Hazard Fires in tunnels
Knowledge and understanding
|Fires in tunnels||
Understand all associated hazard knowledge
A tunnel is a passageway which may be above or below ground however is completely enclosed except for openings at its exits commonly at each end of the tunnel.
Tunnels should be expected to behave in a similar way to basements. There is likely to be limited access to the tunnel and means of access is likely to be the same as the smoke egress route. Smoke movement from a fire is often considered as part of the initial tunnel design. In some cases there will be shafts to the surface at intervals along the tunnel length, allowing smoke to escape. Often fixed ventilation systems will be provided to supply fresh air for people and vehicles, and these too can take smoke out of the tunnel. See National Operational Guidance: Subsurface, height, structure and confined spaces – Uncontrolled ventilation: Underground structures.
There is likely to be a lack of natural ventilation within the tunnel and the tunnel can be considered a highly insulated space. These two aspects work together to compound the effects of fire within the tunnel environment as the highly insulated confined nature of a tunnel means heat is not lost to the atmosphere but tends to remain in the location of the fire. Fire development may be affected by the lack of inflowing oxygen within the smoke-filled tunnel.
The often-curved nature of a tunnel ceiling means that smoke spread along its length will be far more rapid than an equivalent fire within a compartment. The smoke layer height is also likely to be reduced compared to that within a compartment fire of the same magnitude. For a typical small compartment fire, the hot gases and smoke will rise to the ceiling and spread over the roof, gradually filling the compartment. In a tunnel, the ceiling space is, at least initially, far too large to be filled by the smoke.
Left to itself, the smoke and hot gases will spread along the tunnel roof in both directions. Longitudinal ventilation can control the flow of smoke, pushing it all in one direction. If the ventilation air flow is too slow, the hot smoke and gases can push upstream against the main air flow, a phenomenon known as ‘back-layering’. This upstream hot layer will stop traveling once it has cooled down sufficiently for the force of the ventilation air to overcome the buoyant forces remaining in the cooled gases.
Downstream, the hot gases and smoke will travel long distances at roof level, with the layer dropping as it cools. If it cools to the ambient temperature, the gases and smoke will drop to floor level causing a smoke plug.
If the air supply to the fire is coming from the same end as the one from which the hot gases and smoke are escaping, this smoke plug will be drawn back into the tunnel along with the fresh air supply, eventually smoke-logging the tunnel if the production of smoke is maintained.
People involved in an incident in a tunnel or an underground structure may behave in an unexpected or unpredictable manner. This can affect firefighting and rescue operations as the occupants may have remained within the tunnel rather than evacuating.
- Access routes are likely to be the same as the smoke egress route.
- Lack of ventilation or lack of natural light.
- Radiant heat intensified
- Tunnels are highly insulated spaces allowing the possibility for more rapid-fire growth.
- Increased risk of structural damage and spalling
- Excessive travel distances may be encountered limiting the working duration of firefighters
See National Operational Guidance: Subsurface, height, structures and confined spaces
See National Operational Guidance: Tunnel foundation
Further reading: Beard, A and Carvel, R. Handbook of Tunnel Fire Safety (Geotechnical and Environmental), 2011