Abstract: The Mohr-Coulomb (M-C) failure criterion has been widely used in engineering practice, which does not efficiently reflect the rock strength characteristics including the hydrostatic pressure effect, Lode angle effect and intermediate principal stress effect. To overcome this weakness, attempts have been made to develop the three-dimensional failure criteria, among which a recently proposed modified Hoke-Brown (H-B) failure criterion based on a two-parameter deviatoric function has an excellent performance. However, the M-C failure criterion is still widely used in practice. In order to avoid the difficulty resulting from the use of three-dimensional failure criterions in engineering, a new approach is proposed to determine the equivalent M-C strength parameters from the three-dimensional modified H-B failure criterion. The procedure for determining the equivalent M-C strength parameters from the modified H-B criterion is demonstrated in detail, and the equivalent M-C strength parameters influenced by the rock strength characteristics are investigated. Results show that dependence of M-C strength parameters on the hydrostatic pressure effect is nonlinear, and the effect of the intermediate principal stress and Lode angle is related to observation range. For a higher hydrostatic pressure, Lode angle has greater influence on the equivalent M-C strength parameters. As the minimum principal stress rises, the effect of intermediate principal stress on instantaneous cohesion is more significant. Verification of this determination method is conducted based on the true triaxial test data of different rock types, demonstrating its applicability. Findings in this paper have significant sense for practical application of the modified H-B failure criterion in the rock engineering.

Key words: three-dimensional failure criterion, instantaneous Mohr-Coulomb strength parameters, modified Hoek-Brown failure criterion, rock strength characteristics