On the conditions at failure in schistose rocks

The conditions at failure are examined in schistose rocks using the critical plane approach. A general 3D anisotropic failure criterion is introduced, which accounts for the orientation dependency of strength. The onset of shear cracking is governed by Hoek-Brown's empirical function, while the tensile fracture is monitored by Rankine's cut-off criterion. The directional variation of strength parameters (m, s) in Hoek-Brown criterion, as well as the variation in tensile strength, is described by incorporating a second order traceless tensor that defines the bias in their spatial distribution. The orientation of the critical plane on which the failure commences is found by solving a constrained optimization problem. The validity of the proposed failure functions is checked through simulation of a number of compression and tension tests reported in the literature. The influence of the inclination of weakness planes on the variation of the axial strength is also discussed.