Intermediate stress effect in reverse: Strength anisotropy

Because rock is typically sampled in the form of a right circular cylinder, it is convenient from a laboratory testing perspective to produce failure by the application of an axi-symmetric state of stress called triaxial compression, where the major principal stress σ_I = σ_a the axial stress, and the intermediate and minor principal stresses σ_II = σ_III = σ_r the radial stress. For isotropic rock, compression testing σ_I > σ_II = σ_III leads to a conservative estimate of strength parameters, as opposed to triaxial extension, where σ_I = σ_II > σ_III. However, it is shown that even though a rock may exhibit only slight (< 10%) elastic anisotropy, anomalous behavior can occur in strength testing such that a "reverse" intermediate stress effect appears.