# Directional Core Drilling as an Approach to Reduce Uncertainty in Tunneling Construction
## 4 Results
### Bolanos Tunnel  
There was a region in the Bolanos Tunnel with a potential length of nearly 1000 m that required detailed definition of the rock mass characteristics during the excavation stage of the tunnel. In this regard, five vertical boreholes were drilled in the study area. Table 6 sets out the cost assessments of the conventional boreholes and the DCD technique for this particular stretch of the tunnel. Based on the same geological cross-section used to define the conventional vertical boreholes, a single DCD was proposed (Figure 1). \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
100 m \(<\) L \(\leq\) 150 m & 2 & 260 & 6000 & 26,000 & 32,000 \\ L \(\geq\) 150 m & 3 & 530 & 9000 & 53,000 & 62,000 \\ Total conventional & 5 & 790 & 15,000 & 79,000 & 94,000 \\ DCD & 1 & 1600 & 3000 & 320,000 & 323,000 \\ \hline \end{tabular}
\end{table} 
Table 6: Conventional and DCD assessments for the Bolánós tunnel. \begin{table}
\begin{tabular}{l c} \hline  & **Factor** & **Uncertainty Cost (EUR)** \\ \hline Lining segments & +1,004,108 \\ TBM performance & +1,620,000 \\ Entrapment & +900,000 \\ Gas emissions & +350,000 \\ Cutter tools consumption & \(-907,500\) \\ Total risk estimate & 4,781,608 \\ \hline \end{tabular}
\end{table} 
Table 5: Uncertainty factors. The cost distribution for the conventional approach was 16% for transportation and 84% for drilling, resulting in knowledge of only 5% of the tunnel's layout for the area of interest (investigation unit cost of EUR 940 per m). On the other hand, DCD reached 100% definition of the region, and its costs were concentrated on drilling (investigation unit cost of EUR 323 per m), as it can be seen by the red line in Figure 5. In this case, the conventional technique was nearly three times more expensive than the DCD technique. Regarding the geochemical composition of the rock mass, the ampelite slates found in the stretch studied could have generated contamination due to the presence of sulphides and heavy metals. Thus, it was considered the worst-case scenario in the analysis, based on the characteristics defined in Section 3, with a total of EUR 600,000 as an additional cost for the management of the rock mass excavated in the affected area. Moreover, the TBM performance was poorer than planned in the project. The expected rate was 25 m/day, whereas the actual excavation rate was 4 m/day, requiring 22 additional days of excavation, which resulted in an additional cost of EUR 660,000. The presence of a rock mass with squeezing characteristics created an additional risk cost of EUR 900,000. In the case studied, one of the main points of the construction system was the previous treatment of the unstable excavation face using organo-mineral resins. Thus, the resin consumption was higher and, therefore, more expensive. The tunnel in this case study also faced the risk of squeezing due to the presence of ampelite minerals in the slate formation [61]. The unit cost of the treatment could be estimated at EUR 1260 m under normal conditions, and it reached EUR 4620 per m under the squeezing conditions, with 1000 m of poor conditions faced in the analyzed tunnel. Hence, there was a cost overrun of EUR 3,360,000 due to poor knowledge of the rock mass. Table 7 summarizes the overall uncertainty costs of the tunnel. Based on the assessment of the Bolanos tunnel, the DCD technique could have reduced the uncertainty by EUR 5,520,000 in the stretch analyzed (EUR 5520 per m), which represents EUR 6.7 for every EUR 1 invested in DCD. \begin{table}
\begin{tabular}{l c} \hline \hline
**Factor** & **Uncertainty Cost (EUR)** \\ \hline Geochemical composition & +600,000 \\ TBM performance & +660,000 \\ Entrapment & +900,000 \\ Organo-mineral resins & +3,360,000 \\ Total risk estimate & +5,520,000 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Uncertainty factors found in the Bolanos tunnel.