Rock quality designation (RQD): time to rest in peace

Abstract

Rock quality designation (RQD) was introduced by Don Deere in the mid-1960s as a means of using diamond core to classify rock for engineering purposes. Subsequently, it was incorporated into the rock mass rating (RMR) and Q-system classification methods that, worldwide, now play substantial roles in rock mechanics design, whether for tunnels, foundations, rock slopes or rock excavation. It is shown that a key facet of the definition of RQD is ignored in many parts of the world, and it is noted that there are several inherent limitations to the use of RQD. Based on mapping of rock formations by 17 independent professionals at different locations in Australia and South Africa, it is shown that differences in assessed RQD values result in significant errors in computed RMR and Q ratings, and also in geological strength index (GSI) and mining rock mass rating (MRMR). The introduction of a look-up chart for assessing GSI has effectively removed the need to measure, or estimate, RQD. It has been found that GSI values derived from the look-up chart are as valid as those derived by calculation from the original component parameters, and are satisfactorily consistent between professionals from diverse backgrounds. The look-up charts provide a quick and appropriate means of assessing GSI from exposures. GSI is, in turn, a useful rock mass strength index; one new application is presented for assessing potential erosion of unlined spillways in rock. Incorporation of RQD within the RMR and Q classification systems was a matter of historical development, and its incorporation into rock mass classifications is no longer necessary.

Review of Rock-Mass Rating and Tunneling Quality Index Systems for Tunnel Design: Development, Refinement, Application and Limitation

“…Since its development [59], RQD was used in RMR and Q system because of its historical background. Because of some intrinsic limitations and ignoring the basic definition [60][61][62], RQD, along with joints spacing, is substituted by joint frequency in the excavation face or rock core [46]. Koutsoftas [63] analyzed the arguments about the different definitions of RQD in different parts of the world and the use of rock exposures for determining RQD which are considered as the main sources of error.…”

“…The incorporation of this parameter within the two systems was a matter of historical development and its use in these classification system is no longer essential [61]. In rock-mass classification systems, parameters and ratings are applied irrespective of the project, specific characteristics and failure mechanisms [151].…”

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

“…A number of rock mass classification systems have been developed since Terzaghi pioneered his study on the rock load factor classification for tunnel support design [1]. Among these several rock mass classification systems, rock mass rating (RMR) and tunneling quality index (Q) are internationally accepted systems for support design that are widely used in the field of tunneling [2][3][4].…”

“…Because of several inherent limitations, and ignoring the basic definition [4,30], this parameter is not used in the latest version of RMR. Along with the joint spacing, RQD was replaced by joint frequency [15].…”

Discussion of “Rock quality designation (RQD): time to rest in peace”

“…The paper by Pells et al (2017) makes the case that the rock quality designation (RQD) system has essentially outlived its usefulness and is no longer effective as a classification and rock characterization method. The authors present examples to justify their arguments, and in their conclusion section they list four reasons why RQD should "be phased out in rock mass classification.…”