The intact rock constant, mi, is one of the three basic input parameters required for the widely used Hoek-Brown failure criterion. In the absence of laboratory testing, guideline charts with mi values based on rock type have been provided by Hoek and Brown and others. Our analysis of existing databases on rock material properties (such as Sheorey, Douglas and Rocscience) demonstrates that there is a very poor correlation between lithology and mi values. The databases, together with the results of further uniaxial, triaxial and tensile laboratory testing, have been used to assess the relationship between the ratio of uniaxial to tensile strength (R) and mi. The paper concludes that the most accurate method of assessing mi values remains as statistical analysis of data from a full set of triaxial tests plus tensile tests. In the absence of such testing, the use of R as a means of estimating mi is considered to be a preferable alternative to reliance on guideline charts.

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