## 13 Conclusions  
The rock quality designation (RQD) method remains a cornerstone for assessing rock mass quality, offering a simple and widely adopted approach over the past six decades. Despite its limitations, including its sensitivity to borehole orientation and inherent anisotropy, it continues to be integral to rock mass classification systems such as RMR, the Q-system, and the GSI. This study highlighted the following key findings:  
**Limitations of Traditional RQD:** RQD is highly sensitive to borehole orientation, leading to a potential overestimation when joints are parallel to the borehole axis or underestimation when joints intersect perpendicularly. It fails to account for fractures smaller than 10 cm, resulting in an incomplete representation of rock mass quality. In highly fractured zones, RQD values as low as 0% have been observed, even when intact core segments are just below the 10 cm threshold. **Quantitative Advancements in RQD Calculations:** Modified RQD methods, such as weighted joint density (RQD\({}_{\text{wjd}}\)), provide adjusted values (e.g., 70% and 62% for two boreholes in the same domain with different orientations), mitigating the directional bias inherent in traditional RQD. Empirical correlations, such as RQD = 100 \(\times\) (V\({}_{\text{pF}}\)/V\({}_{\text{p0}}\))\({}^{2}\), enable seismic-based RQD estimates, allowing for spatially continuous assessments over inaccessible terrains. **Application of Discrete Fracture Network (DFN) Models:** DFN modeling reduces the directional dependency of RQD by incorporating volumetric fracture intensity (\(\lambda\)v), yielding values that better align with observed rock mass conditions. For example, RQD\({}_{\text{wjd}}\) and DFN-derived RQD values for fractured rock masses showed only a 5-10% variation compared to field measurements. **Correlation with Mechanical Parameters:** Empirical relationships linking RQD to the deformation modulus (E\({}_{\text{m}}\)) and compressive strength (\(\sigma_{\text{cm}}\)) enhance its utility. For example, the relationship E\({}_{\text{m}}\)/E\({}_{\text{r}}\) = 0.0123e\({}^{0.0428\text{RQD}}\) provides a robust means of estimating the deformation modulus across various rock qualities. Similarly, \(\sigma_{\text{cm}}\)/\(\sigma_{\text{c}}\) = 0.00457e\({}^{0.0299\text{RQD}}\) enables the estimation of compressive strength, addressing design needs in geotechnical engineering. **Integration of Seismic Techniques:** Seismic methods, particularly those utilizing P-wave velocities, overcome spatial and accessibility limitations. In one case study, seismic RQD values matched within 10% of traditional RQD measurements across complex terrains, demonstrating the reliability of these methods. **Hybrid Approaches for Comprehensive Assessments:** Combining RQD with other classification systems, such as RMR and the GSI, enhances its practical applicability. For instance, RQD contributes up to 20 points in RMR calculations, directly influencing rock mass quality classifications. For the safe and effective design of rock engineering projects, RQD will continue to play a crucial role, particularly when integrated with advanced techniques like DFN modeling and seismic methods. Future research should focus on further refining these methods and exploring their applications in complex geological settings to enhance the reliability and versatility of RQD-based assessments. Conceptualization, N.B. and B.V; methodology, S.M.D. and S.N. ; writing--original draft preparation, S.N. and S.M.D. ; writing--review and editing, N.B. and B.V.; visualization, S.N and S.M.D. ; supervision, B.V.; project administration, B.V. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. **Conflicts of Interest:** Author [PERSON] was employed by the company Gecko Geotechnics LLC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.