Relationship between RMR and BQ for rock mass classification and estimation of its mechanical parameters

On the basis of rock mass quality classification in China's national standard, Standard for Engineering Classification of Rock Masses (SECRM, GB50218—94), a simplified calculation method for the basic quality index (BQ) of rock mass is proposed. According to the value ranges of various physical mechanical parameters of rock mass in SECRM, the graphs of the relationship between various parameters and BQ are established. Empirical formulas for the relationship between various parameters and BQ are established using the nonlinear fitting analysis method. Based on the equivalent principle of internal friction angles, through comparing the existing equations for describing internal friction angles using BQ and RMR respectively, a relation equation between RMR and BQ is derived. Similarly, based on the equivalent principle of deformation modulus, through comparing the existing equations for describing deformation modulus using BQ and RMR respectively, four relation equations between RMR and BQ are derived. By comparing five relation equations between RMR and BQ with the existing the field test data, the equations for the upper limit and the lower limit lines are found. The results show that the relation equation obtained using the equivalent internal friction angle has good trend, but its value is a little larger, and a correcting relation equation between RMR and BQ is suggested by means of taking medium line between the upper limit and the lower limit lines.