# Development of a Fire Risk Assessment Program for Submerged Tunnels
## 1 Introduction  
Submerged tunnels are emerging as next-generation transportation infrastructure capable of replacing large-scale maritime bridges and subsea tunnels. They are gaining attention as innovative infrastructure solutions that can overcome geographical barriers and have the potential to significantly contribute to the development of integrated road and railway networks. Research and demonstration projects on submerged tunnel technology are actively being carried out around the world, and in particular,, Norway and China have examined its feasibility as part of large-scale national projects ([PERSON] and [PERSON], 2007) [1]. As a cutting-edge tunnel technology capable of stable operation even in deep waters and strongtidal environments, submerged tunnels are attracting increasing attention. Especially as an efficient means for constructing integrated transportation networks connecting roads and railways, they are an innovative infrastructure solution for connecting roads and railways, especially in areas where conventional bridge or overland tunnel construction is limited by deep waterways, narrow straits, or dense urban development. In 1972, a night train fire in the Hokuriku Tunnel in Japan resulted in many deaths due to carbon monoxide poisoning. This incident prompted the Japanese government to strengthen tunnel safety standards by enhancing fire detection and early response equipment and by introducing structural safety designs, including parallel evacuation passages and smoke control systems. In 1999, the Mont Blanc Tunnel fire occurred, where the lack of an adequate ventilation system and emergency response measures led to a significant number of casualties. In 2008, the Channel Tunnel fire emphasized the importance of fire protection and evacuation systems inside tunnels. More recently, in 2025, fires broke out in the I-80 Green River Tunnel in Wyoming and the Caldecott Tunnel in California, United States. In the case of the I-80 Green River Tunnel, the absence of sprinklers and fire suppression systems allowed the fire to spread uncontrollably, and the inadequate ventilation system caused smoke to fill the tunnel quickly, making evacuation difficult. In contrast, the Caldecott Tunnel's ventilation fans effectively minimized smoke spread, and the tunnel was reopened approximately three hours after the fire was extinguished. In response to these incidents, national governments and international organizations have revised safety standards. PIARC's recommendations and updates to NFPA 502 have introduced requirements for longitudinal ventilation, evacuation cross passages, and structural fire resistance. These standards have had a significant influence on the assumptions used in the SFT_QRA model, particularly with regard to evacuation intervals and the classification of fire scenarios. Establishing thorough fire protection systems and response strategies can help minimize damage. However, unlike conventional tunnels, submerged tunnels require even more comprehensive and robust systems. Submerged tunnels differ from conventional ground-level and subsea tunnels in terms of structural and environmental characteristics, and the risks in the event of fire or other disasters tend to be significantly higher compared to those in above-ground or mountain tunnels due to limited ventilation, constrained evacuation routes, and high-water-pressure environments. In particular, complex evacuation procedures, the difficulty of smoke flow control, and the need to ensure the reliability of ventilation systems pose serious safety challenges. Moreover, comprehensive technical evaluations are required, including considerations for long-term maintenance and environmental impact. Despite these challenges, public awareness regarding the safety of submerged tunnels remains low, and research on fire prevention and risk assessment for such tunnels is still insufficient ([PERSON] et al., 2024) [2]. Therefore, the establishment of a fire and emergency response system tailored to these conditions is urgently required.