New strength criteria for rocks under polyaxial compression

Rock mechanics abounds with a great number of strength criteria, among which the Hoek-Brown (HB) criterion is one of the most commonly used. However, it takes no account of the influence of the intermediate principal stress and therefore cannot be used in conditions where the intermediate principal stress does influence the rock strength. In this paper, a comment on the HB criterion and existing enhanced Hoek Brown (EHB) criteria is presented. Then, two general methods to derive new criteria for rocks under polyaxial compression according to the HB criterion are proposed by which existing EHB criteria can be obtained, and three new criteria are introduced using the second method. Comparisons including smoothness and convexity of the failure envelope surface between proposed criteria and existing ones are made. The validities of the new criteria are checked according to polyaxial tests for eight different rock types from published literature and show good agreement with the test data.