Overcoring Methods for Stress Measurements in
Competent rocks
Overcoring methods are measuring in situ stress based on the stress relief around the borehole.
The relief of external forces by overcoring causes the changes in
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borehole diameter for "USBM deformation gage";
strain on the borehole wall for "Triaxial strain cell", or
changes in strain on the flat end of the borehole for "Doorstopper".
If the elastic properties of the rock are known, the changes in borehole diameter or strains can be
converted to in situ stress in the rock.
USBM deformation gage - method
The USBM three-component borehole deformation gage is designed to measure diametral
deformations of an EX-size (1.5" in diameter) borehole during overcoring a concentric borehole
(6" in diameter). The diametral deformations are measured in three directions (60 degree apart)
in the same diametral plane.
The field procedures consist of drilling a concentric EX-size borehole, installation of the
deformation gage, and overcoring a stress relief borehole.
Click the above picture to get the full size drawing.
USBM deformation gage - instrument
The borehole deformation gage consists of six strain-gaged cantilever transducers made out of
heat-treated beryllium copper. At the free end of each cantilever, tungsten carbide button is
mounted to make a contact with the borehole wall. A pair of transducers, placed 180 degree
apart, measure diametral deformation of the borehole.
Adapted from http://www.hydrofrac.com
USBM Deformation gage
USBM deformation gage - theory
If the borehole is oriented in 0y direction and the xz plane is perpendicular to the borehole, the
diametral deformation d at  (angle measured counterclockwise from 0x oriented with the a pair
of buttons) can be represented as follows based on the theory of elasticity and the Kirsch
solution.
The diametral deformations  at =0, 60, and 120 are measured from field testing. Young's
modulus E and Poisson's ratio  are measured from the extracted core. If the stress component,
y, parallel to the borehole is known (usually assumed to be zero for shallow measurements), the
stress components in xz plane can be calculated as follows.
Adapted from http://www.hydrofrac.com
Three channels of diametral deformations logged continuously as the deformation gage is
overcored. As the overcoring bit passes through the plane of measurements the stresses
are relieved and the results are shown as diametral deformations (After Fisher, 1982).
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Adapted from http://www.hydrofrac.com