ABSTRACT: Breakouts along vertical boreholes have often been used as indicators to estimate the orientation and magnitude of one of the principal stresses in rock. According to the usual assumptions, the maximum horizontal (principal) .stress can be evaluated if the vertical stress and minimum horizontal stress are known. Previous investigations have mostly been based on bi-dimensional analysis, using 2D failure criteria. Theoretically, such 2D criteria are only valid for loading conditions corresponding to the test results used to determine material parameters, which is seldom the case around openings. Thus, a multiaxial criterion that can take into account the 3D nature of rock failure is more appropriate for a realistic determination of in situ stress from borehole breakouts. In this paper, the authors introduce a method to estimate the magnitude of both the maximum and minimum horizontal principal stresses from the geometry of single breakout. Using results from the literature, the full application of the proposed method is also shown. The applicability of the method to in situ rock masses is then discussed.

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