ABSTRACT
Experiments in which borehole breakouts were produced in different rocks have revealed three major mechanisms of rock fracturing. In all tests a true triaxial stress condition was applied to cubical rock specimens. Then a central vertical borehole was drilled, which induced borehole-breakout failure, depending on the magnitudes of the far-field stresses. In crystalline granite intra- and trans-granular dilatant microcracking subparallel to the borehole wall and to the direction of the maximum horizontal stress σH preceded the development of breakout failure, resulting in ‘V’-shaped failed zones along the σh spring line. In well-consolidated sandstone breakout failure took a similar ‘V’ final shape. However, microcracks preceding failure were mainly intergranular, and only occasionally intragranular. In a less consolidated but still competent sandstone, failure did occur at the same locations as with the other rock types but, surprisingly, the final breakout shape was a narrow linear fracture perpendicular to σH direction. The mechanism of this failure mode appears to be largely matrix non-dilatational disintegration leading to grain disaggregation.
