Convergence of synthetic rock mass modelling and the Hoek–Brown strength criterion

Synthetic rock mass modelling is based on estimating the mass scale properties of rock directly from properties of the intact material and observed jointing geometry and strength. This technique explicitly represents characteristics of the rock mass that are implicit in the widely applied, empirical Hoek–Brown strength criterion. We demonstrate here that the two approaches converge in mass strength and the strength envelope when the necessary conditions for the empirical method are satisfied. However, the synthetic rock mass approach is advantageous whenever the necessary conditions of the empirical method are not satisfied. As an example, we use the synthetic rock mass method to describe an anisotropic strength envelope for a rock mass with four independent joint sets, where the orientation-specific strength has a significant impact on rock mass stability. In addition we show that rock mass strength is controlled strongly by joint persistence and the resultant connectivity of discontinuities, and only weakly by joint density and intensity.