## 1 Introduction  
Core recovery is a critical metric in geotechnical engineering and geological explorations, quantifying the proportion of a drilled rock column retrieved as intact core during drilling operations. Expressed as a percentage, it serves as an indicator of the rock mass's quality and the effectiveness of the drilling process. The formula for calculating core recovery is as follows:  
\[\text{CR}=\frac{\text{total length of core recovered}}{\text{total length of core run}}100[\%] \tag{1}\]  
The quality of the rock's ability to endure the coring procedure and drilling fluid flushing, as well as the weathering and disintegration of the rock, can all be inferred from core recovery. When no core is recovered due to cavities or core losses, an interval of zero as the core length shall be recorded. In this instance, it is essential to enter the type of core loss in the \"Cavity or Core Loss\" field as listed in Table 1.  
It can be seen that the CR value is important for the analysis of the drill core, but it does not give information about its jointing; thus, it cannot be used for rock engineering design. Rock quality designation (RQD) is a quantitative measure introduced over six decades ago by Deere to assess rock mass quality for geological engineering and rock engineering analyses [1; 2]. It is defined as the percentage of intact drill core pieces longer than 10 cm (4 inches) recovered during a single core run. The RQD value is calculated using the following formula:  
\[\text{RQD}=\frac{\Sigma\text{h}_{10}}{\text{h}_{\text{b}}-\text{h}_{\text{a}} }100[\%], \tag{2}\]  
where \(\Sigma\text{h}_{10}\) is the total length of the pieces longer than 10 cm, and \(\text{h}_{\text{b}}\) and \(\text{h}_{\text{a}}\) are the upper and the lower depths of the depth intervals. The core should be at least 54.7 mm in diameter and drilled with a double-tube barrel [3]. RQD is expressed as a percentage and is a measure of the quality of the rock mass within a borehole. RQD is widely used in core logging since it is simple and quick to measure. It is frequently the sole technique available for determining the jointing density along the core drill hole. Moreover, [PERSON]'s Rock Mass Rating (RMR) and the Q-system developed by [PERSON] et al.--two of the most popular approaches for classifying rocks--use it as raw data [4; 5; 6]. It is also applicable to the calculation of the borehole's Geological Strength Index (GSI), where the block of rock mass is considered to be half of the RQD value [7]. Deere states that fresh fractures that obviously happened during drilling should also be disregarded. In boreholes where jointing surfaces are parallel to the borehole axis, these joints may not intersect the core, leading to an overestimation of rock quality if not properly accounted for. [PERSON]'s guidelines suggest that such boreholes should be considered intact, as the parallel joints do not disrupt the core samples [8; 9]. In scenarios involving highly fractured or weak rock masses, obtaining accurate RQD values can be challenging due to poor core recovery and the presence of numerous discontinuities. Despite these challenges, RQD remains a valuable indicator of rock mass quality. It provides a realistic picture even in cases of poor core conditions caused by a very high fracture intensity or weak rock. Based on his proposal, the possibility of calculation is illustrated by an example using Figure 1. The example of a very poor and excellent borehole from Moragy Granite Formation (Hungary) is presented in Figure 2. \begin{table}
\begin{tabular}{c c} \hline
**Lookup** & **Cavity or Core Loss** \\ \hline CAV & Cavity \\ CL & Core Loss \\ \hline \end{tabular}
\end{table} 
Table 1: Cavity or core loss. Real observations serve as the foundation for the relationship between the RQD value and the rock test classes and are displayed in Table 2. The categorization provided in EUROCODE 7-1 [10] is comparable to this table. The quality of the rock mass in situ should to be represented by RQD.