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Abstract The existence of bedding planes is the fundamental cause of the anisotropy of the mechanical behaviors，the strength and the failure modes of shale formations and is also one of the main reasons for borehole instability of horizontal wells. In order to analyze the mechanical properties of bedding planes and the anisotropy of shear strength，direct shear tests on shale sample with different bedding angles were carried out. The causes for the anisotropy of shear strength were analyzed based on the anisotropy of the mechanisms of shear failure and the concentration factor of shear stresses. (1) The cohesion and the internal friction angle of bedding planes were found to be significantly smaller than those of the matrix of shale；the bedding planes are weak ones in shale formations and have the smallest shear strength. The shear stress-shear displacement curves do not exhibit the characteristic of slip-weakening when shear failure occurs along bedding planes and the residual friction is even slightly higher than the shear strength of the bedding planes. (2) The largest shear strength occurred if the angle ? between the bedding planes and the normal stress is 60°among the four angles of ? = 0°，30°，60°and 90°. The shear stress-shear displacement curves exhibit the characteristic of slip-weakening when the shear failure occurs for the shale samples with ? = 0°，30°and 60°. (3) The failure mechanism of shale is divided into three types: the shear failure across matrix，the sliding across matrix together with the tensile splitting of the weak planes, and the sliding along the bedding planes. The anisotropy of shear strength of shale is governed by the anisotropy of shear failure mechanism. (4) To some extent，the shear stress concentration factor can reflect the shear strength of rock, and it can be used to analyze the anisotropy of shear strength of shale. In the direct shear tests of shale，the shear stress concentration factor is related to the elastic modulus in shear direction and the thickness of shear layers. Under the same normal stress，the shear stress concentration factor is the largest with ? = 90° and the smallest with ? = 60°. But the shear strength of shale is the smallest with ? = 90° and the largest with ? = 60°.
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Received: 16 August 2013
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2014, Vol. 33 
