Review of geotechnical measurement techniques for a nuclear waste repository in bedded salt Page: 93 of 285
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• Mining excavations
• Pillar failures
« Roof failures
• Changes in the ground-water table
• Rock expansion due to heating
a Rock contraction due to cooling
a Other factors.
The analysis of stress changes is often a problem of determining the
reason for the change.
Hard-inclusion gages have been used to monitor stress changes in pillars
in coal mines (Wilson, 1961) and saltmines (Potts, 1964; Dreyer, 1977).
A hard-inclusion gage has two advantages when used in rock susceptible
to creep, such as salt. First, as mentioned earlier, a hard-in. .sion
gage will measure stress independently of the elastic moduli of c salt,
thus making determinations of salt properties unnecessary. Second, the
stresses measured by the inclusion will not be seriously affected by
creep. If a rigid inclusion is placed in rock that is creeping at a
constant rate, the stress level in the inclusion will be raised until
stress equilibrium is reached between the salt and the inclusion. Thus,
at equilibriun, the in-situ stress can be read directly without the need
to correct for creep (Ski 1 ton, 1971).
Soft-inclusion stress monitoring systems include fluid-filled borehole
sections, fluid-filled cylindrical cells for borehole placement, fluid-
filled flat cells encased in cylinders of grout and placed in boreholes
(borehole pressure cells, or BPCs), and fluid-filled flat cells placed
in pre-cut slots. In all cases, rock stress changes cause changes in
fluid pressure 1n the cells. Because a change in cell pressure tends
to counteract the deformations induced by the rock stress changes, the
cell pressure change can be related to a given rock stress change
(Barr 1960),
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Review of geotechnical measurement techniques for a nuclear waste repository in bedded salt, report, December 1, 1979; United States. (https://digital.library.unt.edu/ark:/67531/metadc1093986/m1/93/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.