Modification of the rock mass rating system (RMRmbi) considering three-dimensional rock block size

Traditional measurements of block size (or degree of jointing), such as the rock quality designation (RQD), are questionable due to certain theoretical limitations, including orientation bias and their weakness in considering the joint persistence and three-dimensional shapes of block sizes. This may lead to inaccurate characterizations of rock mass structures and unreliable classification of rock mass qualities. The modified blockiness index (MBi) is a three-dimensional measurement of block size which was developed to overcome these problems. In this study, correlations between MBi and several traditional block size measurements were assessed; based on the MBi, the rock mass rating (RMR) system was modified, and this version was termed RMRmbi. In the first part of this work, multiple simulated experiments were conducted using the GeneralBlock software program and 3DEC (three-dimensional distinct element code), and a large volume of MBi, RQD, joint frequency (JF) and volumetric joint count (Jv) values (artificial data sets) were obtained; subsequently, the correlations between MBi and RQD, JF and Jv were assessed. In the second part, the combined use of RQD and JF in the RMR system was replaced with MBi, and hence, the RMRmbi system was developed; based on the artificial data sets, the viability of RMRmbi was preliminarily supported. At the end of this study, the correlations between MBi and RQD, JF and Jv were verified based on actual data; the RMRmbi was applied to real cases, and a comparison between RMRmbi and RMR was conducted. The results showed that (i) the MBi can capture the influence of joint persistence; (ii) the RMRmbi system can overcome the theoretical limitations caused by RQD and JF; and (iii) simulated experiments showed that the RMRmbi values are more reliable, thus validating the accuracy of RMRmbi and revealing its great potential for future application.