A tunnel is an underground or underwater passage, typically enclosed except for openings at each end. It can be dug through earth or rock or laid under water. Tunnels vary by construction type (cut-and-cover, NATM, TBM), shape, and purpose (road, rail, metro, sewage).

Figure 1 Tunnel

According to Hilti’s internal research, the total investment value for tunneling projects in the period 2024 to 2028 will exceed $780b, with the biggest proportion of investment in Asian and European countries.

Figure 2 Total investment value in tunnel

This article discusses some key aspects of passive fire protection in tunnel projects excluding underground stations and their accesses.
One of the main considerations when designing and maintaining a tunnel is fire risk. On average, there are three tunnel fires per month worldwide. 75% of the fires in tunnels are in road tunnels and 25% in rail tunnels. According to a publication from the Swedish National Testing and Research Institute, the duration of fires in tunnels can be from hours to days, depending on the severity and the response efforts. Tunnel designs are becoming more complex: incorporating roundabouts, road, rail and metro transfer stations, and multiple services requirements for mechanical, electrical and ventilation systems. All these factors increase the risk of fire.

The main causes of fires in tunnels include vehicle collisions, overheating, electrical failures and leakages of fuel or oil from vehicles as represented below.


In case of a fire, it's crucial to have safe escape routes and access for fire-fighting teams. Passive fire protection is essential for this.

The codes and standards framework for fire protection in tunnels projects is complex.
Many cities and metropolitan areas across the world have specific codes and standards for tunnel projects and maintenance. In addition to these local codes, tunnel projects also need to adhere to national and international codes and standards. National and international codes provide a strong framework for construction and health and safety topics while local codes and standards, in addition, highlight environmental and other local requirements. Some of the most recognised global codes and standards for tunnels are:

• NFPA 502 standard for road tunnels, bridges, and other limited access highways provides comprehensive guidelines on fire protection and life safety.
• PIARC provides guidelines and best practices for road tunnel safety, including fire protection measures.
• EU Directive 2004/54/EC sets minimum safety requirements for tunnels in the Trans-European Road Network, including fire safety measures.
• ISO 834 standard for fire resistance tests on building materials and structures, which is also applicable to tunnel linings.
• EN 1363-1 standard for fire resistance tests, provides general requirements for fire protection in tunnels.
• NFPA 130 standard covers fixed guideway transit and passenger rail systems, including fire protection and life safety in tunnels.

Figure 3 Time-Temperature curves for fire in tunnels

There are different time-temperature curves internationally which are developed to effectively standardise fire scenarios in tunnel environments.

Challenges and Solutions for Fire Safety in Tunnels
Fires in tunnels have particularities that make evacuation, firefighting and rescuing very challenging: access and egress are limited and make very difficult for firefighters to reach the fire and people to evacuate; long distance is needed to reach an escape route. Ventilation problems can quickly cause smoke and toxic gases to accumulate, reducing visibility and air quality. The confined space and materials in tunnels can lead to unpredictable fire behaviour, requiring fire fighters and rescue teams to have specialised knowledge and to apply specialised tactics. Availability of resources such as water supply, ventilation systems and firefighting equipment in tunnels are key elements to facilitate rescue and firefighting operations.
To reduce the risk of fires spreading along tunnels, compartmentation is a key design element, especially on the access and escape routes, ventilation shafts and other service areas. When selecting passive fire protection solutions for tunnels, it is crucial to consider several key parameters such as fire resistance, reaction to fire, and smoke control: durability to reduce maintenance effort; water resistance, mold, and mildew to cope with humid environments. Some local standards, particularly in big metropolitan areas, require other attributes like rodent resistance, fatigue and capacity to resist high pressure.

Hilti is committed to making construction better and tunnel projects are no exception.
For years Hilti have been developing the most innovative pre-formed passive fire protection solutions for both temporary and permanent applications in tunnel projects such as cable transits and cable sleeves for electrical applications and collars for metal and plastics pipes in mechanical applications.
Sustainability and the environment play a key role on Hilti’s innovation with Life Cycle Assessments (LCAs) and Environmental Product Declarations (EPDs) providing Data Transparency on Product Carbon Footprint. Material Health Certificates such as C2C Material Health Certificates and Health Product Declarations provide assessments and screening of material ingredients in accordance with Green Building Schemes. Finally, VOC Emissions and Content Certificates show that the products have been tested and fulfil Green Building Standards for VOCs.

If you would like to find out more about HILTI and its products, visit the Hilti online shop, visit HOL

References
• https://tunnel-fire.com/passive-fire-protection-in-tunnels/
• https://iopscience.iop.org/article/10.1088/1757-899X/741/1/012095
• https://www.scor.com/sites/default/files/pc_nl_tunnels.pdf
• https://www.cetu.developpementdurable.gouv.fr/IMG/pdf/cetu_guide_protection_passive_mars2017_en.pdf
• https://iopscience.iop.org/article/10.1088/1757-899X/741/1/012095/pdf
• https://nap.nationalacademies.org/read/14562/chapter/6#21
• https://www.thefreelibrary.com/Road+tunnel+ventilation+and+fire+control-- regulations%2c+standards%2c+and...-a0201378209
• https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32004L0054
• Tunnel boring machine - Wikipedia
• New Austrian tunneling method - Wikipedia
• Cut-and-cover tunneling method
• Fire Development in Large Tunnel Fires
• NFPA – National Fire Protection Association
• ISO – International Organization for Standardisation
• PIARC – World Road Association
• PCF – Product carbon footprint
• VOC – Volatile organic compound