# Directional Core Drilling as an Approach to Reduce Uncertainty in Tunneling Construction
## 4 Results
### Burata Tunnel  
The study of the tunnel's rock mass conditions was completed using 29 conventional boreholes (27 that were vertical and 2 that were horizontal). Three DCDs would be sufficient, theoretically, to determine the conditions over the whole tunnel length, with one in each portal and the third above the tunnel layout. Tables 1 and 2 set out the costs of both types of drilling techniques.  
Considering that each vertical borehole obtains information for a 20 m length of the tunnel and that horizontal boreholes provide information for their equivalent lengths, an investment of EUR 442,500 would determine conditions for 20% of a tunnel's total length (790 of 4000 m). Thus, the investigation unit cost was EUR 560 per meter of tunnel characterized. Further, 20% of the total cost was related to transport and preparation (not  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Type of borehole** & **Number of** & **Total Length (m)** & **Average Length** & **Transportation and** & **Drilling Cost** & **Total Cost** \\  & **Boreholes** & **(m)** & **Perparation (EUR)** & **(EUR)** & **(EUR)** & **(EUR)** \\ \hline Vertical & 3 & 140 & 46.7 & 9000 & 14,000 & 23,000 \\ \(\rm S\) m \(\leq 100\) m & 4 & 310 & 77.5 & 12,000 & 31,000 & 43,000 \\ \(\rm 100\) m \(\leq\) L \(\leq 150\) m & 12 & 1525 & 127.1 & 36,000 & 152,500 & 188,500 \\ \(\rm L\geq 150\) m & 8 & 1330 & 166.3 & 24,000 & 133,000 & 157,000 \\ Horizontal & 2 & 250 & 125 & 6000 & 25,000 & 31,000 \\ Total & 29 & 3555 & & 87,000 & 355,500 & 442,500 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Analysis of the conventional approach for the Burata tunnel.  
drilling). On the other hand, the DCD approach enables the definition of 100% of the tunnel's length with an investment of EUR 882,600 and an investigation unit cost of EUR 220 per m, which focused nearly all of the cost on drilling. The conventional technique unit cost was 2.5 times higher than the DCD unit cost.  
The economic analysis of the uncertainty related to the tunnel supports was based on actual data from the project. We used data from [PERSON] (2016) to define the type of support required (e.g., bolts, steel sets, and shotcrete) and considered the over-excavation based on [PERSON] et al. (2017) and [PERSON] et al. (2018). Table 3 summarizes the uncertainty costs related to each element that could be found for the Burata tunnel. Average permeabilities of K = 10 \(-\) 6 m/s and K = 10 \(-\) 7 m/s were considered for the residual soil and granite, respectively.  
Regarding the excavation time, an important variation was found when the time increased from an initial 650 days to 950 days. This increase was caused by a lack of knowledge about the whole tunnel layout, which included a longer length of residual soil conditions instead of granite. Thus, the fixed cost rose incrementally by 46%, reaching EUR 3,600,000.  
In this case study, the uncertainty created additional costs for all the factors due to the greater length of the residual soil conditions instead of the granite that was expected initially. Altogether, the associated cost due to uncertainty was EUR 23,808,334, which could have been eliminated by using a DCD technique, with an additional initial cost of EUR 442,100. When the DCD cost is compared to the total cost of the tunnel, at 2.2% of the budget, it remained within an adequate range for a project with complex conditions ([PERSON] et al., 2017). Every EUR 1 allocated to DCD usage would have reduced the uncertainty by EUR 54, which was in line with the USNCTT ([PERSON] et al., 2017), involving an uncertainty reduction of EUR 5952 per m of tunnel excavated. This value was quite relevant since the overall construction cost of the tunnel was EUR 10,000 per m.