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Uncontrolled ventilation: Underground structures

Hazard Knowledge

Underground structures, especially tunnels, may quickly be filled with poor quality air due to the impact of the incident or operational activities. Maintaining effective ventilation will be a key consideration for the incident commander.

The purpose of ventilation is normally for keeping the users of infrastructure comfortable, keeping equipment cool or assisting with the control of hot gases or fumes. For the purposes of fire and rescue service operations, ventilation systems can be divided into two principal types:

  • Natural: caused by the flow of air through the infrastructure's openings
  • Mechanical: where a ventilation system serves a specific function, such as cooling, removing fumes or controlling smoke or fire

It should be noted that ventilation systems are more common in highway tunnels due to high concentration of contaminants. Rail transit tunnels may have ventilation systems in the stations or at intermediate fan shafts, but during normal operations rely mainly on the piston effect of the train pushing air through the tunnel to remove stagnant air. Some rail transit tunnels have emergency mechanical ventilation that only works in the event of a fire.

Mechanical ventilation system types

Tunnel ventilation systems can be categorised into four main types:

  • Longitudinal ventilation
  • Semi-transverse ventilation
  • Fully-transverse ventilation
  • Single point extraction

If the ventilation is mechanical, it should normally be known whether it has been provided, designed and rated for fire and rescue service purposes. If it has been provided to control smoke or fumes in an emergency then the incident commander can have confidence that it will provide adequate protection for those evacuating the premises under the guidance of the infrastructure manager, and for responders being committed.

Where mechanical ventilation is unidirectional, or where pre-prepared plans define a default air flow direction, this creates a preferred entry point at the inlet end of the tunnel. Plans should then have responders committed in this direction.