ABSTRACT
Proper estimation of the in-situ intact rock strength as well as an estimation of the in-situ stresses forms the basis for most geomechanical designs of underground openings. Borehole failure constitutes a well accepted stress orientation indicator, but its use for stress magnitude estimation is controversial. Indeed, in order to obtain a proper estimate of stress magnitude from borehole breakouts, an accurate and independent estimation of the in-situ strength of the borehole walls is required. Zoback et al. (2003) suggest that the UCS obtained from core samples provides an acceptable estimate of borehole wall strength. In relatively high stress environments, however, damage is created in the core while drilling, which may significantly reduce the core strength (e.g. Martin and Stimpson, 1994). Such core damaging processes must be expected in stress environments where breakouts form. Consequently, an underestimation of UCS due to core damage could lead to an underestimation of the magnitude of in-situ stress from breakout back-analyses. At depth, both the core and the borehole wall experience complex but not identical stress paths during drilling. Since these paths are not identical and may or may not result in damage and strength reduction in comparison to that of the rock remote from the wellbore, the
damage and strength reduction of the core may not be equal to that in the wall.
