A Risk Assessment Model for NATM Tunnel Construction Incorporating Site Conditions

This study develops a quantitative risk assessment framework that explicitly incorporates site-dependent variability in NATM (New Austrian Tunneling Method) tunnel construction projects. The underlying motivation is that identical risk factors can exhibit substantially different risk levels depending on project-specific site conditions. Conventional risk assessment approaches, which rely primarily on probability and impact ratings, are inherently limited in their ability to capture such variations across different project environments. To address this gap, key site condition factors affecting NATM tunnel construction were systematically identified and integrated into the existing risk assessment framework through a structured scoring and weighting process. Eight site condition factors were selected based on an extensive review of domestic and international literature, underground safety evaluation reports, tunnel design standards, geotechnical information databases, standard cost data, and expert consultation. These factors—Geotechnical Condition, Construction Schedule Float, Construction Budget Contingency, Spoil Bank Location, Likelihood of Civil Petitions, Underground Water Level, Environmental (Noise, Vibration), and Site Accessibility (Traffic Constraints)—were each quantified using a five-level scale ranging from 0.6 (very favorable) to 1.4 (very unfavorable). Subsequently, a composite site condition index was derived by combining the assigned scores with corresponding weights, and this index was incorporated as an adjustment coefficient into the conventional risk scoring system. The results demonstrate that, when the composite site condition index is considered, both the final risk magnitude and management priority vary depending on site-specific conditions, even for identical risk factors. This indicates that the proposed framework provides a more refined representation of actual project environments than traditional probability–impact-based approaches. The model can also serve as an effective decision-support tool for developing risk mitigation strategies tailored to specific site characteristics. Accordingly, the proposed model enhances the accuracy of risk assessment in tunnel projects and facilitates the rational identification of critical risks requiring prioritized management. However, because certain evaluation criteria rely on expert judgment, further validation through diverse real-world case studies and improvements to the objectivity of the evaluation framework remain necessary.