## 3 Limitations of RQD  
A rigorous rule in the first interpretation stipulates that a core's minimum length for inclusion is 10 cm. Despite its apparent simplicity, RQD is greatly impacted by the relative direction of the borehole (or scanline) with respect to the orientation of the fractures. An in-depth analysis of the RQD factor's limits was provided by [PERSON] [11; 12], who also discussed the original definition's contradictions and orientation dependency. This also implies that the anisotropy of the rock mass cannot be completely reflected by the conventional method of establishing a rock quality designation (RQD), which makes it inaccurate in reflecting the quality of the rock [13; 14]. Figure 5 shows the measurement results of [PERSON] [11], which illustrate the potential errors of using RQD. There can be a significant difference between RQD = 0% and RQD = 100% within a drilling section, even with slight variations in joint spacing. For example, RQD = 0% may occur not only when the core is entirely fractured but also when the intact core lengths are just below 10 cm. Conversely, RQD = 100% can also result with subtle changes in spacing, as seen when all core sections are over 10 cm long. These small changes can lead to drastically different RQD values, emphasizing the sensitivity of joint spacing on RQD calculations. Figure 6 demonstrates that in rare cases (in the real world), subtle changes in discontinuity spacing can greatly affect RQD (i.e., from 0% to 100%) when uniformly distributed at approximately 10 cm. Another drawback of RQD is that it provides no information regarding the quality of new rock fragments shorter than 10 cm in length or core bits resembling soil-like materials. RQD's simple measurement technique has certain inherent limitations and only offers a partial evaluation of the quality of the rock mass. [PERSON] [9] states that the RQD calculation should take into account the real drilling-run length based on field measurements, whereas  
Figure 5: The graphic presentation of the jointing shown in Figure 3 and Table 3. The RQD measured in sections is compared to the RQD measured every meter. Figure 6: Examples of minimum and maximum RQD values along drill cores at different joint densities [11]. the core length in a laboratory is usually restricted to 1.5 m. This approach can be used as a reference for quantifying RQD within a specific range, leading to a more accurate prediction of rock mass characteristics in engineering projects. According to [PERSON] et al. [15], it is not possible to fully capture the true quality of a rock mass using the original definition of RQD. As such, in real-world applications, a high RQD value does not always translate into a high-quality rock mass. Since RQD is a directionally dependent metric, the orientation of the borehole can have a big impact on its value. By way of example, Figure 7 illustrates the dependence on directionality of RQD for an anisotropic rock mass. The RQD values found in drill hole A is likely to be significantly lower than that of drill hole B. The density of the joints along the drill core may be significantly impacted by the angle formed by the bore hole and the primary joint set. The accuracy of RQD values can be influenced by the expertise of the engineer responsible for logging the core. The engineer's ability to correctly identify and differentiate between natural fractures and those caused by drilling processes plays a crucial role in determining the RQD. Misidentifying these fractures can lead to inaccuracies in rock mass classification. Additionally, RQD does not account for critical factors like incipient fractures--small cracks that may not be fully developed--and veins, which can significantly affect the overall strength and integrity of the rock mass. RQD may provide an inaccurate or overly optimistic evaluation of rock mass quality by ignoring these factors, which could have an effect on engineering project design and safety [16].