# Directional Core Drilling as an Approach to Reduce Uncertainty in Tunneling Construction
## 4 Results
### Pajares Variant Lot 3 Tunnel  
The study of this specific part of the tunnel layout was completed by using only two conventional boreholes, with a total length of 2050 m drilled. The problems found in drilling the boreholes have been described by [PERSON] and [PERSON] (2016). On the other hand, one DCD of 1175 m would have been sufficient, theoretically, to determine the characteristics of the rock mass in the tunnel section studied (Figure 4). \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Type of Borehole** & **Number of** **Boreholes** & **Total Length (m)** & **Average Length** & **Transportation and** **Preparation (EUR)** & **Dilling Cost** & **Total Cost (EUR)** \\ \hline From the portal & 2 & 3000 & 1500 & 6000 & 600,000 & 606,000 \\ Surface above the tunnel & 1 & 1368 & 1368 & 3000 & 273,600 & 276,600 \\ Total & 3 & 4368 & & 9000 & 873,600 & 882,600 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Analysis of the DCD approach for the Burata tunnel. \begin{table}
\begin{tabular}{l c} \hline \hline  & **Factor** & **Uncertainty Cost (EUR)** \\ \hline Material related to support & +6,496,040 \\ Fixed costs & +3,600,000 \\ Support treatments & +12,571,200 \\ Water drainage & +1,141,094 \\ Total risk estimated & 23,808,334 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Uncertainty factors. In this case, a large geological structure needed to be defined. Assuming that the conventional drilling provided information about the tunnel between the two boreholes (approximately 250 m of the tunnel's layout), there was an investigation unit cost of EUR 844 per m. On the other hand, a DCD technique, with the layout exposed in Figure 4, is proposed. It would require 1175 m of directional drilling and would have an investigation unit cost of EUR 377 per m in order to determine 100% of the tunnel's layout. Thus, the unit cost of the conventional exploration boreholes is approximately 2.2 times higher than the cost of the DCD technique. The design and manufacture of the precast lining segments was a very relevant challenge, as has been pointed out by several authors [55; 56; 57]. There was a cost increment related to the lining (EUR 1,004,108) due to rock mass conditions that were worse than initially defined by the conventional drilling, and the cost increased from the planned EUR 4,202,085 to a real cost of EUR 5,206,193. \begin{table}
\begin{tabular}{l c c c c c} \hline  & **Number of** & **Total Length** & **Transportation and** & **Drilling Cost** & **Total Cost** \\  & **Boreholes** & **(m)** & **Preparation (EUR)** & **(EUR)** & **(EUR)** \\ \hline Vertical (S-78) & 1 & 950 & 3000 & 95,000 & 98,000 \\ Inclined (ST-7) & 1 & 1100 & 3000 & 110,000 & 113,000 \\ Total conventional & 2 & 2050 & 6000 & 205,000 & 211,000 \\ DCD & 1 & 1175 & 3000 & 317,500 & 320,500 \\ \hline \end{tabular}
\end{table} 
Table 4: Conventional and DCD techniques analysis for the Parajes variant tunnel. Figure 4: Area of study in the Lot 3 Pajares variant tunnel. In addition, the advance of the TBM was unexpectedly reduced from 15 m/day to 6 m/day, according to the real advancing rate data [58; 59], increasing the duration of the excavation by 54 days due to a poorer performance, which represented a cost increase of EUR 1,620,000 due to unexpected conditions. The cutter tools consumption was lower in the real conditions due to less abrasive characteristics [60]. There was an expected consumption of 0.7 pcs/m, and in the real conditions, the consumption was 0.04 pcs/m (595 and 232 cutter tools, respectively). Considering the average cost of a cutter at the time of the tunnel's construction (EUR 2500 per piece), EUR 907,500 was saved. The risks of entrapment and gas emissions were other potential setbacks that did not occur, despite that they were prone to occur in the actual geological conditions found, and therefore, they must be counted in the global assessment of the project [42; 43]. The potential entrapment, with an average of 30 days, would have a total fixed cost of EUR 900,000.