Anisotropic mechanical properties and brittleness evaluation of layered phyllite

In order to study the mechanical properties and anisotropy characteristics of layered phyllite, rock mechanical tests of phyllite were conducted under different bedding inclinations and different confining pressures. The anisotropic mechanical properties in strength, deformation, brittleness and failure modes of phyllite samples were comparatively analyzed. Some findings were as follows. (1) As the bedding inclination beta increases, the strength and deformation characteristic curve of the rock sample is U-shaped; and the strength and plasticity increase, the anisotropy gradually decreases and stabilizes with the increase of confining pressure. (2) The anisotropic Saeidi criterion and the modified Ramamurthy criterion can well predict the failure strength of rock specimens under different bedding inclinations. (3) A comprehensive brittleness index of phyllite samples at different bedding inclination under different confining pressures is proposed based on the stress strain curve characteristic and energy equilibrium. The brittleness index is lower, and the shear failure is more likely to occur when the bedding inclination beta is about 45 degrees. The relationship between brittleness and the failure mode is revealed, and the decreasing order of brittleness is concluded as follows: tensile splitting along bedding plane > tensile splitting through bedding plane > shear along bedding plane > shear through bedding plane. (4) The failure modes of phyllite are related to the bedding inclinations and confining pressures. Under uniaxial compression conditions, complex networks of fractures are formed easily after splitting failure of rock samples; and under high confining pressure conditions, a single shear failure plane is generally formed along the bedding plane or through multiple planes after rock failure.