Asymmetric borehole breakouts at the URL

Investigations into borehole breakouts in brittle rock usually treat the phenomenon as a two-dimensional problem. In these cases, failure is assumed to initiate at the point of maximum tangential stress concentration at the borehole wall. According to linear elastic theory for a circular hole in an elastic medium, the maximum stress concentrations around the hole are diametrically opposed for anisotropic far-field stress conditions. At the 420 Level of the URL, boreholes drilled at angles not coincident with a principal direction typically developed breakouts that were asymmetric, or not diametrically opposed. Numerical modeling of two large-diameter boreholes demonstrates that the asymmetry in azimuth and depth of borehole breakouts in highly stressed granite can be explained by the maximum deviatoric stress pattern ahead of the advancing borehole.