Comparison of existing methods and a new tensile strength based model in estimating the Hoek-Brown constant mi for intact rocks

The constant mi is a fundamental parameter required for the Hoek-Brown (HB) failure criterion in estimating the strength of rock materials. In order to calculate mi values triaxial tests need to be carried out, however, triaxial tests are time-consuming and expensive. Therefore, simplified models are proposed to estimate mi 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 paper, existing models (Guidelines, R index and UCS based model) are compared and their prediction performances are assessed using the published triaxial test data of five common rock types. In addition, the tensile strength (TS) based model is also proposed by analyzing the rock database. In order to evaluate the reliability of existing and proposed models, mi values estimated from the various models are used in the HB criterion to estimate the strength of intact rocks. The estimated intact rock strength is then compared with the experimental intact rock strength using existing triaxial test data. Results show that mi values calculated from the proposed TS based model yield the best strength prediction performance and can be reliably used in the HB criterion for estimating the intact rock strength when triaxial test data are not available.