Evaluating Carbon Emissions during Slurry Shield Tunneling for Sustainable Management Utilizing a Hybrid Life-Cycle Assessment Approach

By inergency On Mar 25, 2024

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1. Introduction

The world will now encounter the biggest wave of climate hazards over the next two decades with a global warming temperature of 1.5 °C (2.7 °F), thereby posing a grave and mounting threat to both nature and human wellbeing [1]. Human-induced climate change is causing unequivocal and widespread impacts, some of which are long-lasting and, in some cases, irreversible. Increasingly, droughts, floods and heatwaves will be triggered by increased weather extremes in the wake of climate change. The human influence on climate change is reflected in both the growing scientific literature and in people’s perception worldwide. Accelerated actions will, thus, be required to respond to the increasing risks brought by climate change. Making rapid and deep cuts in greenhouse gas (GHG) emissions is central to reducing risks and vulnerability from climate change. GHG emissions are usually seen as the prime villains in the debate on global warming [2]. A global challenge calls for global efforts. Realizing this pressing issue, many countries have been mapping out strategies or measures in an effort to address climate risks and to meet their carbon reduction targets. For instance, the United States (US), by not only funding projects that focus on reducing carbon emissions but also by investing in technologies that help unleash their potential for increasing the remanufacturing or recycling of industrial materials, is heading to a net-zero greenhouse gas economy according to the US Department of Energy (DOE) [3]. China has unveiled working guidelines that aim for the goal of peaking carbon emissions by 2030 and achieving carbon neutrality by 2060, at the same time as honoring its climate commitment. As the guidelines show, China’s CO₂ emissions per unit of GDP are projected to be lower by 18% compared with the 2020 level by 2025 [4]. Equally, according to European Climate Law, it is anticipated that the European Union (EU) will increase its GHG emissions reduction target for 2030 to at least 50% compared with 1990 levels and has presented its objective of achieving net-zero GHG emissions by 2050 [5].

The infrastructure construction sector, characterized by a gargantuan consumption of energy and materials, is regarded as a significant source of GHG emissions, as well as one of the major contributors to global carbon emissions [6,7]. Together with the building sector, the infrastructure construction sector accounts for 38% of global carbon emissions [1], demonstrating the key role of these sectors in cutting emissions. Notably, growing populations and rapid urbanization, particularly in Asia and Africa, will further facilitate the surging demands for infrastructure construction. Many construction sites are highly resource- and energy-consuming. This highlights the urgency of addressing construction-related emissions, which are being released into the atmosphere, as we continue to extract and manufacture materials and products for construction projects.

Tunnel construction, commonly referred to as the highest energy- and material-dense sector in transportation infrastructure, intensively consumes prodigious amounts of energy and materials [8]. China’s road tunnels are developing rapidly. Over the past 100 years, China has built 15,025 road tunnels with a total length of more than 14,000 km. From 2013 to 2016, the total length of Chinese road tunnels increased by 4434 km. According to [9], over the last decade, governments and researchers have shown more concern over the GHG emissions associated with tunnel construction. With the number of Chinese tunnels growing, studies on GHG emissions from tunnel construction are, therefore, urgently needed.

Process-based life-cycle assessments (P-LCAs), one of the LCA methods, have been widely used to evaluate the GHG emissions from tunnel construction. For example, Li et al. [10] used the P-LCA method to calculate the GHG emissions during the construction of new highway tunnels in China and ascribed most of the endogenous GHG emissions to the equipment powered by fossil fuel combustion motors for tunnel construction. These fossil fuels include gas, diesel, oil, coal and asphalt. The GHG emissions generated by diesel consumption are at a high level, with a proportion of over 90% in comparison with other fuel types in tunnel building. Subsequently, Miliutenko et al. [8], in Sweden, using the P-LCA method, analyzed the carbon emissions from a road tunnel. From the perspective of life cycles, this study took into consideration the emissions during not only the construction phase but also the operational life. Similarly, Huang et al. [11], applying the LCA method to quantify the emissions of rock tunnels in Norway, concluded that GHG emissions should be taken into account from the early planning phase to the end-of-life phases. During the same year, Huang et al. [12], in Shanghai, described the GHG emissions in a tunnel, pointing out the differences between the emissions of different machines and the importance of driving speed for fuel consumption during the construction period.

Rodríguez and Pérez [13], based on the LCA method, developed a simplified calculation model for analyzing the GHG emissions of tunnels under construction and applied it to estimate the GHG emissions associated with tunnels excavated in medium–low-strength rock masses by using roadheaders or hydraulic breaker hammers. To detect the impacts of varying parameters on emissions, they employed the model in several cases and carried out an analysis of seven different tunnels in Asturias (northwest Spain). From the results that about 80% of GHG emissions were related to the concrete used in the tunnel, they attributed the largest contribution to GHG emissions to the production of concrete used in both supporting and lining the tunnel under construction. However, it was thanks to the applied simplification that the scope of the study was, to a certain extent, limited. For instance, when it came to the GHG emissions related to excavation, rock waste removal or auxiliary services, simply the emissions related to the work of machinery fueled by energy, both electricity and fuel, were calculated, whereas they dismissed materials from their consideration. And when estimating the GHG emissions associated with the support installation and lining, their work was exclusively concerned with emissions related to the manufacturing and transport of the materials, purely including concrete and steel, while those emissions related to the energy use of construction machines were neglected.

Finally, it has to be pointed out that P-LCA has become a globally accepted method for evaluating carbon emissions. Yet there are few studies that focus on carbon emissions during the slurry shield tunnelling. Most research on GHG emissions during the tunnel construction focuses on road tunnels or highway tunnels. The number of works on GHG emissions from the slurry shield tunnelling that we can find in the specialized literature is rather low. Moreover, the system boundary of using P-LCA is quite subjective. In other words, different researchers who use P-LCA will select different system boundaries when studying the same tunnel project. Therefore, it can be seen that most researchers who adopted the P-LCA method tended to disregard inputs that may not be substantial in order to make the method more applicable to their case study, thereby making it less possible to gain and acquire a more rounded assessment.

To encapsulate, there exist three main problems in studies on GHG emissions at the tunnel construction stage. First, the system boundary selection when using the P-LCA method is subject to the quality of inventory analysis of the case project. Often, due to the loss of data, inputs that play a relatively minor role in tunnel construction have been overlooked frequently, and those GHG outputs remain unclear. Second is to introduce a new method of solving the problem that studies focusing on the emissions from tunnel construction have always been restricted by limitations due to the use of one single method. Although various methods of calculating the GHG emissions caused in the construction phase of the life cycle have been proposed, these methods cannot be developed for the accurate measurement of emissions from tunnel construction. The third is to provide an estimation method when the carbon emissions factors do not include indirect emissions. For example, some previous studies have failed to consider indirect inputs like upstream fuel.

For this reason, we have detected a gap in terms of studies or results related to emissions during tunnel construction by a hybrid method and thereby generated an idea of a combinatorial strategy for addressing the emissions that have been ignored. This study is intended to define a hybrid LCA method to calculate GHG emissions, suggesting different LCA methods, including process-based LCA and I-O LCA, to determine the amount of energy and materials used in different phases of the tunnel under construction. The hybrid LCA method described herein can be seen as a combination of process and the I-O LCA method. The article first explored and analyzed the strengths and limitations of these two methods, which aims to produce a comprehensive analysis that includes economy-wide effects in addition to on-site construction activities. After comparative analysis of two methods, a hybrid LCA model will be developed and introduced to estimate the GHG emissions during the tunnel construction. This hybrid model has been utilized for calculating the GHG emissions of a tunnel in China. Section 4.2 and Section 4.3 describe the main findings of this research and present possibilities for capturing and reducing GHG emissions.

Herein lie the main features of the proposed hybrid method:

Flexible: It can be used as two different methods to estimate the GHG emissions during different phases of a tunnel. Instead of one single method, we opt for one method or the other or both, depending on site conditions, data category, local market, etc. It will be possible to adjust the carbon emissions factors according to the country or region specifics.
Effective and time-saving: The task of calculating the GHG emissions, especially by P-LCA, is data-intensive and time-consuming. With the I-O LCA method based on the monetary value and conversion factor to CO₂, we can make a rapid calculation of GHG emissions. The P-LCA method is not efficient enough to achieve the calculation target under severe missing data circumstances. This hybrid approach may be effective by completing missing data in the life-cycle inventory.
Easy to implement: No special skills are required for its practice. The simplicities of the proposed hybrid method are based on the following: (a) the budget quota and energy consumption of tunnel construction; (b) the use of the conversion factor method to convert the amount of energy and materials consumed into GHG emissions; (c) focus not on the whole life cycle but on the construction phase of a tunnel; (d) only account for the CO₂ emissions and not for other gas emissions.
Provide a holistic view of GHG emissions: This hybrid approach could provide a rounded analysis of the emissions associated with materials production, materials materialization, materials transportation and off-road machinery. It can not only address those emissions arising from auxiliary materials (grease, bentonite, PVC pipe and rubber material) but also provide a method to estimate the emissions during the upstream fuel when carbon emissions factors do not consider indirect emissions.

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