## 10 Common Use of RQD for Rock Mass Classification
### Q-System  
The Q-system, a rock mass classification method, was introduced in 1974 by [PERSON] et al. [6], at the Norwegian Geotechnical Institute (NGI) in Norway. This system was designed to assess rock mass properties and determine necessary tunnel support measures. The development of the Q-system involved a detailed analysis of 212 case histories of hard rock tunnels in Scandinavia, providing a robust foundation for its application in geotechnical engineering [5].  
According to its definition, the rock mass quality (Q) value can be calculated from the following equation [6]:  
\[Q\text{=}\frac{RQD}{Jn}\frac{Jr}{Ja}\frac{Jw}{SRF} \tag{51}\]  
RQD: rock quality designation (10-100);  
Jn: joint set number (0.5-20);  
Jr: joint roughness number (0.5-4);  
Ja: joint alteration number (0.75-24);  
SRF: stress reduction factor (1-20);  
Jw: joint water reduction factor (0.05-1);The Q-system's use of an open logarithmic scale ranging from 0.001 to 1000, in contrast to a simpler linear scale up to 100, adds complexity to its application, making it more challenging to use effectively [5; 40; 70].  
Relative block size is taken into account by RQD/Jn, the first quotient in Equation (51). According to Table 10, the joint set number, Jn, is the rating for the quantity of joint (discontinuity) sets in the same domain. A joint set description for the borehole from the Moragy Granite Formation is presented in Figure 25.  
When calculating Jn, the term \"joint\", which is taken from the original Q-system, can refer to any kind of discontinuity or geological structure. All sets of discontinuities or geological structures (not simply joints) must be taken into consideration while evaluating Jn. In sedimentary and metamorphic rocks, this includes bedding and foliation, respectively. In addition to appearing more frequently as joint sets, a joint or discontinuity might occur alone as a random joint [71]. A family of nearly parallel, similarly spaced discontinuities with comparable physical or geomechanical characteristics is called a joint set (or discontinuity set).  
Relative block size (RQD/Jn), as illustrated in Figure 26, can be used to decrease the restrictions noted by [PERSON] [11] and differentiate between various rock masses with comparable RQD values. Relative block size (RQD/Jn) is plotted in the function of both RQD and Jn in Figure 27.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Joint Set Description** & **Jn** \\ \hline A & Massive, no, or few joints & 0.5–1 \\ B & One joint set & 2 \\ C & One joint set plus random joints & 3 \\ D & Two joint sets & 4 \\ E & Two joint sets plus random joints & 6 \\ F & Three joint sets & 9 \\ G & Three joint sets plus random joints & 12 \\ H & Four or more joint sets, random, heavily jointed & 15 \\ J & Crushed rock, earth–like & 20 \\ \hline \hline \end{tabular} Note: Not just joints but all sets of discontinuities or geological structures must be taken into consideration.  
\end{table}
Table 10: In parameters.  
Figure 25: Joint set description based on Table 10 for borehole from MöAgy Granite Formation.  
Based on the relative block size, the drill core classification is recommended according to Table 11.