Abstract: In order to improve the applicability of Hoek-Brown failure criterion and reduce subjectivity in the determination of geological strength index (GSI) value for anisotropic rocks, a modified Hoek-Brown failure criterion is proposed, which considers the variation of the GSI value with the bedding angle of anisotropic rocks. Triaxial compression test data of anisotropic rocks with different bedding angles were first collected. The results show that the peak strength of anisotropic rocks exhibits a U-shaped relation with bedding angle . Then, the rock specimen with the bedding angle  = 0º is defined as the intact rock. The uniaxial compressive strength c and material parameter mi of intact rocks are obtained from data fitting using Hoek-Brown failure criterion. The corresponding GSI values under different bedding angles are calculated. The relationship between GSI and bedding angle  is fitted using Gaussian function, and a new strength model of anisotropic rocks is established based on Hoek-Brown failure criterion. Finally, the proposed model is verified by comparing the peak strength obtained from the GSI-softening model. It is found that the modified Hoek-Brown failure criterion is suitable for predicting the strength of anisotropic rocks with different bedding angles and under various confining pressure. The physical meaning of the new parameters in the model is also discussed.

Key words: anisotropic rock, bedding angle, Hoek-Brown failure criterion, GSI-strength model