Brazilian Tensile Strength of Anisotropic Rocks: Review and New Insights

Strength anisotropy is one of the most distinct features of anisotropic rocks, and it also normally reveals strong anisotropy in Brazilian test Strength (BtS). Theoretical research on the BtS of anisotropic rocks is seldom performed, and in particular some significant factors, such as the anisotropic tensile strength of anisotropic rocks, the initial Brazilian disc fracture points, and the stress distribution on the Brazilian disc, are often ignored. The aim of the present paper is to review the state of the art in the experimental studies on the BtS of anisotropic rocks since the pioneering work was introduced in 1964, and to propose a novel theoretical method to underpin the failure mechanisms and predict the BtS of anisotropic rocks under Brazilian test conditions. The experimental data of Longmaxi Shale-I and Jixi Coal were utilized to verify the proposed method. The results show the predicted BtS results show strong agreement with experimental data, the maximum error is only similar to 6.55% for Longmaxi Shale-I and similar to 7.50% for Jixi Coal, and the simulated failure patterns of the Longmaxi Shale-I are also consistent with the test results. For the Longmaxi Shale-I, the Brazilian disc experiences tensile failure of the intact rock when 0 degrees <= beta(w) <= 24 degrees, shear failure along the weakness planes when 24 degrees <= beta(w) <= 76 degrees, and tensile failure along the weakness planes when 76 degrees <= beta(w) <= 90 degrees. For the Jixi Coal, the Brazilian disc experiences tensile failure when 0 degrees <= beta(w) <= 23 degrees or 76 degrees <= beta(w) <= 90 degrees, shear failure along the butt cleats when 23 degrees <= beta(w) <= 32 degrees, and shear failure along the face cleats when 32 degrees <= beta(w) <= 76 degrees. The proposed method can not only be used to predict the BtS and underpin the failure mechanisms of anisotropic rocks containing a single group of weakness planes, but can also be generalized for fractured rocks containing multi-groups of weakness planes.