The estimated intact rock strength is then compared with the experimental intact rock strength using existing triaxial test data. 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. In addition, the tensile strength (TS) based model is also proposed by analyzing the rock database. 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. These models are in general proposed using statistical technique and the reliability of prediction relies on the quantity and quality of the data used. 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. In order to calculate mi values triaxial tests need to be carried out, however, triaxial tests are time-consuming and expensive. The constant mi is a fundamental parameter required for the Hoek-Brown (HB) failure criterion in estimating the strength of rock materials.
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