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The overcored specimen is inserted into the biaxial pressure cell to measure Young's modulus based on the thick-walled cylinder solution. Poisson's ratio is usually assumed to be known for a specific rock or measured in the laboratory using an extracted EX-size core.

Overcoring methods

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

  • 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.

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 Dd at q (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 DD at q=0, 60, and 120 are measured from field testing. Young's modulus E and Poisson's ratio n are measured from the extracted core. If the stress component, sy, 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.

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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Hooker, V.E. and D.L. Bickel, Overcoring equipment and techniques used in rock stress determination, USBR IC-8618 (1974).

Hooker,V.E, Aggson, J.R., and D.L. Bickel (with appendix by W.I. Duvall), Improvements in the three-component borehole deformation gage and overcoring technique, USBR RI-7894 (1974).

Fischer, D.J, Near surface stress measurements in a candidate rock mass for superconductive magnetic energy storage, MS Thesis, University of Wisconsin-Madison (1982).

Goodman, R.E., Introduction to rock mechanics, 2nd edition, John Wiley and Sons (1989).




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