Laboratory study of borehole breakouts in Lac du Bonnet granite: a case of extensile failure mechanism

We subjected boreholes drilled vertically in the center of cubical specimens of Lac du Bonnet granite to constant vertical (Sv) and least horizontal (Sh) farfield stresses, and increased the maximum horizontal stress (SH) until breakouts occurred. A thin section study revealed that extensile cracking behind the borehole in two zones aligned with Sh preceded the development of breakouts. Progressive detachment of thin long flakes bounded by these densely spaced subparallel cracks led to 'dog ear' shaped breakouts. Breakout mechanism and appearance match those found in the field. Results also suggest the potential for using breakout occurrence to estimate SH if Sh is known or vice versa.