Shear behavior of inherently anisotropic rocks

The effect of schistosity plane orientations on failure mechanism and effective shear strength response of inherently anisotropic rocks are dealt with in this paper, with an emphasis on shear parameters of the weak planes. The study employed an oblique shear apparatus to apply shear and normal stress on specimens at three predetermined shear inclinations. The concurrent application of confinement and shear loading was made to the apparatus. Both stresses were developed as a result of vertical loading, confined stress conditions and hence their failure mechanisms.The results indicate a transition state of shear strength values, fluctuating from a minimum to a maximum based on β angle, due to the anisotropic effects of schistosity plane orientation coupled with confinement stress. β is the angle between strike of weak planes and the predetermined shear loading plane. Hence, the anisotropic effect of weak planes orientation is a significant occurrence that must be noted in the analysis and failure mechanism studies. It was also observed that different shearing failure modes were built up depending on the orientations of, a family, weak planes with respect to confinement stresses as: (1) tensile fracture across the weak planes (non-sliding mode), (2) tensile-split along the weak planes (non-sliding mode), and (3) sliding failure along the weak planes and/or combination of these mechanisms. The experimental results demonstrated that, the failure modes were changed from non-sliding into sliding along weak planes when dip of weak planes increases. It was also noticed that due to the influence of confinement stresses, failure occurred through rock material together with tensile splitting of weak planes. This was more prominent for higher values of strike direction of weak planes (β). Investigations were extended to examine the influence of effective shear stresses on the failure mechanisms considering various dip and strike directions of weak planes.

► Investigation of shear response of inherently anisotropic rocks.
► Oblique shear experiments applying shear and normal stress.
► Proposed empirical expression for determination of effective shear strength of schistose rocks.
► Failure modes of specimens considering weak planes orientations and confinement stresses.