A multiaxial stress criterion for short- and long-term strength of isotropic rock media

Natural and induced stress states around man-made openings in rock generally have different magnitudes along the principal axes. Accordingly, appropriate stability analyses should be based on expressions that take into account the full stress tensor. Based on the main features of compressive and tensile failure of isotropic media, the authors propose a general multiaxial criterion for describing the short-term failure (STF) strength as well as the damage initiation threshold (DIT) of rocks and rock masses. Time and size effects are explicitly included in the formulation. The influence of time is described by an extension of the subcritical crack growth theory in which the DIT is introduced. This damage initiation threshold represents the stress state below which there is virtually no crack growth, and hence corresponds to the long-term rock strength. Size effects, on the other hand, are treated using a generalized version of existing power-law expressions in which limiting values have been imposed for the relative size of representative elements. Application of the criterion to rock masses is done using simultaneously the size effect function for the case where no new types of defects are introduced, and a function of the RMR geomechanical classification to take into account the influence of new types of discontinuities such as joint sets. Sample applications of the criterion, using simplified calculations for rock structures, are shown and briefly discussed.