A new internal friction angle–based approach for estimating Hoek–Brown constant m_i and its comparison with those estimated from some current methods

One of the two independent key parameters of the Hoek–Brown failure criterion, m_i, is calculated using 3-axial test data consisting of at least five major and minor principal stresses and the regression method. In the absence of 3-axial test data, the guideline chart, which provides m_i values, is commonly used in practice. However, m_i values calculated from regression analyses have a much greater spread than that proposed in the Guideline. Therefore, simplified models, based on some strength properties, have been proposed to calculate m_i values. But some of these prediction models were based on a few types of rock and limited data. In this study, an alternative method for predicting m_i using internal friction angle (ϕ), which is determined from classical 3-axial test, is proposed. First a brief description of the proposed method was given and a summary on the existing methods for predicting m_i was discussed with their limitations, and the data selection criteria used by the authors were also described. In order to assess the prediction performance of the proposed method, a database, consisting of 125 data sets and representing a total of 26 different rock types, was compiled from a well-known dataset and published sources. Results of the comparisons between the proposed method and some of the current prediction methods mainly suggested that the proposed method yields the best m_i predictions. It is also noted that for sedimentary rocks, the ranges of m_i estimated from this method were closer to those proposed in the Guideline chart.