Empirical Evaluation of Rock Mass Rating and Tunneling Quality Index System for Tunnel Support Design

Abstract

The primary output of the rock mass rating (RMR) and tunneling quality index (Q) system is a preliminary tunnel support design, as these methods are empirically developed and updated for this purpose. In this study, these internationally accepted design tools are evaluated to improve results for tunnel support design. The rating system is simplified and improved for some parameters through the use of equations to replace the discrete/lump characterization with a continuous rating. Recent developments in characterization and support are used in proposing the back analysis approach of rock mass quality calculation from tunnel span and installed support. This approach is used for two tunnel projects which experience high stresses. Approximately 90% of the tunnel sections show that actual supports have rock bolt spacing and shotcrete thickness which are heavier than those indicated by RMR₈₉, indicating a system limitation. Another assessment using RMR₁₄ indicates that its rating is higher than that of RMR₈₉. A strong correlation exists between them, as supported by the literature and data from analyses of 462 tunnel sections. Despite its new version, RMR₈₉ still preserves its importance. Evaluating the different correlations between RMR and Q through published data indicates that the rock mass fabric index gives comparatively better results.