## 4 CO2 Emissions Analysis and Indicators in Sub-Projects
### Discussion  
Collectively, these findings indicate that the CO2 emissions distributions among the four sub-projects differ significantly, necessitating unique CO2 reduction strategies for each, as presented in Table 7.  
This study has identified the patterns of CO2 emissions across a variety of sub-projects, including tunnel reinforcement, internal structure construction, and shield excavation. Understanding these emission patterns is critical for developing targeted reduction strategies that can be implemented to mitigate the environmental impact of construction projects. In tunnel reinforcement, one effective strategy to reduce CO2 emissions involves increasing the tunnel length while simultaneously reducing the number of working shafts. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Project** & **Statistical Objects** & **Statistical Unit** & **Value** & \begin{tabular}{c} **Maximum** \\ **Variance** \\ \end{tabular} \\ \hline Shield Tunnel & Shield Tunnel & \begin{tabular}{c} CO2 emissions per \\ unit investment \\ \end{tabular} & 3025 & 11.60\% \\ Shield excavation & Shield excavation & \
\begin{tabular}{c} CO2 emissions per \\ unit investment \\ \end{tabular} & 1868 & 19.80\% \\ Segmental lining & Concrete segments & CO2 emissions per unit volume & 1180 & 1.60\% \\ Internal structure & Concrete structures & CO2 emissions per unit volume &
\begin{tabular}{c} y = 2735.5x + 488.48 \\ (y-value, x-steel content) \\ \end{tabular} & 2.40\% \\ \multirow{4}{*}{Tunnel reinforcement} & Cement soil mixing pile & CO2 emissions per unit volume & 317 & 1.90\% \\ Jet grouting pile & CO2 emissions per unit volume & 523 & 3.30\% \\ Secondary Grouting & CO2 emissions per unit volume & 493 & 0.00\% \\ \hline \hline \end{tabular}
\end{table} 
Table 7: CO2 emissions indicators for shield tunnel. Figure 12: Comparison of CO2 emission indicators for shield excavation. This approach not only optimizes the construction process by minimizing the need for additional entry and exit points but also reduces the carbon footprint associated with the construction of each shaft. Additionally, efforts should focus on avoiding or minimizing crossings through challenging terrain, as such areas often require more energy-intensive construction methods and materials, leading to higher CO2 emissions. Decreasing the volume of secondary grouting, which is typically used to fill voids and stabilize the tunnel, can further contribute to emission reductions. Employing reinforcement methods that are known for their lower carbon output, such as advanced geosynthetic materials, can also play a key role in this context. For internal structure projects, adopting designs that save on material usage, such as columnar structures, can significantly reduce wall usage. By diminishing the structural volume per unit of length, it is possible to enhance CO2 savings. These designs not only decrease the amount of concrete and steel required, both of which have high embodied carbon, but they also allow for more efficient use of space, potentially reducing the overall project footprint. It is important to tailor the most effective reinforcement strategies to the specific type of structure being constructed, ensuring that the chosen method provides the necessary strength and stability with the least environmental impact. When it comes to shield segments, substituting traditional, high-emission steel segments with carbon-efficient alternatives, like those made from steel fiber, can substantially lower CO2 emissions. Steel fiber segments offer similar or even superior performance characteristics while having a smaller carbon footprint due to their production processes and the reduced need for additional support materials. Shield excavation projects must carefully select synchronous grouting materials and work towards reducing the volumes of grout used. The choice of grouting material can greatly affect the overall emissions, as some types may require more energy to produce and apply.