Should the Use of Rock Quality Designation Be Discontinued in the Rock Mass Rating System?

In 2013, an updated rock mass rating system (RMR13) was introduced, in which the combined use of rock quality designation (RQD) and fracture spacing (FS) in the commonly used version (known as RMR89) was replaced with the fracture frequency (FF). Multiple studies have suggested that the incorporation of RQD into the RMR system is no longer necessary, and this stance is extremely controversial. In this study, to investigate whether the use of RQD should be discontinued in the RMR system, both RMR89 and RMR13 were applied to a large number of cases involving artificial and real data sets, and comparisons of RMR89 and RMR13 were conducted. The artificial data sets were constructed through developing theoretical discrete fracture network (DFN) models, and the real data sets were collected from 187 actual cases worldwide. The comparison between RMR89 and RMR13 was based on the following steps. (1) The combined ratings of RQD and FS in RMR89 (R_RQD+FS) and the rating of FF in RMR13 (R_FF) were used to determine the fracturing degrees of the theoretical DFN models. Additionally, the classification abilities of the two subsystems were analysed and compared, and the correlation coefficient (r) and included angle cosine value of two vectors (λ_ij) were calculated. (2) Two specific case studies were performed, and an analysis of variability between RMR89 and RMR13 was conducted. (3) In comparative analyses of 187 real cases, R_RQD+FS, R_FF, RMR89 and RMR13 were adopted to assess the structures and basic qualities of rock masses. Moreover, their results and classification abilities were compared, and r and λ_ij were calculated. The conclusions of the study were as follows: (1) the assessment results of theoretical DFN models show that R_RQD+FS and R_FF exhibit a similar ability to distinguish rock mass structures; (2) the two specific case studies performed suggest that the discrepancy between RMR89 and RMR13 truly exists; (3) the real case studies indicate that RMR89 and RMR13 differ in their sensitivities to rock mass fracturing and basic quality, and RMR89 is superior, but the application potential of RMR13 cannot be denied; and (4) the introduction of RMR13 is not a sufficient reason to cease the use of RMR89, which remains highly practical. In real cases, individuals should select a proper classification system by considering the geological conditions and their experiences and preferences.