## 4 CO2 Emissions Analysis and Indicators in Sub-Projects
### Internal Structure  
For internal structural projects, CO2 emissions are strongly influenced by the length of the shield tunnel, as well as the form and reinforcement ratio of the internal structure. However, conducting a comprehensive analysis of all these factors in CO2 emission studies presents significant challenges. Thus, this analysis focuses on CO2 emissions per unit of length, unit of project volume, and steel content.  
As shown in Table 4, Tunnels 5 and 6, with diameters of 11.36 m, exhibit very similar project volumes per unit of length. In contrast, Tunnels 1 and 3, which have diameters of 15 m, display a notable difference in project volumes per unit of length, with a variation of up to 8 m\({}^{3}\). This discrepancy is primarily attributed to Tunnel 2 utilizing walls with  
Figure 10: CO2 emission analysis chart of tunnel reinforcement.  
-flue slab structures for internal partitions, whereas Tunnel 1 employs column structures without flue slabs, resulting in an approximately 30% difference in their internal structures.  
When comparing Tunnels 2 and 3, although their unit-of-length CO2 emissions from the internal structures are similar, there is an approximately 9% difference in their unit-of-length project volumes. This discrepancy is primarily attributed to the variations in steel content within the internal structures. Consequently, this study establishes a linear relationship between steel content and CO2 emissions per unit of volume of the internal structure.  
As shown in Figure 11, a close correlation exists between CO2 emissions per unit of volume of the internal structure and the steel content. The specific functional relationship is provided in Equation (6):  
\[\text{y}=2745.5\text{x}+487.3\ (\text{R}^{2}=0.974) \tag{6}\]  
where y represents the CO2 emissions per unit of volume (kgCO2e/m3) and x represents the steel content.  
The findings in this chapter emphasize the importance of adopting material-efficient structural forms, such as utilizing columns instead of walls where feasible, to achieve significant CO2 reductions. Furthermore, the reinforcement ratio of the internal structure has a substantial impact on CO2 emissions. Therefore, it is essential to apply the most  
\begin{table}
\begin{tabular}{l c c c c c c} \hline  & **Length** & **CO2 Emissions** & **Project Quantity per** & **CO2 Emissions per** & **CO2 Emissions** & \\  & **(m)** & **(tCO2e)** & **Meter** & **Unit Volume** & **per Meter** & **Steel Content** \\  & & & **(m3)** & **(kgCO2e/m3)** & **(kgCO2e/m)** & \\ \hline Tunnel 1 & 2008 & 45,546 & 21.86 & 1038 & 22,682 & 0.204 \\ Tunnel 2 & 1539 & 38,787 & 27.05 & 932 & 25,198 & 0.154 \\ Tunnel 3 & 2316 & 59,141 & 29.82 & 856 & 25,536 & 0.132 \\ Tunnel 4 & 520 & 13,272 & 29.81 & 856 & 25,523 & 0.132 \\ Tunnel 5 & 2501 & 26,694.8 & 12.71 & 840 & 10,675 & 0.133 \\ Tunnel 6 & 768 & 7742.6 & 12.71 & 793 & 10,082 & 0.116 \\ \hline \end{tabular}
\end{table}
Table 4: Internal structure engineering CO2 emissions analysis table.  
Figure 11: CO2 emissions per unit of volume of internal structure.  
appropriate reinforcement strategies for different structural types. A holistic approach that accounts for both the structural form and reinforcement ratio can lead to the most carbon-efficient construction design.