## 5 Conclusions  
To tackle the challenges and enhance the precision of CO2 emissions accounting in shield tunnels, this study introduces a Cost-based Multilevel CO2 Emissions Accounting Method. This method comprises a CO2 emissions coding system grounded in cost analysis and a corresponding accounting technique. It facilitates a comprehensive examination of CO2 emissions distribution within shield tunnels and elucidates the variations in emissions among different tunnels.  
This study assessed CO2 emission distribution patterns in shield tunnels across six demonstration projects, and the average CO2 emissions of 15 m diameter shield tunnels is 86,706 kgCO2e/m. It revealed that as the diameter of the shield tunnel increases, the CO2 emissions per unit of length during the materialization phase rise accordingly, with a predominance of emissions originating from machinery over materials. In terms of overall tunnel metrics, CO2 emissions per 10,000 yuan of investment emerged as a more consistent indicator, registering an approximate value of 30.25% (or 3025 kgCO2e/10,000 yuan).  
Furthermore, this study explored the sub-project level, developing tailored CO2 emissions estimation indicators for shield tunnels and their sub-projects. These indicators quantify CO2 emissions per unit of investment and per unit of length. The research also proposed specific carbon reduction measures for various sub-projects to address the considerable variations and ambiguous distributions of carbon emissions in shield tunnel accounting.  
Future research should focus on calculating CO2 emissions across various projects to identify specific carbon reduction strategies. By integrating lower-carbon initiatives in the planning phase, it is possible to decrease CO2 emissions during the construction of shield tunnels.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14209552/s1](https://www.mdpi.com/article/10.3390/app14209552/s1)]([https://www.mdpi.com/article/10.3390/app14209552/s1](https://www.mdpi.com/article/10.3390/app14209552/s1)).
* [2]**Author Contributions:** Conceptualization, S.Z., B.Z. and X.X.; methodology, S.Z., B.Z. and X.X.; software, S.Z. and B.Z.; validation, S.Z. and B.Z.; formal analysis, S.Z., B.Z. and X.X.; investigation, S.Z. and B.Z.; resources, B.Z.; data curation, B.Z.; writing--original draft preparation, S.Z. and B.Z. writing--review and editing, S.Z., B.Z. and X.X.; visualization, S.Z. and B.Z.; supervision, B.Z. and X.X.; project administration, B.Z. and X.X.; funding acquisition, X.X. All authors have read and agreed to the published version of the manuscript.
* [3]**Funding:** This work was supported by National Key R&D Program of China (grant no. 2023 YFC3806701; 2023 YFC3806705), the National Natural Science Foundation of China (grant nos. 52378408, 52038008), and the Science and Technology Innovation Plan of Shanghai Science and Technology Commission (grant no. 22 dz1203004).
* [4]**Institutional Review Board Statement:** Not applicable.
* [5]**Informed Consent Statement:** Not applicable.
* [6]**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.
* [7]**Conflicts of Interest:** The authors declare no conflict of interest.