5. The Hoek-Brown Constant m_i

The constant m_i is a fundamental parameter required for the Hoek-Brown (HB) failure criterion in estimating the strength of rock materials. In order to calculate m_i values triaxial tests need to be carried out, however, triaxial tests are time-consuming and expensive. Therefore, simplified models are proposed by many researchers to estimate m_i values using rock properties that are easily obtained at an early stage of a project when triaxial test data are not available. These models are in general proposed using statistical technique and the reliability of prediction relies on the quantity and quality of the data used. In this Chapter, existing models (Guidelines, R index, UCS based model and tensile strength based model) are reviewed and compared and their prediction performances are assessed using the published triaxial test data of five common rock types. The estimated intact rock strength is then compared with the experimental intact rock strength using existing triaxial test data. Results show that m_i values calculated from the UCS and TS based model can be reliably used in the HB criterion for estimating the intact rock strength when triaxial test data are not available.