Tunnel Fire Dynamics

For structural firefighters, response to a fire in a tunnel is an uncommon experience. On Friday, February 14, 2025, at 11:37 firefighters in Green River, Wyoming responded to a multiple vehicle accident and fire in the Twin Tunnel on Interstate 80. Working on a 10-Minute Training tactical decision game based on this incident got me thinking about fire dynamics and the similarities and differences between tunnel fires and fires in buildings.

This post examines tunnel fire dynamics from the perspective of a structural firefighter, building on an understanding of fire dynamics in buildings.

Enclosed Fires

Open fires occur outside of an enclosure. The behavior of open fires is influenced by the type, quantity, and configuration of the fuel involved along with weather (particularly wind) and terrain. An enclosure fire is typically seen as a fire in a room or rooms within a building. The size, ventilation, type of compartment linings, and the type, amount, and configuration influence compartment fire dynamics.

Just like there are similarities and differences between fires enclosed in highly compartmented residential buildings and large, open, “big box” commercial or warehouse occupancies, the same is true of tunnels.

Similarities

Similarities between fires in buildings and tunnels include fuel load, ventilation, and characteristics of the compartment linings.

Fuel Load

The type, amount, and configuration of fuel influences the potential total energy that can be released as well as the heat release rate (HRR) or energy released per unit of time. Consider the differences between legacy fuels encountered in buildings prior to the 1970s and modern fuels. Legacy fuels were largely comprised of wood and natural fibers. Fuels encountered in the modern fire environment are largely hydrocarbon based. Hydrocarbon based fuels generally have a higher energy content than legacy fuels.

Influence of Ventilation

In 1917, British scientist William Thornton discovered that the consumption of oxygen by a fire is directly related to the amount of energy released from the fuel. Regardless of what product may be burning, whether it is a cellulose or hydrocarbon fuel, for each kilogram of oxygen used for combustion, approximately 13.1 MJ of heat energy will be released.

Fires in buildings as well as those in tunnels may be fuel or ventilation limited. With a fuel limited fire, fire development is limited by the type, quantity, and configuration of the fuel. Within enclosure, a fire will remain fuel limited if there is an adequate amount of atmospheric oxygen. This is dependent on the size of the compartment and ventilation (exchange of the atmosphere inside the compartment with outside air).

If the compartment is relatively small and there are no significant openings to the outside atmosphere, a developing fire can quickly reduce the concentration of oxygen inside the compartment, limiting fire development and potentially resulting in the fire self-extinguishing. If there are openings in the compartment, the fire may continue to develop, with the heat release rate (HRR) or energy released per second being limited by the available ventilation.

Ventilation factors influencing fire development inside an enclosure include:

  • Compartment Volume: Larger compartments contain more atmospheric oxygen than smaller compartments, as such a fire will remain fuel limited for a longer period of time,.
  • Size, Number, and Location of Openings: Ventilation is significantly influenced by the total cross-sectional area of the openings, the number of openings, and their location. For example, a low inlet and high outlet is more efficient than a single opening or even multiple openings of similar size at the same level.
  • External Influences: Wind can have a significant impact on ventilation, depending on its speed and direction in relation to ventilation openings. Consider that doubling the velocity of air through an inlet doubles the volume of air provided to the fire.

Compartment Linings

A fire developing inside an enclosure also heats the linings of the compartment through convective and radiant heat transfer. Radiative feedback from compartment linings then radiates thermal energy back into the compartment, heating yet uninvolved fuel packages.

Radiative feedback is influenced by the thermal capacity of the compartment linings. Thermal capacity is the amount of heat energy needed to change the temperature of a material by a specific amount. Thermal capacity is influenced by the density of the material, its conductivity, and mass.

Differences

Buildings typically have four walls, ceiling and floor, providing a complete enclosure with windows and doors providing limited potential ventilation openings. In some cases, buildings are highly compartmented (residential occupancies) while in other cases, compartmentation is limited (e.g., “big box” occupancies). Tunnels on the other hand are long enclosures that are open at each end.

Configuration

Buildings can have various internal compartment sizes, shapes, and ceiling heights, ventilation openings, and potential creating complex flow paths for heat and smoke.

Tunnels typically have more limited and controlled ventilation (e.g., mechanical ventilation systems) with fewer openings to the outside. The “long and confined” geometry can lead to unidirectional smoke movement if ventilation systems are activated incorrectly or if airflow is influenced by slope and ambient conditions (Ingason, Li, & Lönnermark, 2015). Tunnels often have a long, relatively uniform cross section and limited height, which can cause faster smoke propagation along the tunnel length, making smoke distribution more predictable in some ways but also more dangerous due to the potential for rapid, unidirectional spread (Ingason et al., 2015).

Fuel

Fuel loading in buildings varies with occupancy. With tunnels, fuel loading is dependent on the use of the tunnel (roadway, rail, etc.). Fires in tunnels, particularly road tunnels most commonly involve vehicles. Firefighters generally have experience dealing with vehicle fires. However, things change a bit when vehicles are on fire in an enclosure. Consider the following range of heat release rates based on the type of vehicle involved in fire (Ingason, Zhen Li & Lönnermark, 2015):

  • Passenger Cars: Approximately 2–5 MW
  • Busses: Approximately 15–30 MW
  • Heavy Trucks: Approximately 30–100 MW
  • Cargo Tanks (Flammable Liquids): Up to 200–300 MW

In the Green River incident, it is reported that ten passenger cars and sixteen heavy trucks were involved in the accident with six heavy trucks and two passenger vehicles involved in the fire. While the fuel loading provided by the vehicles involved in fire is important, heat release rate is significantly influenced by ventilation when the fire occurs in an enclosure such as a tunnel.

Ventilation and Smoke Movement

Ventilation in tunnel fires can be complex. Some tunnels have mechanical ventilation systems for smoke management, but others do not. To keep a bit more focused, consider ventilation in a 1200’ (366 M) long tunnel with no mechanical ventilation and the tunnel openings at the same elevation, similar to the tunnel in the Green River Twin Tunnel Incident.

Natural ventilation depends on the buoyancy of hot smoke, differential in the elevation of ventilation openings, and the effects of wind. In a tunnel with no slope, and the absence of wind, smoke will transition to a horizontal flow towards both tunnel portals (openings). As the smoke flows towards the tunnel openings, the smoke layer loses buoyancy and will become thicker, descending towards the tunnel floor. In this case, as smoke exits both ends of the tunnel, air from outside the tunnel enters and flows towards the fire in a bi-directional flow.

If one opening of the tunnel opening is at a significantly higher elevation than the other, this may result in a unidirectional air flow within the tunnel. Flow of smoke and air in the tunnel may also be influenced by wind. If wind direction is aligned with the tunnel and wind speed is sufficient, a unidirectional flow may be developed. While equally applicable to tunnel fires and fires in buildings, the term backlayering refers to upstream smoke movement against the flow of ventilation.

In the Green River Twin Tunnel Incident, the east and west portals of the westbound tunnel were at similar elevation. The wind impacted the west portal at a shallow angle across the opening (not in alignment with the tunnel). Wind speed and direction were insufficient to create a unidirectional flow of smoke and air from west to east. This is indicated by incident photos indicating a bi-directional air flow at both tunnel openings.

In a tunnel fire it is possible to develop a flow of smoke against the effects of wind and slope, this is referred to as backflow. Backflow is dependent on multiple interrelated factors including the HRR, tunnel geometry (cross section, length, and slope), buoyancy (tied to HRR and gas temperature), obstructions and friction (e.g., vehicles in the tunnel), fire location within the tunnel, and external influences such as wind speed and direction) (Ingason, Zhen Li & Lönnermark, 2015).

Fire Development

In a building with relatively small compartments and adequate ventilation, a compartment fire can progress from ignition and an incipient fire through the growth stage, and through flashover to a fully developed fire within the compartment. Flashover is a rapid transition from the growth to fully developed stage in which all of the contents and combustible compartment linings rapidly ignite. In large compartments such as open plan offices and “big box” occupancies, flashover does not occur due to differences in the temperature of the smoke layer throughout the compartment. As smoke travels through a large compartment temperature is reduced as thermal energy is transferred to the compartment linings and compartment contents. In large compartments a flame front progressively spreading across the enclosure, leading to regions of high and low temperatures as the fire progresses, is referred to as a traveling fire.

In a tunnel, smoke from the initial burning fuel package rises in a plume and forms a ceiling jet as hot gases and flames spread laterally. As the smoke travels along the ceiling, the smoke temperature decreases rapidly with distance mainly due to heat loss to the tunnel lining. As the temperature becomes lower, the smoke layer is less buoyant, and smoke drops lower to the floor of the tunnel.

Note: Adapted from Ingason, H., Zhen Li, Li, & Lönnermark, A. (2015). Tunnel fire dynamics. https://bit.ly/4bhkI7Y.

As with fires in buildings, tunnel fires begin with the ignition of a single fuel package (most often a vehicle). Extension to other fuel packages (vehicles) depends on their proximity to the initial fire and heat transfer predominantly through convection and radiation. Given general tunnel configuration and ventilation flashover appears to be impossible in a tunnel fire (Ingason, Zhen Li & Lönnermark, 2015). Fire development and spread within a tunnel are like (but not exactly like) a traveling fire in a large compartment. Depending on the type and arrangement of vehicles in the tunnel, fire spread may simply be from vehicle to vehicle.

Depending on the size and number of vehicles involved in a tunnel fire and the extent of ventilation the fire may remain fuel limited or with larger fires may become ventilation limited (Ingason, Zhen Li & Lönnermark, 2015). One important hazard with ventilation limited fires (in buildings or tunnels) is that an increase in ventilation results in increased heat release rate (HRR). For example a change in wind direction bringing it into alignment with the tunnel and/or an increase in wind speed may significantly increase HRR and dramatically increase downstream temperature.

Strategic and Tactical Considerations

Life hazard can be significant in a tunnel fire with the length of the tunnel and rapidly developing fire conditions potentially compromising civilian occupants’ ability to escape and presenting considerable challenges to firefighting operations.

Critical Factors

Tunnel fires and fires in buildings have many similar fixed and variable critical factors.

Fixed Critical Factors

  • Building (the configuration of the tunnel).
  • Occupancy type (road or rail tunnel).
  • Arrangement (single bore or multiple bore, interconnections between bores, length of the tunnel).
  • Resources (e.g., water supply, fixed fire protection systems, response plans).

Variable Critical Factors

  • Life (trapped occupants).
  • Fire (what is burning, size of the fire, burning regime).
  • Actions (impact of tactical operations).
  • Special circumstances (time of day, traffic, weather conditions)
  • Resources (e.g., resources responding and on-scene, staffing)

Arrangement and Access

Unlike buildings, limited access to a fire is the main problem for tunnels. When a tunnel has multiple, parallel bores that are interconnected to provide emergency egress and access, an unaffected bore can be used for apparatus access and emergency access doors can limit the distance between the entry point and the fire. However, in single bore tunnels or if there are no interconnections between parallel bores (as in the Green River Twin Tunnel Incident) the tunnel portals provide the only points of access.

In Effective Firefighting Operations in Road Tunnels, Kim, Lönnermark, & Haukur (2010) identify that access to a fire in a tunnel may be made from upstream or downstream in the flow path from the fire, depending on conditions. As with fires in buildings, the downstream flow path presents increased risk and as wind speed and alignment with the tunnel increases, this area is likely to be untenable.

Penetration depth and limited air supply are the determining factors in tunnel firefighting and rescue operations. Blue Card standard operating guidelines limit penetration depth in structural firefighting to 175’ (53 m) inside the hazard zone to provide sufficient air supply for deployment of attack lines, time for firefighting operations, and maintaining sufficient air supply to exit prior to activation of the end of service time indicator (EOSTI) low air alarm (at 1/3 of total air supply). The International Fire Academy (Switzerland) is regarded as Europe’s leading centre of excellence for the management of (fire) incidents in tunnels. “The biggest challenge of underground transport systems (UTS) are the great penetration depths… From a penetration depth of about 80 mm (262’), the stresses and personal risks of the firefighters equipped with breathing apparatus increase drastically” (IFA, n.d.). In the Green River Twin Tunnel Incident, the total length of the tunnel was 1200’ (366 m).

Ventilation

Some tunnels are equipped with ventilation systems designed for smoke control in the event of a fire. Others do not. Kim, Lönnermark, & Haukur (2010) identify positive pressure ventilation as one tactic for control of ventilation during firefighting operations. However, given the typical road tunnel size, this necessitates a large apparatus mounted fan (rather than typical gasoline or electric powered fans carried on engine or truck company apparatus).

Chance Favors the Prepared Mind!

If you have tunnels in your response area, have you given thought to the strategic and tactical challenges presented by fire incidents? If you don’t have tunnels, what other types of incident could challenge your capabilities?

References

The International Fire Academy. (n.d.). Great penetration depths: the core problem of UTS operations [webpage]. Retrieved March 10, 2025, from https://bit.ly/3XD6sR8.

Ingason, H., Zhen Li, Li, & Lönnermark, A. (2015). Tunnel fire dynamics. Retrieved March 10, 2025, from https://bit.ly/4bhkI7Y.

Wyoming Highway Patrol. (2025). [Facebook post]. Retrieved March 10, 2025, from https://bit.ly/4gHJDT6.

Kim, H., Lönnermark, A. & Haukur, I. (2010). Effective firefighting operations in road tunnels. Retrieved March 10, 2025, from https://bit.ly/41BI2sm.