• Document: COMPARING THE RMR, Q, AND RMi CLASSIFICATION SYSTEMS
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Reference: www.rockmass.net March 2008 COMPARING THE RMR, Q, AND RMi CLASSIFICATION SYSTEMS PART 1: COMBINING THE INPUT PARAMETERS USED IN THE THREE SYSTEMS by Arild Palmström, Ph.D. RockMass as, Oslo, Norway The main rockmass classification systems make use of similar rockmass parameters. It is therefore possible to combine the input parameters to three of the systems in a set of common parameter tables. This enables the ground quality to be found directly in these systems from only one set of input parameters. Thus, the estimated rock support found in one system can be easily be compared and checked in the other systems. This leads to more reliable rock support estimates, provided the actual ground is within the limitations of the systems and that the ground characterization is properly made. 1. INTRODUCTION As pointed out Barton and Bieniawski in T&T February, 2008, rock engineering classification systems play a steadily more important role in rock engineering and design. The main classification systems for rock support estimates, the Q and the RMR (Rock Mass Rating) systems, use some of the most important ground features or parameters as input. Each of these parameters is classified and each class given values or ratings to express the properties of the ground with respect to tunnel stability. Also, the NATM (New Austrian Tunnelling Method) and the RMi (Rock Mass index) support method use similar parameters. EXTREMELY VERY EXT. EXC. EXC.POOR VERY POOR POOR FAIR GOOD GOOD POOR GOOD GOOD 100 +50% GOOD VERY +25% NGI CASE STUDIES GEOM. CASE STUDIES 80 OTHER CASE STUDIES GOOD INDIAN CASE STUDIES Rock Mass Rating - RMR -25% 60 -50% FAIR Common correlation 40 RMR = 9 ln Q + 44 POOR 20 POOR VERY 0 0.001 0.01 0.1 1 10 100 1000 Rock Mass Quality - Q Figure 1. A commonly used correlation between the RMR and the Q-index where deviations from the common correlation are shown. As seen, for Q = 1, RMR varies from less than 20 to 66. Note that the Q system applies logarithmic scale while RMR has a linear scale (revised after Bieniawski, 1976). For arriving at appropriate results in rock engineering and design, Bieniawski (1984, 1989) advises application of at least two classification systems when applying such empirical tools. However, many users are practising this recommendation by finding the value (or quality) in one classification system from a value in another using some sort of transition equation(s). The most known of these transitions, between Q and RMR is presented in Figure 1. As seen, the equation used here is a very crude approximation, involving an inaccuracy of ± 50% or more. Thus, severe errors may be imposed, resulting in reduced quality of the rock engineering works, or even errors, which may lead to wrong decisions. Another error may be imposed from the fact that the two systems COMPARING THE RMR, Q, AND RMi CLASSIFICATION SYSTEMS 2 Part 1: Combining the input parameters used in the three systems Reference: www.rockmass.net March 2008 have different limitations. The paper “Classification as a tool in

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