Road Tunnels Manual

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Ventilation principles

The design of tunnel ventilation systems aims to select the best choice in order to face the following challenges:

  • dilution of air pollutants (inside tunnels),
  • environmental issues (outside tunnel),
  • smoke control in case of fire.

Taking into account the volume of air required for the dilution of pollutants as well as other factors, such as tunnel length, location, type of traffic, environmental laws, and not least, fire safety considerations, an assessment can be performed and the ventilation system can be chosen for each particular tunnel.

1. Types of ventilation systems

Many different types of ventilation systems can be identified in road tunnels including, among others, the following:

  • natural ventilation; which can be induced by the air temperature and meteorological conditions and/or by traffic;
  • mechanical ventilation, which can be
    • longitudinal,
    • massive point or point-flow extraction
    • fully transverse,
    • semi-transverse (and reversible semi-transverse),
    • partial (pseudo) transverse;

(combinations of the above systems are possible and, in some cases, unavoidable):

  • air cleaning combined with mechanical ventilation.

A description of the main characteristics of each of these systems can be found in Chapter V "Ventilation for fire and smoke control" of the 1999 PIARC report 05.05.B, Chapter 4 “Ventilation” of the PIARC report 2007 05.16.B Systems and equipment for fire and smoke control and Chapter "Classification of ventilation Systems" of 2011R02 PIARC report Road Tunnels: Operational Strategies for emergency ventilation.

Criteria and methodologies for the design and dimensioning of tunnel ventilation can be found on the page "Design and Dimensioning", which are based on the main basic ventilation principles, applicable to normal operation and fire scenarios, as described in the following sections.

2. Ventilation during normal operation

The emissions of CO, particulate matter and NOx (the sum of NO and NO2) are considered the reference pollutants from internal combustion driven vehicles in road tunnels.

The required amount of fresh air for a given traffic situation in the tunnel depends on the number and type of vehicles in the tunnel, the average emission per car in this traffic and the admissible concentration for this particular emission (see PIARC report 2019 R02 Vehicle emissions and air demand for ventilation).

For those contaminants affecting human health, exposure time dependent threshold values can be imposed by jurisdictions. The pollution dosage depends on the travel time required to pass through the tunnel. In the absence of national regulations, Chapter 4 “Design and Operational pollutant values” of the PIARC report 2019 R02 PIARC report “Vehicle emissions and air demand for ventilation” recommends admissible concentration and operation (normal and maintenance) limits.

Additional information on NOx emissions and derived recommendations in relation to tunnel ventilation in road tunnels can be found in the PIARC report 2000 05.09 “Pollution by nitrogen dioxide in road tunnels”.

Moreover, for environmental reasons, the ambient air quality at tunnel portals is often required to adhere to certain thresholds of pollutants, mainly NO2. This can be achieved by portal air emissions management. The requirements for in-tunnel air quality or ambient air quality at the tunnel portals may determine, in some cases, the capacity requirements of the ventilation system.

The page on "Design and dimensioning" describes the main criteria to be considered for the design and sizing of ventilation systems in road tunnels for normal operation.

In addition, during the operation of the tunnel, the control of the ventilation system is established according to set points, generally lower than admissible concentration limit values, so tunnel ventilation is engaged prior to pollutant levels exceeding criteria. For extreme circumstances, tunnel closure threshold values that should never be exceeded are defined for safe operation of the tunnel. Further information on design of ventilation control systems can be found on the page "Control and monitoring".

3. Fire Scenarios

An understanding of how smoke behaves during a tunnel fire is essential for every aspect of tunnel design and operation. This understanding will influence the type and sizing of ventilation system to be installed, its operation in an emergency and the response procedures that will be developed to allow operators and emergency services to safely manage the incident.

Chapter 1 "Basic principles of smoke and heat progress at the beginning of a fire" of the PIARC report 2007 05.16.B "Systems and equipment for fire and smoke control" presents details of the general behaviour of smoke and the major influences that affect its propagation in a tunnel, while Chapter 3 describes real case experiences on the influence on smoke control of tunnel ventilation operation in case of fire.

Since a tunnel can be used by different vehicles such as cars, buses, trucks, special vehicles, etc., which may have different loads (persons, non-flammable, flammable, explosives, toxic goods, etc.), possible tunnel fires may differ in terms of quantity and quality. In most cases they are relatively harmless, small tunnel fires with minor temperature and smoke development, but very dangerous tanker fires with high temperatures, enormous smoke production and the danger of explosion may occur. Therefore, it is not possible to prescribe the temperature and smoke development for every possible kind of tunnel fire.

The development and dispersal within the tunnel of smoke resulting from fires depend mainly on the following factors:

  • possibly reduced supply of oxygen to the fire site,
  • heat release,
  • heat convection,
  • longitudinal slope,
  • type of ventilation,
  • dimensions of the traffic space and possible obstructions,
  • thrust caused by any moving vehicles,
  • meteorological influences (wind strength and direction).

Basically it can be said that due to the heat released around the fire site, the smoke rises to the ceiling, and that it continues its flow in one direction when the longitudinal velocity is high, with or without backlayering (see the section on "Dimensioning" of the page on "Design and dimensioning") and in both directions when the longitudinal velocity is low. Thus, there should be some smoke-free space just above the road surface in the vicinity of a fire - at least for a short period of time.

Detailed information on smoke and pollutants production during real fires and large scale fire tests can be found in Section II.4 "Choice of design fires" and Section III.4 “Smoke development and dispersal of smoke in fire tests” of the 1999 PIARC report 05.05.B "Fire and Smoke control in Road Tunnels" and in Appendix 2 "Fire tests" of the 2017 R01 PIARC report "Design Fire characteristics for Road Tunnels".

Tunnel ventilation is a key element to mitigate the consequences of a fire in a tunnel. The page on "Design and dimensioning" describes the main criteria to be considered for the design and sizing of ventilation systems in road tunnels for fire scenarios.

In addition, the page on "Ventilation Strategies" provides additional information on the best operational strategies in case of fire.

4. Other hazards

A very important risk factor when dealing with fire safety in a tunnel is whether vehicles transporting dangerous goods are allowed or not. The criteria to decide when such transport should be allowed are not usually covered in the topics related to tunnel ventilation. Further information on the assessment an mitigation of the risks associated to the transport of dangerous vehicles in road tunnels can be found on the page "Hazards due to DG-transport".

Reference sources

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