Upfront Carbon Footprint of Deep Microtunneling Vertical Nodes in Hualien Gravel Strata

Trenchless technologies are essential for urban sewerage infrastructure; however, standard Life Cycle Assessment (LCA) boundaries often overlook the upfront carbon footprint of vertical nodes (working shafts and precast manholes), particularly under deep excavation and difficult geological conditions. To bridge this research gap, this study establishes a comprehensive upfront carbon (Stages A1–A5) assessment model based on the EN 15804 standard, calibrated against empirical microtunneling inventory data from Hualien, Taiwan, characterized by deep excavations (10–12 m) and hard gravel strata (SPT N > 50). The empirical results reveal a dual carbon challenge: a geologically induced energy surge during construction (Stage A5, contributing 42.5% of total assessed upfront emissions) and an embodied carbon lock-in within high-strength permanent structures (Stages A1–A3, contributing 51.1%). To isolate these effects, a progressive four-scenario matrix was evaluated. Scenario simulations demonstrate that adopting high-volume supplementary cementitious materials (SCM) concrete manholes (50% cement replacement: 37.5% GGBS and 12.5% fly ash) achieves a material reduction factor (Rmat) of 15.0% in Stages A1–A3, fully offsetting the isolated 3539 kgCO2e construction-energy increment imposed by the hard gravel strata, although total upfront emissions in Scenario IV remain slightly higher than the baseline Scenario I when evaluated across the complete A1–A5 boundary. This study provides an expanded LCA framework and empirical evidence for integrating geological constraints and low-carbon material specifications into underground infrastructure procurement and engineering design.