Experimental and theoretical study of the anisotropic properties of shale

●Quantization of shear strength parameters of bedding planes and matrix of shale.●Maximum failure strength is not obtained in the matrix (β=0°) but for β=60°.●Expression of shear stress concentration factor of direct shear model is derived.●Anisotropic shear strength is evaluated based on shear stress concentration factor.

The effect of bedding plane orientations on shear strength and failure mechanisms of shale is investigated, with an emphasis on the shear parameters of the bedding planes. This study is based on direct shear tests conducted on directional shale samples obtained from formations which exhibit well-developed stratification. The tested cylindrical specimens were prepared by coring at 30° intervals from the direction of the bedding planes. The results indicate that the anisotropy of the shear strength of shale can be readily observed due to the anisotropic effects of bedding plane orientation and normal stress. The shear parameters, such as strength, cohesion and internal friction angle reach their maximum and the minimum values at β=60° and β=0°, respectively, where β is the angle between the bedding planes and the coring orientation, which is also the direction of normal stress during the laboratory tests. It is demonstrated that the bedding planes are weak not only with regards to cementing strength, but also with regards to friction. Three different shear failure modes were built up depending on the shearing angle β: (1) sliding failure across the bedding planes, (2) sliding failure along the bedding planes and (3) sliding failure across the bedding planes combined with tensile splitting along the bedding planes. An expression of the shear stress concentration factor of the direct shear model is derived to evaluate the anisotropy of shear strength.