Decarbonizing the Geological Premium: A Scenario-Based Carbon Footprint Assessment of Low-Carbon Vertical Nodes in Microtunneling Projects

Trenchless technologies are critical for global urban sewerage construction; however, existing Life Cycle Assessments (LCA) predominantly focus on horizontal segments under standard soft soil conditions, neglecting the massive embodied carbon of "vertical nodes" in extreme geology. Based on EN 15804, this study conducts an upfront carbon (A1–A5) inventory and scenario analysis of microtunneling shafts in Hualien, Taiwan, characterized by deep excavations (10–12 m) and hard gravel formations (SPT N > 50). The research reveals a dual climate challenge induced by extreme geology: the "Geological Premium" resulting from increased machinery energy consumption, and the "Forced Carbon Lock-in Effect" triggered by the necessity of high-strength permanent structures. Empirical results demonstrate that the product stage (A1–A3) of vertical nodes accounts for 51.1% of total emissions, while the construction stage (A5) contributes 42.5%. Consequently, a material-based compensation mechanism is proposed. Scenario simulations verify that introducing geopolymer precast manholes (50% cement replacement) generates a "Green Premium" that effectively neutralizes the construction's geological premium. This study fills the LCA gap for underground infrastructure, providing scientific support for integrating geological variables and low-carbon materials into Green Public Procurement (GPP) policies.