The classification of rock masses and evaluation of their mechanical properties is of vital importance in both rock mechanics and rock engineering. The most widely used definition of Rock Quality Designation (RQD) is by Deere which was developed in 1964. It is defined as the borehole core recovery percentage incorporating only pieces of solid core that are longer than 100 mm in length measured along the centreline of the core. The RQD which is used to estimate the rock mass shear strength, deformation parameters and bearing capacity of foundations. A rock mass classification study was hence undertaken in 17 structural regions in the unlined spillways of major dams in South Africa and Australia. This was used to highlight its origin, inconsistent changes in its definition from place to place, its limitations, its incorporation into rock mass rating (RMR), ‘Q values’ and as well as the geological strength index (GSI). The inherent limitations associated with the use and measurement of RQD include; improper handling, ignorance of the sound and hard criterion, under-sampling due to rock mass having joints occurring near-parallel to borehole. The studies have shown that it is unnecessary to use RQD in the determination of RMR and GSI since when it produces erratic results, these indices also incur a certain percentage error. GSI can be accurately estimated using the Hoek’s look-up chart and fracture frequency should be used instead of RQD. The studies have also shown a large variation in RQD interpretation due to the inconsistency in its definitions resulting in a variety of criterion to be considered in rock mass strength and classification, hence, should be done away with.
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