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Breakouts are zones of failure of the borehole wall that symmetrically form at the azimuth of the least principal horizontal stress (Fig. 15.1). Breakouts are the best indicators of maximum horizontal stress when enough data are available. The breakouts frequently are extended in the direction of the borehole axis and can be described by orientation in the borehole (θb), opening angle (2ϕb), and radial depth (rb) (Lund and Zoback 1999).

Two hypotheses are used to explain the formation of breakouts. The first hypothesis suggests that if the difference in magnitude of two horizontal principal stresses is high enough, compressive shear failure will cause breakouts to form in the borehole wall. The opening angle can be empirically related to the far-field in-situ stresses:

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