## 1 Introduction  
Achieving net-zero carbon emissions globally by 2050 poses a significant challenge. Various cross-industry initiatives have been mobilized in response to decentralization objectives [1]. China pledges to peak its CO2 emissions by 2030 [2]. To attain this goal, every sector is required to establish feasible CO2 reduction targets. The transportation network in China, the largest and fastest-growing worldwide, spans over six million kilometers. This network includes the most extensive systems of high-speed rail and highways globally, presenting substantial opportunities for CO2 reduction [3]. Although the construction sector in developed regions such as the US and Europe has reached peak carbon emissions, those from transportation continue to escalate. In this context, shield tunnels represent the most carbon-intensive mode of transport infrastructure [1, 4], emitting more than 40 times the CO2 per unit of length compared to highways [5]. The construction industry, akin to the transportation sector, has advanced notably in CO2 emissions accounting. Owing to the high standardization across design, construction, and maintenance, along with precise accounting and thorough research [6, 7, 8, 9, 10], projects can be categorized into distinct structures for individual accounting. The methods and systems for CO2 emissions factor accounting are well-established [11, 12] and bolstered by current standards and software. For shield tunnel facilities, the comprehensive and precise accounting of carbon emissions throughout the project's life cycle poses a significant challenge, particularly due to the scarcity of comparative cases for shield tunnels with different diameters under similar conditions. [PERSON] et al. (2015) observed that greenhouse gas emissions are higher during the construction phase than during the operational phase [13]. [PERSON] and [PERSON] (2021) identified four tunnel conditions, ranging from very favorable to unfavorable, with emission rates starting from 5000 kgCO\({}_{2}\)e/m to 15,000 kgCO\({}_{2}\)e/m for conventional tunnel excavation [14]. Further research by [PERSON] on highway tunnels revealed that casting and lining, rock support, and road engineering are the processes with the highest emissions during the construction phase, with the shotcrete process having a significantly greater environmental impact than other processes [15]. They estimated that each meter of a Chinese highway tunnel emits between 82 and 113 tons of CO\({}_{2}\). [PERSON] et al. estimated regional CO\({}_{2}\) and NOx emissions from road transport in Korea [16]. [PERSON] et al. conducted carbon emission accounting for cross-sea tunnels, finding an emission intensity of 11,000 CO\({}_{2}\)eq/m [17]. Additionally, some scholars have proposed a sustainability assessment method for shield segments [18], while [PERSON] et al. explored the relationship between greenhouse gas emissions and rock strength in mountain tunnels [19], [PERSON] et al. reported that the total life-cycle GHG emissions per construction length of the entire Shanghai Metro amounted to 109,642.81 t CO\({}_{2}\)e [20]. It is clear that most current research is centered on measuring carbon emissions. Numerous scholars have focused on carbon emission predictions, primarily targeting aggregate emissions rather than those from specific projects [21; 22; 23; 24]. [PERSON] and [PERSON] analyzed CO\({}_{2}\) emissions during the construction phase of subway projects in Fuzhou and Chengdu, identifying discrepancies exceeding 20% between modeled and actual emissions [25; 26]. Similarly, [PERSON] and [PERSON] investigated carbon emissions from highway tunnels, reporting substantial emissions [27; 28]. In shield tunneling, varying geological conditions and complex surrounding environments necessitate adjustments in construction plans, materials, and equipment usage. Consequently, despite uniform emission factors, carbon accounting outcomes can significantly diverge for comparable projects [29; 30; 31; 32].