## 3 Applying the Methodology to the Example of Dabar Power Tunnel
### Quantitative Analysis
#### 3.3.3 Derivation of Threshold Values in Terms of Overbreak Volume  
The key criterion to distinguish the type of overbreak is that a favourable orientation of the discontinuity combined with a high quality of rock mass cannot lead to a geological overbreak. In that case, the overbreak would be characterised as technological.  
In the next step, all mapped sections with RMR values greater than 60 were divided into two categories: those with a favourable orientation of discontinuity (0 and \(-\)2 points from the RMR classification) and those with an unfavourable orientation of discontinuity (\(-\)5, \(-\)10, and \(-\)12 points). The locations of all mapped excavation faces with a favourable discontinuity orientation and with one or two sets of joints were isolated. In these sections, due to the favourable orientation of discontinuities, the occurrence of overbreaks was characterised as technological, with an average volume of 1.60 m\({}^{2}\). This was considered the threshold value to distinguish between technological and geological overbreaks for RMR values greater than 60. This means that for all cases with RMR > 60 and overbreaks \(\leq\)1.60 m\({}^{2}\), the cause is of a technological nature, as indicated in Table 3.  
Based on the established threshold values (2.5 m\({}^{2}\) and above for geological overbreaks and 1.6 m\({}^{2}\) and below for technological overbreaks), the total volume of over-profile excavation can be calculated for the length of the unsupported tunnel excavation, which in our case was 3.0 m. In sections in which the measured overbreaks were greater than 1.60 m\({}^{2}\) and less than 2.5 m\({}^{2}\), a detailed inspection of geodetic measurements was conducted. The trends generally showed that even in the wider area encompassing neighbouring sections, at a length of approximately 3 m, the overbreak excavation was always greater than 1.60 m\({}^{2}\), which implied the geological type of overbreak for most cases.  
In sections in which the measured overbreaks were less than 1.60 m\({}^{2}\), a detailed inspection of geodetic measurements was conducted to calculate the overbreak volume along the 3 m length (if the overbreak excavation was slightly larger than 1.60 m\({}^{2}\) at any station in the previous 3 m, it was given a value of 1.60 m\({}^{2}\)).  
The total volume (m\({}^{3}\)) of technological overbreaks was obtained as an integral of the volume attributed to technological causes along the tunnel layout. In certain cases, in which there was a mismatch between the location of geodetic surveying and geological mapping, some adjustment and extrapolation of the data were necessary. However, given the statistical nature of the analysis, the inaccuracies which were noted were regarded as negligible for the final estimate.  
The calculated areas of overbreaks per meter length of the tunnel for different threshold values (below 1.6 m\({}^{2}\), between 1.6 and 2.5 m\({}^{2}\), and above 2.5 m\({}^{2}\)) are presented in Table 4. The calculated volume of technological overbreaks, according to the previously defined procedure, is also presented in Table 4. The volume of geological overbreaks was calculated by subtracting the volume of technological overbreaks from the total volume of overbreaks.  
\begin{table}
\begin{tabular}{c c} \hline
**RMR (\%)** & **\textgreater{}60** \\ \hline
**Overbreak area (m\({}^{2}\))** & **\textless{}1.6** \\ \hline \end{tabular}
\end{table}
Table 3: Threshold values for determining technological overbreaks.