## 4 Discussion
### Possible Technical Solutions
#### 4.1.1 Performance Adjustment  
Maintaining the current TBM tunnel facilities (including Stage III engines in the locomotives) while adhering to the directive's OELV limit would impact TBM performance. One approach is to decrease TBM performance, resulting in fewer trips to transport materials and segments, and leading to reduced NOx emissions and lower gas concentrations in the tunnel atmosphere. For the case analyzed, where the TBM production was maintained at an average of 18 rings/day at its maximum length, 18 rings/day can be considered a realistic advancement rate. To estimate the NO2 average concentration inside the tunnel, theoretical calculations were performed based on specific design parameters and measures following the below premises:  
- Maximum tunnel length: Lmax = 6880 m;
- Average performance (average number of rings/day) = 18 rings/day;
- Number of locomotives in the tunnel (\(N_{L}\)): variable, from 1 to 4 locos in the tunnel simultaneously;
- Locomotive speed: vL = 15 km/h;
- Rings transported per trip = 1 ring;
- Unloading time of the material in the TBM: \(t_{0}\) = 15 min;
- Duration of the excavation cycle + ring placement: tC = 40 min;
- Locomotive Power: PL = 200 kW; \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Current OELV** & **Proposed OELV Directive 2017/164/EU** \\  & **Gas Sensor NO2** & **Gas Sensor NO2** \\ \hline Maximum & 4.58 ppm & 4.58 ppm \\ Average & 1.02 ppm & 1.02 ppm \\ Measurements & 36 & 3457 \\ \(>\)OELV & 1067 & 437 \\ Measurements & & \\ \(>\)10\% OELV to 50\% & 2692 & 302 \\ OELV & & \\ Measurements & & \\ \(<\)10\% OELV & 842 & 440 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Number of NO2 measurements in relation to the current OELV and the proposed OELV. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Current OELV** & **Proposed OELV in the** \\  & **D. 2017/164/EU** \\ \hline  & **Gas Sensor NO2** & **Gas Sensor NO2** \\ \hline N\({}^{\text{o}}\) days with concentrations \(>\) OELV & 0 & 209 \\ N\({}^{\text{o}}\) days with concentrations \(>\) & 50\% OELV to OELV & 46 & 46 \\ N\({}^{\text{o}}\) days with concentrations \(>\) & 199 & 48 \\
10\% OELV to \(<\)50\% OELV & 104 & 46 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Number of days with NO2 daily average in relation to the current OELV and the proposed OELV. Herein, we present a simplified calculation method to quantitatively analyze the issue. Our aim is not to conduct an exhaustive ventilation calculation but to provide a straightforward illustration of the limitations imposed by the new regulations and their practical implications. Although this method is theoretical in nature, the calculation parameters are derived from practical experience. Suppose, at a given moment, that the tunnel's length is denoted as \(L\) (meters), the locomotives' speed as \(v\) (km/h), and a time \(t_{0}\) (hours) is taken for the ring installation maneuver. In this case, the time that a locomotive spends inside the tunnel for each ring, denoted as \(t_{i}\) (hours per ring), is calculated as follows:  
\[t_{i}=t_{0}+2\;\frac{L}{1000\;v} \tag{1}\]  
If \(n_{A}\) rings are installed per day, the total time that the locomotives spend inside the tunnel, denoted as \(t_{L}\) (hours per day), can be expressed as  
\[t_{L}=n_{A}\;t_{i} \tag{2}\]  
The number of locomotives inside the tunnel simultaneously is determined by the need to ensure rapid ring installation to facilitate excavation. For short tunnels, a single locomotive (\(N_{L}=1\)) is sufficient. As the tunnel's length increases, the number of locomotives is incrementally raised, up to a maximum of \(N_{L}=4\). When the tunnel reaches its maximum length, it can accommodate three to four locomotives simultaneously, with an average taken as \(N_{L}=3.5\). Let us assume that a locomotive consumes an average power, \(P_{M}\) (kW), during its work in the tunnel and transports one ring in each cycle.