The theoretical treatment of the failure behaviour of bulk solids is often based on the simplifying assumption that the material acts as an isotropic continuum, regardless of the direction in which the stresses are acting. This article describes the results from measurements with both a standard Jenike shear tester and a biaxial tester which indicate that the yield behaviour of a cohesive solid can show significant anisotropic effects depending on the directions of the principal stresses at steady-state and incipient failure. Moreover, it can be concluded from those measurements that neither the flow function nor the density of a cohesive bulk solid can be regarded as an invariant with respect to stress history without taking into account possible anisotropic effects.
Saraber, F., Enstad, G. G., & Haaker, G. (1991). Investigations on the anisotropic yield behaviour of a cohesive bulk solid. Powder technology, 64(3), 183-190. https://doi.org/10.1016/0032-5910(91)80132-3