###### Abstract  
Rock quality designation (RQD) is a parameter that describes rock mass quality in terms of percentage recovery of core pieces greater than 10 cm. The RQD represents a basic element of several classification systems. This paper studies scale effects for RQD measurements using synthetic rock masses generated using discrete fracture network (DFN) models. RQD measurements are performed for rock masses with varying fracture intensities and by changing the orientation of the simulated boreholes to account for orientation bias. The objective is to demonstrate the existence of a representative elementary length (REL, 1D analogue of a 3D representative elementary volume, or REV) above which RQD measurements would represent an average indicator of rock mass quality. For the synthetic rock masses, RQD measurements were calculated using the relationship proposed by [PERSON] and [PERSON] and compared to the simulated RQD measurements along the boreholes. DFN models generated for a room-and-pillar mine using mapped field data were then used as an initial validation, and the conclusion of the study was further validated using the RQD calculation results directly obtained from the depth data collected at an iron cap deposit. The relationship between rock mass scale and assumed threshold length used to calculate RQD is also studied. rock quality designation (RQD); discrete fracture network (DFN); representative elementary length (REL) +
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considerations for directional dependence and scale effects can improve the prediction of RQD. RQD values may also depend on the engineer's experience logging the core and the ability to distinguish natural fractures from drill- and core handling-induced fractures. Measured RQD values are sensitive to the length of the core run [9]. The same author recommended a core run no greater than 1.5 m. While such a limited core length may be helpful in identifying poor-quality zones, it is argued that it cannot represent the rock mass as a whole. When using RQD as an input parameter in rock mass classification systems, it may be helpful to use \"_variable core run lengths_\" to estimate a more representative RQD value for the entire geotechnical/structural domain under consideration. As discussed by [PERSON] [10], most core logging is performed by measuring the joints along each meter of the core. This approach might introduce measurement errors if there are alternating sections with lower and higher densities of joints. The uncertainty in predicting the mechanical behavior of a naturally fractured mass is associated with scale effects [11]. Therefore, when using RQD or any classification systems incorporating RQD for rock mass classification, it is essential to acknowledge scale effects and their impact on rock engineering design, particularly when applying classification ratings across different problem scales [12]. Using variable core run lengths would allow us to better account for the transition from small-scale anisotropic rock mass behavior to large-scale isotropic (even though heavily jointed) rock masses. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Condition** & **RQD** \\ \hline A. & Very Poor & 0-25 \\ B. & Poor & 25–50 \\ C. & Fair & 50–75 \\ D. & Good & 75–90 \\ E. & Excellent & 90–100 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Rock quality designation (RQD) [4]. Figure 1: Schematic representation of RQD measurement and calculation for a core run [9]. Several researchers have questioned the use of RQD in rock classification systems ([PERSON] et al., 2017 [13]; [PERSON] et al., 2022 [14]). In this paper, we review the RQD method and take advantage of its limitations to understand how rock mass quality may be interpreted differently as the scale of the problems varies. We introduce the concept of representative elementary length (REL) as a 1D analogue to the 3D representative elementary volume (REV) for a naturally fractured rock mass when the problem at hand is transformed into a continuum problem.