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In civil engineering, tunneling is one of the few areas where new horizons are constantly being explored. It is an exciting and rapidly evolving field in which pioneering processes are commonplace and innovative ideas continue to rewrite the rules.

The Kaisermuhlen Tunnel in Vienna is considered as the safest road tunnel in the world. With a total length of  2,150 m and a daily traffic load of 100,000 vehicles, it is one of the most heavily used tunnels in Austria. It is considered to be the safest road tunnel as it is provided with the world’s best tunnel installations. 

The tunnel uses more than 2,000 lamps for illumination and 104 fans for ventilation, while 7.5 km of sensor cables are installed for automatic fire monitoring, and two ventilation buildings with ventilators assure a complete exchange of air in 10 minutes.

A total of 22 emergency call niches with spacing of 200 m and nine emergency call columns in front of the portals enable contact with the tunnel guard. 50 robot video monitors control the tunnel. Radio amplifiers and antenna cables at the tunnel roof enable communication for the service teams. A special pipeline fed from the river Danube is provided for water for firefighting.

Kaisermuhlen Tunnel in Vienna is considered as the safest road tunnel in the world due to provision of world's best tunnel installations
Kaisermuhlen Tunnel

The tunnel control room operates day and night, and the tunnel guard is assisted by 13 automatic fire programs and more than 150 traffic programs. A concise system of traffic guidance and information helps the users to react to unexpected situations. Three spare power stations and accumulators, as well as a mobile and a stationary generator guard against complete blackouts.

In this article, we discuss the different installations required for the operation of a tunnel, such as traffic control installations, installations for telecommunication, ventilation, fire protection, illumination of tunnels, and drainage. 

1. Installations for Traffic Control

Installations for traffic control inside the tunnel include:

  1. Road signs should be provided.
  2. Traffic lights should be placed for tunnels. It should be placed at the portals, U-turn (turn-around) niches, and at trafficable cross-overs.
  3. Traffic guide equipment such as floor labeling and side reflectors should be provided.
  4. Provision for height control to catch oversized vehicles before they enter the tunnel should be provided, for e.g., photo-sensors can be used for this purpose.
  5. Provision for video-monitoring should be provided for tunnels longer than 1,500 m.
  6. Traffic density should be monitored. The entire tunnel length, as well as the areas in front of the portals, should be monitored.
  7. Modern sensors should be installed to warn about traffic slow down, for instance, due to a fire.
Placement of traffic lights at the portal of tunnel
Road signs and traffic lights in a tunnel

2. Installations for Telecommunication

The following major equipment should be provided in a tunnel for telecommunication.

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2.1 Equipment for Emergency Calls

The equipment for emergency calls have to be provided in tunnels of more than 500 m in length with a spacing of 150 m. Portals and U-turn niches should also be equipped with emergency call facilities. Telephone boxes should be provided with glass doors that can be opened toward the tunnel.

Provision of service telephones at every service station in a tunnel
Emergency call facilities inside a tunnel

2.2 Service Telephones

Service telephones have to be provided in tunnels of more than 1,000 m length at every service station and also in every control room. They are dispensable if radio communication facilities are provided.

2.3 Radio Communication

In tunnels longer than 1,000 m or with a high traffic density, radio communication should be provided for the fire brigade, police, and road administration, as well as for traffic announcements. Radio broadcasting equipment and loudspeakers should be placed inside a tunnel for making announcements to the tunnel users.

3. Ventilation

With reference to tunnel ventilation, two different systems of ventilation have to be provided. The first one is ventilation during construction (i.e. during the heading of the tunnel), and the second one is service ventilation (i.e. during the operation of the tunnel). Expenditures on the latter amount up to 30% of the total construction costs.

3.1 Ventilation During Construction

Ventilation during construction is provided with the aim of oxygen supply. The O2-content of air should not fall below 20% of the occupied air volume. Below 18 % volume, breathing is not possible, and protection masks should be used. The lack of oxygen is due to:

  1. Combustion motors
  2. Breathing
  3. Oxidation of wood, coal, etc.
  4. Dissolution of groundwater because it dissolves more O2 than N2

The main reason behind providing oxygen supply is to clean the air during construction. The following pollutants have to be removed during construction

  1. Dust from rock excavation and shotcreting
  2. Combustion motors
  3. Blasting fumes
  4. Gas egression from rock
  5. Radon decay products
Horizontal shaft provided for the ventilation during the tunnel construction
Ventilation during construction

Fine SiO2 dust with a particle diameter less than 5 μm gets deposited in the lungs and can cause lethal illnesses like silicosis by hardening the lung tissue. Radioactivity due to radon and its decay products should not exceed 1,000 to 3,000 becquerel per m3 air.

Methane concentrations between 4 and 14 % volume may cause explosions. Ventilation also serves cooling purposes. One should take into account the geothermy. Apart from geothermy, the hydration of concrete can be an additional source of heat. The following ventilation alternatives can be considered:

  1. Supply: supply with fresh air
  2. Extraction: extraction of polluted air (to be preferred for shotcrete applications)

Supply and extraction of air during construction of the tunnel is provided using ductings. Mostly, ductings are extendable tubes made of synthetic or steel sheets. Their cross-section amounts to 1/60 to 1/30 of the tunnel cross-section. The required amount for fresh air is 20 m3/mint/person.

3.2 Service Ventilation

The ventilation is intended to guard against pollution, guarantee visibility and, in case of fire, secure the escape routes, ensure the entrance of rescue teams and reduce damage. The required fresh air supply should be calculated in accordance with the anticipated traffic flow to guarantee that the following concentrations are not exceeded:

CO concentration < 100 ppm
NOx concentration < 25 ppm
Opacity: extinction coefficient< 7 x10-3 m-1
Air velocity (averaged over the cross section) < 10 m/s
Limitations on the concentration of pollutant inside a tunnel

In rail tunnels (in particular metros), cooling, i.e., the removal of warm air (e.g., due to locomotives), is also provided through ventilation.

Road traffic produces the maximum pollution at speeds of 10 to 15 km/h. The increase in the number of vehicles equipped with catalyzers results in a considerable reduction of fresh air requirements of road tunnels, which, depending on the ratio of trucks and on the slope, amounts to between 30 and 50%.

Four types of ventilation techniques are used worldwide for ventilation of tunnel after construction; these techniques are discussed below.

3.2.1 Natural Ventilation

Natural ventilation is accomplished by the pressure difference between the portals and also by the piston action of the vehicles.

3.2.2 Fans

Fans should be provided at a spacing of ten times the tunnel diameters and produce longitudinal ventilation. The blowing direction can be reversible. Large ventilators achieve a higher thrust related to installation costs.

Longitudinal ventilation: Fans
Fans installed inside a tunnel for ventilation

3.2.3 Semi-Transverse Ventilation

Fresh air is supplied from special pathways perpendicular to the tunnel’s longitudinal axis, whereas the polluted air escapes from the portals. This is appropriate for tunnels with 2 to 4 km length and medium traffic load. Alternatively, the used air is extracted through special ducts, while fresh air enters through the portals. Thus, the worst air quality is found in the middle of the tunnel. The air velocity reaches its maximum near the portals.

3.2.4 Transverse Ventilation

Fresh air is introduced, and polluted air is extracted perpendicular to the tunnel axis. Mostly, the ducts having a cross-section area up to 25 m2 are used for transverse ventilation. The ducts are designed from an aerodynamics point of view, while the area of duct is considered from a maintenance point of view.

Long tunnels may require ventilation shafts or ventilation adits that are driven parallel to the tunnel. The ventilation system shown in Table-1 is a standard ventilation system, which is developed according to the international standards for the equipment and service of road tunnels. However, if the tunnel is located in a location where the air is more polluted, then in addition to Table-1, electrostatic filters should be used.

A vertical shaft provided for transverse ventilation of tunnel
Transverse ventilation of a tunnel
Tunnel length in kmTunnel length in km 
Bidirectional single tubeOne-directional two tubesVentilation
< 0.4< 0.7natural ventilation with CO-warning
< 2< 4fans
< 4< 6fans and ventilation shaft
< 0.5< 2reversible
< 1< 2semi-transverse ventilation
< 2< 6transverse ventilation
Table-1: Ventilation system used in tunneling

4. Fire Protection

Fire combat measures should exist actively and passively. Active measures aim at the extinction of fire by means of fire detectors, fire extinguishers, sprinklers, emergency ventilation, and telecommunication devices.

Passive measures aim at minimizing the damage, for e.g., by means of fire-resistant concrete, synthetic materials which do not produce toxic gases when burning, safe electric cables placed below the carriageway, transverse drain pipes that collect the leaking fuel, use of materials of low porosity, and also clear signals indicating the escape routes.

Active measures for fire protection of a tunnel
Fire extinguishers and sprinklers installed inside a tunnel

5. Illumination of Road Tunnels

Road tunnels must be sufficiently illuminated. The luminance is gradually reduced from the portal towards the interior of the tunnel. Generally, a tunnel is divided into three main sections with respect to illumination.

The first two are the entrance and transition sections. The length of the entrance section is the length for the vehicle to stop. In the transition section, the luminance is gradually reduced. Its length depends on the following quantities:

  1. Luminance at the end of the entrance section
  2. Luminance in the inner section
  3. Adaptation of the eye to changing illumination conditions

The fundamental luminance applies to the remaining tunnel section. Luminance should be 3 cd/m2, where a reduction of 30% takes dirt and aging into account (unit of luminance is cd/m2). The luminance should be uniform within the tunnel.

Installation of solar panels for tunnel illumination
Illumination of road tunnels

To facilitate the adaption of the eye, the portals should be as dark as possible. Therefore, galleries and illumination reduction constructions should be used, if affordable. To achieve maximum illumination and to enable easy cleaning, the tunnel inner walls should be covered with a bright and reflecting, but not dazzling coating, which should be cleaned at regular intervals.

A good solution is to mount enamel panels on the tunnel wall. The objectives of interior finish are:

  1. Identification of alignment by means of the difference in luminance between walls and road surface
  2. Enabling drivers to assess the distance from the walls
  3. To hide wires and pipes that divert the attention of the drivers

During power blackouts, the emergency call niches and some intermediate points, as well as the cross-overs, should be illuminated.

6. Drainage

The drainage is a very important element of a tunnel. When groundwater interacts with the tunnel, it becomes important to collect and divert the water to maintain the workability of a tunnel. The following waters should be collected and diverted:

  1. Groundwater (for ecological reasons, the withdrawal of groundwater should be as low as possible)
  2. Day water (precipitation or melting ice entering from the portals)
  3. Service water (e.g., from washing)

Contrary to the mixed system, where all types of water are put together, groundwater, on one hand, precipitation water (entering from the portals) and service water (used for maintenance or firefighting) on the other hand are withdrawn in separate pipes called as separate system (dual drainage system). Provision for non-propagation of fire should be taken for the case of effusion of inflammable fluids.

Longitudinal drains are installed on the sides of the tunnel. Their diameter depends on the slope (at least 0.5%) and on the pipe material and should be at least 15 cm. Cleaning and flushing shafts should be provided with a spacing of 50-65 m. Water is removed from the carriageway by means of longitudinal drains with a diameter ≥ 20 cm, slope ≥ 0.5%, and cleaning shafts with spacing of, e.g., 110 m. A possible aggressivity of the water, as well as the danger of freezing, should be taken into account.

FAQs

What is the purpose of mounting enamel panels on a tunnel wall?

The objectives of mounting enamel panels on the tunnel wall are:
1. Identification of alignment by means of the difference in luminance between walls and road surface
2. Enabling drivers to assess the distance from the walls
3. To hide wires and pipes that divert the attention of the drivers

What are the active and passive measures of fire protection in a tunnel?

Active measures aim at the extinction of the fire by means of fire detectors, fire extinguishers, sprinklers, emergency ventilation, and telecommunication devices. Whereas passive measures aim at minimizing the damage, e.g. by means of fire-resistant concrete, by the use of synthetic materials, etc.

What are the types of tunnel ventilation?

Types of tunnel ventilation are mentioned below:
1. Natural ventilation
2. Fans
3. Semi-transverse ventilation
4. Transverse ventilation

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