Abstract: In order to describe the anisotropy characteristics of rock strength, a modified Hoek-Brown yield criterion based on the microstructure tensor is proposed, in this process, the microstructure tensor of the material is introduced, considering the change of uniaxial compressive strength with the bedding dip ?. Conventional triaxial compression tests are conducted on shale samples with different bedding orientations (0°, 15°, 30°, 45°, 60°, 75° and 90°) under different confining pressures (0, 5, 10, 15 and 20 MPa). The results show that the peak strength of shale and the bedding dip ? have a "U"-shaped curve relationship. When the confining pressure is low, it shows a "Shoulder"-shaped curve relationship. Elastic modulus Et and Poisson’s ratio ν are not sensitive to confining pressure, and the relationship between their average value and the bedding dip angle ? is inverted "U" shape and "shoulder" shape, respectively. The micro and macro failure modes of shale are further discussed, and the results show that its failure mode is controlled by both confining pressure and bedding dip. The comparison between experimental data and fitting results indicates that the modified Hoek-Brown yield criterion can simulate the anisotropy of shale strength satisfactorily. Meanwhile, the experiment data of uniaxial and triaxial compression tests on Koteshwar phyllite, Koteshwar slate and chlorite schist are conducted to further verify the application of the proposed modified Hoek-Brown yield criterion in the simulation of most transversely isotropic rock with bedding structure. The simulation effect of the peak strength of the yield criterion is evaluated in the case of more and less test data.

Key words: layered shale, transversely isotropic, Hoek-Brown, yield criterion