Abstract
We subjected vertical boreholes drilled in cubical specimens of Cordova Cream to a 3-dimensional stress field, and increased one or both of the horizontal principal stresses until breakout failure occurred. Acoustic emission, borehole strain, and visual monitoring of the borehole wall during testing revealed four stages of breakout development: crack initiation, extension, breakout formation, and stabilization. Thin section analysis suggests that breakout formation in Cordova Cream is the result of shear failure mechanism. The relationships at breakout stabilization between the two horizontal principal stresses and between the span at the borehole wall and the least horizontal stress suggest a potentially simple method of estimating the far-field state of stress if ‘breakouted’ borehole geometry is accurately mapped.
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