Due to the large experience accumulated in the use of the RMRb (Rock Mass Rating), as well as to the simplicity of the estimation of the GSI (Geological Strength Index) and to the importance of GSI as input data in the Hoek & Brown failure criteria, both the RMRb and GSI are widely used in geotechnical engineering practice. This article analyzes the relationship between both classifications using in situ data corresponding to different types of rocks collected from different outcrops in Spain. Currently available correlations between RMRb and GSI have been compiled and analyzed in order to compare them with the results of the analysis conducted in this study. Finally, the best (most suitable) statistical relations between RMRb and GSI, depending on the type and quality of rock media, are shown and they are used to establish general correlations. To conclude recommendations are presented, suggesting the use of a particular expression and its limits of applicability.
Rock masses are generally an inhomogeneous, inelastic, discontinuous, and anisotropic medium; such properties make its characterization rather complex and difficult. Geomechanical classifications, such as the Q index (Barton, 1974), the Rock Mass Rating (Bieniawski, 1973/1979), and the Geological Strength Index (Hoek, 1994) are a commonway of defining rock mass behavior, especially in the early stages of a project and for tunnels.
These ratings are computed using several observable and measurable characteristics of the rock mass. The most common geomechanical classifications are the one mentioned before (Q index, the Rock Mass Rating and the Geological Strength Index). They use several parameters (type and spacing of joints, unconfined compressive strength, etc.) to provide a single value that serves as a measure of the "quality" of the rock mass.
To be able to use them in practice, it seemed interesting to correlate them using the same rock mass as reference. In this sense, the relation between Q index and the Rock Mass Rating has been widely discussed in the literature, based on extensive experimental campaigns (including as well those proposed by their authors).