Borehole stability in densely welded tuffs Page: 4 of 71
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Failure of the host rock at locations where seals are installed may
allow the development of bypass flowpaths around seals. This report
presents an investigation of conditions that might induce compressive
failure around boreholes in densely welded Apache Leap tuff. Triaxial
and polyaxial tests have been performed on tuff cylinders and rectangu-
lar blocks containing coaxial cored holes. Nominal test hole diameters
are 14 mm for triaxial testing and 25.4 mm for biaxial testing.
To induce borehole breakout failures requires the application of
stresses that exceed significantly the stresses corresponding to elasti-
cally calculated boundary stresses equal to the uniaxial compressive
strength of the tuff. The results confirm the strength increase of
welded tuff with density. Failure patterns are influenced by the heter-
ogeneity of the rock: soft inclusions fail first. Such failures tend
to remain localized. The stronger surrounding matrix provides the
strength needed to maintain hole stability. The results suggest that an
elastic analysis of hole stability in welded tuff may inherently provide
a significant safety margin. This conclusion needs to be qualified by
the fact that all experiments reported on have been conducted on bore-
holes with very small diameters.
It would be desirable to conduct similar borehole stability experiments
on holes with larger diameters. Of particular importance would be an
evaluation of the influence of flow layers in tuff on borehole stabil-
ity. Effects of flow layers have been minimized in the present investi-
gation by preparing all samples normal to the flow layers. Also
desirable would be an investigation of the influence of environmental
conditions, especially temperature and moisture content, and of the
strength under sustained long-term loading. Finally, a more comprehen-
sive analysis of the results should be performed, including evaluation
of the numerous more recently published theoretical models for borehole
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Fuenkajorn, K. & Daemen, J.J.K. Borehole stability in densely welded tuffs, report, July 1, 1992; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc625054/m1/4/: accessed February 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.