Revision of using Rock Quality Designation (RQD) in Rock Mass Rating System (RMR) – a Case Study of Alborz Tunnel

Empirical and numerical methods are commonly used when underground engineering structures are designed. Empirical methods are generally preferred by rock engineers and geologists due to their practicality. In designing tunnel supports, rock mass classification systems, RMR, Q, and RMi have been used by many researchers and gained universal acceptance. These classification systems have been originally obtained from many tunneling case studies and they have been applied to many construction designs. It is necessary to select the appropriate parameters as inputs to these systems in order to accurately describe the rock behavior. Therefore, researchers tend to look more closely at the input parameters. RQD and FS, two input parameters to RMR, and recently introduced parameter FF as their alternative, has been discussed in this paper. Also, the applicability of FF has been investigated on Alborz tunnel data as a case study. Traditionally, parameter ratings were determined from the presented tables for RMR. However, some users were not aware that the ratings in these tables were the average values for the ranges shown and not the minimum values. For improved accuracy, it is better to use the recommended graphs, showing the continuous value of the ratings. Therefore, the presented graph was used to determine the value of FF parameter as a combined rating of RQD and FS. RMR13 has obtained by replacing FF rating instead of the total rating for RQD and FS in RMR89. The values obtained for FF rating were highly correlated with the total rating for RQD and FS. Also, the total rate of RMR13 was similar to RMR89 with a correlation coefficient of 0.996, the values obtained for RMR13 was approximately 2 percent higher than these for RMR89. In general, using both versions of RMR are practical and acceptable, but the choice of each to use, depends on the project conditions and system user capabilities.