Carbon emission mechanism and influence of design-oriented railway tunnel engineering

Tunnels are critical transportation infrastructure, with >80% of their lifecycle carbon emissions from the design phase. Therefore, low-carbon design is a pathway to achieving “zero carbon” goals. However, multi-source and heterogeneous design information creates challenges because of tunnel carbon emission data silos. This study proposes a carbon emissions-structure-design framework with a multi-layered integrated structure for tunnel carbon footprint assessment, clarifying the relationships among design parameters, structural characteristics, and carbon emissions. Additionally, a design structure matrix-carbon footprint model is established to analyze the relationships between low-carbon design elements (LDEs) and the lifecycle carbon footprint. A model is developed to examine the nonlinear mechanisms by which LDEs affect carbon emissions. Case studies indicate that carbon emissions during the construction phase primarily arise from tunnel boring machine excavation, slag transportation, shotcreting, and tunnel lining. They are significantly influenced by LDEs, such as the surrounding rock grade, tunnel radius, advance rate, and slope, which exhibit threshold effects. In the operational phase, carbon emissions are dominated by train traction energy consumption, which increases with speed and decreases with radius. This is in contrast to the construction phase, where larger radii lead to higher emissions. This study integrates tunnel design parameters with lifecycle carbon emissions to overcome the limitations of traditional segmented approaches. The findings provide a decision-support framework for source-level emission reduction during the design phase, enabling engineers to predict carbon emissions for parameter combinations and offer a new strategy for achieving carbon neutrality in transportation infrastructure.