Review of geotechnical measurement techniques for a nuclear waste repository in bedded salt Page: 95 of 285
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Pressure charges recorded by the cell monitor only the sum of the changes
of the two principal rock stresses acting in the plane normal to the
borehole axis. Thus, changes in stress can occur that will not be sensed
by the cell, the most serious case occurring where an increase in the
maximum princiial stress is offset by an equal decrease in the minimi*
principal stress. All cells should be installed with their fluid pres-
sure preset at a value above that of the maximum principal stress. If
the rock stress decreases, the borehole diameter will expand, possibly
to such a degree that the cell membrane begins to retard the fluid
expansion rather than the rock. In this instance, cell pressures will
reflect the elastic nature of the membrane and not of the rock. The
sensitivity of a cell decreases with an increase in rock modulus, and
increases with an increase in cell stiffness, where stiffness is defined
as the pressure.'”olume ratio of the cell system. For this reason, most
cells are made as stiff as possible.
A flat cell grouted into a cylindrical borehole is called a Borehole
Pressure Cell (BPC), and is a type of flat fluid cell developed by the
U.S. Bureau of Hines (Panek and Stock, 1964). In these cells, only rock
stress changes acting normally to the flat face of the cell can be monitored;
however, two cells oriented at right angles can monitor the biaxial
stress changes acting normal to the borehole axis.
Open hole stress monitoring can be done with overcoring probes such as
U.S. Bureau of Mines borehole deformation gage. These probes monitor the
deformations in the perimeter of the borehole that occur over time. Any
changes in regional rock stress will be reflected by a distortion of the
borehole wall. Using the theory of elasticity, one can convert these
distortions into rock stress changes; however, the elastic properties
of the rock must be known. If rock stresses exceed the failure stress
of the rock, then failure will occur around the borehole perimeter, and
the open-hole readings will be erroneous.
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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/m1/95/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.