## 4 Life-Cycle Assessment (LCA)
### Material Production  
This section describes the environmental impact (i.e., CO\({}_{2}\) emissions) and energy consumption caused by material produced during the construction process. Because the rail was constructed after the tunnel's construction, the tunnel and the rail parts will be separately considered as follows. Table 6 shows the material characteristics of the tunnel construction process the data for which are collected from the project report [29]. Concrete is the main material for tunnel support, including both temporary and permanent support. The lining, acting as the permanent support of the tunnel, is made of pre-stressed reinforced concrete. Accelerator, a mixture made of bauxite, sodium, and unslaked lime, is mixed with the concrete to enhance the cement's mechanical properties. A waterproof rubber belt is employed for waterproofing because a great deal of water is contained in the rock mountain and waterproofing should be applied after tunnel excavation for protection purposes. Both the temporary and permanent supports utilise rebars to enhance the strength of the raw concrete. For simplicity, fuel consumption during the tunnel excavation process is also accounted for in this material production portion. The working period is assumed to be 600 days with eight working hours per day. The total fuel consumption is taken as 8 L/hr. Table 7 shows the CO\({}_{2}\) emissions and energy consumption of material production for the tunnel's construction. It is clear that the main contribution of CO\({}_{2}\) emissions is concrete production while the waterproof rubber belt consumes the most energy. These features are mainly related to the nature of the material production processes. Table 7 summarises the material CO\({}_{2}\) emission and energy consumption for rail construction. As shown in Figure 1, ballast, sleepers, rails, and elastomeric pads are the main components of the rail system. Ballast is constructed of gravel with an average density of 2000 kg/m\({}^{3}\). The average thickness and width of the cross-section of the rail are 0.3 and 3.95 m, respectively. These dimensions can help evaluate the total volume and mass of ballast used for this rail. Sleepers are made from pre-stressed reinforced concrete and are distributed at a centre-to-centre distance of 0.6 m. The weight of a single sleeper is around 280 kg. Rails are made from steel with a weight of 60.4 kg/m. The steel fastening system is used to connect rails to sleepers and the weight of each fastener is about 2.4 kg. Elastomeric pads, which are placed between the rail and sleeper, act as dampers that can absorb energy caused by trains and reduce vibrations during operation. They are made of ethylene-vinyl acetate and a single sleeper weighs 1 kg. Similar to the tunnel materials, for simplicity, fuel for ballast extraction, railway profiling, and track stabilisation is also accounted for in this portion. The working period is assumed to be 280 days with 8 working hours per day. The total fuel consumption is also taken as 8 L/hr. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Quantity** & \
\begin{tabular}{c} **CO\({}_{2}\) Emissions** \\ **Coefficients (gCO\({}_{2}\)/kg)** \\ \end{tabular} &
\begin{tabular}{c} **Energy Consumption** \\ **(MJ/kg)** \\ \end{tabular} \\ \hline Concrete & 14.9 ton/m & 885.1 & 3.71 \\ Accelerator & 113 kg/m & 1306.8 & 31.65 \\ Waterproof rubber belt & 9.0 m/m & 580.9 & 79.29 \\ Rebar & 2033.4 kg/m & 1060 & 14.38 \\ Fuel & 4 L/hr & 72.09 & 56.26 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Material characteristics for tunnel construction. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Total Mass (Ton)** & **CO\({}_{2}\) Emissions (Ton)** & **Energy Consumption (\(\times\)10\({}^{6}\) MJ)** \\ \hline Concrete & 41,898.8 & 37,085 & 155 \\ Accelerator & 317.8 & 415.3 & 10.1 \\ Waterproof rubber belt & 7703.8 & 4475.1 & 610.8 \\ Rebar & 5717.9 & 6061.1 & 82.2 \\ Fuel & 16.32 & 1.2 & 0.92 \\ \hline \hline \end{tabular}
\end{table}