 2007

Added to The UNT Digital Library

  • Sept. 22, 2016, 2:13 a.m.

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  • March 23, 2018, 3:37 p.m.

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Rigby, Douglas B. Influence of Lithophysal Geometry on the Uniaxial Compression of Tuff-Like Rock, report, June 13, 2007; United States. (digital.library.unt.edu/ark:/67531/metadc884255/: accessed December 11, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.