Measurements of and correlations between block size and rock quality designation (RQD)

Various measurements of the block size or degree of jointing (i.e. density of joints, RQD, block volume, joint spacing) are described. It is concluded that the RQD measurements are encumbered with several limitations and that this parameter should be applied with care. These limitations influence the engineering results where RQD is applied in classification systems, numerical modelling and other engineering assessments.The three-dimensional block volume (V_b) and the volumetric joint count (J_v) measurements give much better characterizations of the block size. As the block size forms an important input to most rock engineering calculations and estimates, it is important to select the most appropriate method to measure this parameter.Correlations between various measurements of block size have been presented. It turned out difficult to find any reliable correlation between RQD and other block size measurements. An adjusted, better equation between RQD and J_v than the existing is presented, though still with several limitations.More efforts should be made to improve the understanding on how to best measure the block size in the various types of exposures and patterns of jointing.