A fracture and permeability analysis was performed on the Santana Tuff because of its similarity to the
Topopah Springs unit at the Yucca Mountain site. The
Topopah Springs unit is the proposed horizon for the spent nuclear fuel repository. Because of the impossibility of completely characterizing the flow properties of the unit without destroying the
characteristics that make it desirable as a repository, other ash flow tuffs must be studied. The Santana Tuff and the Topopah Springs tuff both are rhyolitic in
composition, nonwelded to densely welded and fractured.
Fractures were examined at six outcrop locations
spanning a five mile area. Stereonets and rose diagrams were constructed from over 312 fracture
orientations. Although the composite data showed two
major orientations of nearly vertical fractures, fracture trends at individual outcrops showed a variety
of preferred orientations. Over 900 surface permeability measurements were
taken using a mini-permeameter. The samples were
categorized by three observed types of surface
weathering: fresh, weathered, or varnished. Fracture
surfaces were generally classified as weathered. The
average permeabilities for the samples are 55.33 millidarcies, 5.03 millidarcies, and 3.31 millidarcies,
respectively. The one-way statistical analysis performed on the data indicated that the permeability
of fresh tuff surfaces is significantly different than
both the permeabilities of the weathered and varnished tuffs, using both a least significant difference and
greatest significant difference test. However, no
difference was shown to exist between the weathered and
varnished tuff permeabilities.
Samples of fresh, weathered, and varnished tuffs
were examined by X-Ray Defraction, the Scanning Electron Microscope, and in thin section. The SEM analysis showed surface differences between the three weathering classifications. The weathered and
varnished samples were similar, exhibiting a platy,
lamellate texture. The fresh surfaces were irregular
and jagged. In thin section, a thin rind of dark
minerals (FE-oxides) is observed on the edges of the
varnished samples and in microcracks. This fills surface pores and causes the reduction in permeability. Two other zones of weathering have been identified in
some of the samples, which may also cause changes in permeability. Tuff permeabilities were also analyzed for directional dependence. After an ash flow tuff is
deposited and cooled, it may undergo flattening of pumice fragments and glass shards. These flattened
fragments can be identified in handsamples, and are
indicative of the direction of flow emplacement. The
analysis showed that permeability is enhanced parallel
to the emplacement direction, which is generally horizontal. Cut surfaces showed a 30% decrease in
permeability perpendicular to flow direction. On
varnished surfaces, this trend is still evident, although decreased in magnitude. This is expected because of the clay particles which make up the desert varnish. This study indicates that the formation of low
permeability weathering rinds in association with
vertical fractures may inhibit infiltration at the
surface. It may accelerate infiltration at depth and allow more fluid to penetrate vertically into the tuff. In the event that fluid is absorbed into the matrix, it will travel horizontally, along the enhanced
permeability parallel to the emplacement direction. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/6777 |
| Date | 11 December 2009 |
| Creators | Fuller, Carla Matherne |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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