Road Tunnels Manual

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1.2. General design of the tunnel (new tunnel)

Section 1.2 relates to the design of new tunnels. The design concerning the refurbishment and the safety upgrading of tunnels under operation is presented in section Renovation - upgrading of existing tunnels.

1.2.1 Horizontal and vertical alignment

The design of the horizontal and vertical alignment of a road or highway section, which includes a tunnel, constitutes a major and fundamental first stage in the creation of a new tunnel, to which the necessary attention is seldom given.

The consideration of the "complex system" which constitutes a tunnel has to start at the early stage of the design of the general alignment, which is seldom the case. It is however at this stage that technical and financial optimisations are the most important.

It is essential to mobilise from the earliest stage of the design a multidisciplinary team made up of very experienced specialists and designers, who will be able to identify all the project's potential problems, despite inevitably incomplete preliminary information. This team will be able to make good and reliable decisions for the major choices, and then consolidate these elements progressively taking into account the availability of additional information.

The objective of this section is not to define the rules regarding tunnel layout design (several countries' design handbooks are referred to in section Regulations - Recommendations) but essentially to sensitise the owners and the designers to the necessity of a global and multicultural approach, from the early stages of the design, and to the importance of essential experience that is paramount to the success of the project.

1.2.1.1 Countries without "tunnel culture"

In these countries owners and designers have a certain apprehension about tunnels. They very often prefer "acrobatic road layouts" passing along ridges, with steep gradients, huge retaining walls or very long viaducts, and sometimes tremendous consolidation works (which are very expensive and not always effective over a long period of time), in order to cross zones with active landslides.

Numerous examples of projects including tunnels and alignment variations designed with a global “system” approach demonstrate, in comparison with approaches refusing systematically the construction of tunnels:

  • construction cost savings may reach between 10% and 25% in areas with mountainous conditions,
  • important savings of operation and maintenance costs can be achieved. The reliability of the route can be improved, in particular in zones of instability or active landslides, or subject to severe climatic conditions,
  • environmental impact is significantly reduced,
  • the level of service for the users is improved, and the operating conditions, in particular in winter (in countries subject to snowfall) are made reliable by the reduction of the gradients required by passages along ridges.

The assistance of an external assessor makes it possible to mitigate the insufficiency or the lack of "tunnel culture", and to improve the project significantly.

1.2.1.2 Countries having a tradition of construction and operation of tunnels

The concept of "complex system" is seldom integrated upstream, to the detriment of the global optimisation of the project. Too often the "geometry" of the new infrastructure is fixed by layout specialists without any integration of the whole of the constraints and tunnel components.

It is however essential to take into account from this stage all the parameters and interfaces described in paragraph 1.1 above, and in particular:

  • the general geology and hydrogeology of the area (with the available level of knowledge) as well as preliminary appreciation of the geological difficulties and the potential risks concerning the methods, costs and construction duration,
  • the potential geomechanical, hydro-geological, hydrographical conditions at the tunnel portals and along the accesses,
  • the risks and hazards related to winter conditions for countries subjected to noteworthy snowfall, in particular:
    • the risks of avalanche or formation of snow-drifts and the possibilities of protecting the access roads and the portals against these risks,
    • the maintenance conditions of access roads in case of significant snowfalls to guarantee the reliability of the route. This provision may condition the altitude of the tunnel portals, the maximum slopes of the access roads, and if necessary the place available to arrange surfaces for chaining and unchaining in the vicinity of the portals,
  • the environmental conditions at the tunnel portals and on the access roads. The impact can be significant in urban environments (in particular because of the noise and the discharge of polluted air), as well as for interurban tunnels,
  • the gradient of the approach ramps:
    • the least expensive tunnel is not always the shortest tunnel,
    • the suppression of a special lane for slow vehicles is difficult to manage in the vicinity of a tunnel portal, and keeping such a lane in a tunnel is generally very expensive,
    • the gradient of the access roads can have a very strong impact on the capacity of the route in terms of traffic volume and winter reliability.
  • the possibility of incorporating adits as lateral accesses (ventilation - evacuation and safety - reduction of the construction works schedule), or as vertical or inclined shafts (ventilation - evacuation and safety),
    • these particular access points, their impact on the surface (in particular in urban environments: available space - sensitivity to the discharge of polluted air - etc), their year-round accessibility (e.g., exposure to avalanches) may constitute important constraints for the design of the horizontal and vertical alignment. Conversely they very often contribute to the optimisation of the construction and operation costs,
    • these particular access points may have a major impact on the construction and operation costs, and on the size of the cross section (potential optimisation of the ventilation and the evacuation facilities),
  • the methods of construction which may have a major impact on the design of the horizontal and vertical alignment, for example:
    • crossing under a river with a bored tunnel constitutes an essentially different project to that of a solution by immersed prefabricated boxes,
    • interfaces with a viaduct at the tunnel portal,
    • the imposed construction deadline may have a direct impact on the layout, in particular to allow driving from both tunnel portals as well as intermediate drives, using adits,
  • the geometrical characteristics of the layout and the longitudinal profile of the tunnel for which it is also necessary to integrate the following elements:
    • limitation of gradients, which have a major impact on the sizing of the ventilation system and on the reduction of the traffic volume capacity of the tunnel,
    • the hydraulic conditions of underground drainage during the construction and the operation period, which affect the vertical alignment,
    • reduced lateral clearance (construction of additional widths is very expensive) which require particular analysis of the visibility conditions and particular vigilance in the choice of the radii of the curves for the horizontal alignment,
    • the best choice of the radii in order to avoid alternating cross-fall slopes, and their major impact on water collecting and drainage systems from the carriageways, interfaces with sleeves for the installation of cables, water pipes for fire fighting, which can even lead to an increase in the dimension of the cross section,
  • all usual constraints related to the occupation of the underground space, in particular in urban environments: subways - car parks - foundations - structures sensitive to settlements,
  • construction and operation costs:
    • the least expensive tunnel is not necessarily the shortest one,
    • an additional investment in civil engineering can be overall more economic over the tunnel lifetime if it enables a reduction of the costs for construction, operation, maintenance and heavy repairs (in particular ventilation), or if it makes it possible to postpone for several years the date of traffic capacity saturation (impact of the gradient in the tunnel and on the accesses),
  • the coordination between the horizontal and the vertical alignments must be carefully studied in a tunnel in order to support the level of comfort and safety of the users. The visual effect of the changes of slopes in the vertical alignment, in particular in high points, is highlighted by the limited visual field of the tunnel and by the lighting,
  • the conditions of operating with uni- or bidirectional traffic have to be taken into account in the design of the layout, in particular:
    • the usual conditions of visibility and legibility,
    • the possibility of arranging lateral accesses (adits) or vertical accesses (shafts), in particular for: optimisation of ventilation and the cross-section, improvement of safety (evacuation of the users and access of the emergency teams by avoiding the construction of an expensive parallel gallery),
  • the layout in the vicinity of the portals:
    • the tunnel portals constitute singular points of transition, and it is necessary to take into account human behaviour and the physiological conditions. It is essential to preserve a geometrical continuity to make it possible for the user to preserve his instinctive trajectory,
    • a rectilinear tunnel is not desirable, in particular along the approach of the exit portal. It may be necessary to reinforce the exit lighting over a long distance,
  • underground junctions at or very close to the tunnel portals:
    • interchanges inside a tunnel or outside in the immediate vicinity of the portals are to be avoided,
    • if they are unavoidable, a very detailed analysis must be made to determine all the constraints and particular consequences to be taken into account (layout - cross-section - exit or merging lanes - risk of backward traffic flow - evacuation - ventilation - lighting - etc) to ensure safety in all circumstances.

1.2.2 The functional transverse profile

1.2.2.1 The issues

The functional transverse profile constitutes the second major stage of the design of a tunnel after selecting the alignment. As for the first stage, the "complex system" approach must be taken into account in a very attentive way, as upstream as possible with an experienced multidisciplinary team. All of the parameters and interfaces described in section Tunnel is a complex system must be considered.

This second stage (functional transverse profile) is not independent of the first stage (alignment), and it must obviously take into account the resulting provisions. The two stages are interdependent and very closely linked together.

Moreover, as mentioned in paragraph 1.1.2.2 above, the process of the first two stages is iterative and interactive. There is no direct mathematical approach to bring a single response to the "complex system" analysis. There is also no uniqueness of answer but a very limited number of good answers and a great number of bad answers. The experience of the multidisciplinary team is essential for a good solution to be identified quickly.

The examples quoted in paragraph 1.2.1 above illustrate that the provisions of the "functional transverse profile" can have a major impact on the design of the horizontal and vertical alignments.

Experience shows that the analysis of the "functional transverse profile" is very often incomplete and limited to the sole provisions of civil engineering, which leads inevitably to:

  • in the best case, a project that is not optimised from the functional, operational and financial points of view. Experience shows that potential optimisations can reach in exceptional cases 20% of the construction costs,
  • in the most frequent case, an inadequate consideration of the functions, their constraints and their impacts on the project. These functions will have to be integrated in the following stages of the project by implementing late and often very expensive solutions,
  • in the worst case, fundamental design errors with an irremediable and permanent impact on the tunnel, on its conditions of operation and safety, as well as on its construction and operation costs.

1.2.2.2 Principal provisions

The major parameters of the "functional transverse profile" are as follows:

  • Traffic volume - nature of the traffic - operation organisation - urban or non-urban tunnel, in order to determine:

    • the number and width of the lanes, according to the traffic and the type of vehicles admitted to the tunnel,
    • the headroom (according to the type of vehicle),
    • the hard shoulder, emergency stopping lane or lay-by, according to the volume of traffic, the mode of operation, i.e. uni- or bidirectional, the statistical rate of breakdowns,
    • a possible central separator and its width in the event of bidirectional operation,
  • Ventilation has a major impact which depends on:

    • the selected system of ventilation, itself depending on many other parameters (see section Ventilation),
    • the space required for the ventilation ducts, for the installation of axial fans, jet fans, secondary ducts, and all the other ventilation equipment,
  • Evacuation of the users and the access of the emergency and rescue teams which depend on the numerous factors detailed in chapter Structural facilities related to Operation and Safety,
  • The length and the gradient of the tunnel. These parameters intervene in an indirect way through the ventilation, the concepts of access and safety,
  • The networks and equipment for operation are also very often determining factors in the dimensioning of the functional cross section, taking into account their number, the space they require, the essential protection associated with them to guarantee the operational safety of the tunnel, and the relatively limited space under the walkways and hard shoulders to locate them. The following networks are in particular concerned, which have a dimensional impact:
    • separated or combined sewer system(s) - collection of polluted liquids from the roadways and associated siphons. The absence of variation in the crossfall, associated with the conditions of the alignment (see § 1.2.1.2) allow a simplification and an optimisation of the functional transversal profile,
    • water supply network for the fire fighting system, fire hydrants, and if necessary their protection against freezing,
    • all networks of cables of high and medium voltage, as well as low voltage currents. It is essential to take into account on the one hand, the cables necessary at the time of the tunnel opening and their protection against fire, as well as the provisions allowing their partial or total replacement, and on the other hand the provisionsfor the inevitable addition of other networks throughout the tunnel's life,
    • the particular needs in the short or medium term for external networks likely to pass through the tunnel,
    • all interactions between networks and needs (technical or legal) for spacing between some networks,
    • all of the signalling for operation: signalling and signage - lane signals - panels with variable messages - regulation indications - safety indications - directional indications,
  • Localised functional interfaces: underground sub-stations - underground ventilation plant - safety recesses - shelters - etc. It is essential to take into account the provisions for operation and the maintenance, and in particular the construction of lay-bys for maintenance interventions and the safety of the operating teams,
  • Construction methods and geological conditions have an impact on the functional transverse section (independently of the dimensioning of the civil engineering structures), for example:
    • the underwater crossing mentioned in section 1.2.1.2 above. The solution with immersed precast boxes enables a very different design and arrangement of the ventilation system, the evacuation galleries or the access of the emergency teams, in comparison with the arrangement for the same equipment in the case of a bored tunnel,
    • a tunnel bored with a TBM (tunnel boring machine) makes surfaces available under the roadway which can be used for example for ventilation, for the users' evacuation, or for the access of the emergency services. This can allow optimisations (removal of connection galleries or a parallel gallery) which can be financially very important if the tunnel is located under groundwater level in permeable materials.

1.2.3 Safety and Operation

1.2.3.1 General provisions

PIARC's recommendations are numerous in the fields of safety and operation for the finalisation of safety studies, the organisation of operation and emergencies, as well as the provisions for operation. The reader is invited to refer to theme : see chapter Safety and Chapter Human factors regarding tunnel safety.

This present chapter primarily treats safety and operation interfaces within the "complex system". The tables of section 1.1.5.2 above indicate the degree of interdependence of each parameter compared to the various subsets of the project.

A certain number of parameters have a major impact from the upstream stages of the project onward. They must be analysed from the first phases of the design and deal in particular with:

  • volume of traffic - nature of the traffic (urban, non urban) - nature of vehicles (possibly tunnel dedicated to one category of vehicles) - transport or not of dangerous goods,
  • evacuation of the users and access of the emergency teams,
  • ventilation,
  • communication with the users - supervision system.

These major parameters for the design of the tunnel are also the essential factors of the "hazard analysis", and drafts of the "intervention plan of the emergency teams". This is why we consider that it is essential that a "preliminary risk analysis", associated with a preliminary analysis of an "emergency response plan" should be carried out in the initial stages of the preliminary design. This analysis makes it possible to better describe the specific features of the tunnel and the functional and safety specifications which it must satisfy. It also contributes to a value engineering analysis, to a better design and to the technical and financial improvement and optimisation.

These parameters and their impacts are detailed in the following paragraphs

1.2.3.2 Parameters relating to the traffic and its nature

These parameters have an impact mainly on the functional cross-sectional profile (See 1.2.2), and through it a partial impact on the layout:

  • the volume of traffic affects the number of lanes, ventilation and evacuation. It also affects the impact of breakdown vehicles and their management when stopped: requirement for a lateral stopping lane or not, for lay-bys, and organisation of particular provisions for repair service,
  • the nature of the traffic, the type of vehicles and their distribution affect the evacuation concept (cross-passages, evacuation galleries, their dimensioning, their spacing) according to the volume of people to be evacuated,
  • tunnels dedicated to particular categories of vehicle relate to the width of the lanes, headroom and ventilation,
  • the passage or not of dangerous goods has an important impact on the ventilation system, the "functional cross-section", fluid collection and dewatering measures, diversion routes, the environment of the tunnel portals or ventilation stacks, the protection of the structures against the consequences of a major fire, as well as on evacuation and the organisation of the emergency services and the provision of the fire brigade in specific means and material.

1.2.3.3 Evacuation of the users - access of the emergency teams

This is a fundamental parameter concerning the functional provisions and the general design. This parameter also often affects the alignment (direct exits to outside) and construction provisions: cross-passages - under gallery - parallel gallery - shelters or temporary refuges connected to a gallery.

Its analysis requires an integrated approach with the ventilation design (in particular the ventilation in case of fire), volume of traffic, risk analysis, drafting of the emergency response plan (in particular investigation of the scenarios ventilation / intervention) and construction methods.

It is necessary from a functional point of view to define the routes, their geometrical characteristics and spacing in order to ensure the flow of able-bodied and disabled people.

It is essential to insure the homogeneity, the legibility and the welcoming and calming character of these facilities. They are used by people in situations of stress (accident - fire), at the self-rescue stage (before the arrival of the emergency services). Their use has to offer a natural, simple, efficient and calming character in order to avoid the transformation of the state of stress into a state of panic.

1.2.3.4 Ventilation

Ventilation facilities designed as a pure "longitudinal ventilation" system have little impact on the "functional cross section" or on the "alignment".

This is not the case for "longitudinal ventilation" facilities equipped with a smoke extraction duct, or for "transverse ventilation" systems, "semi-transverse" or "semi-longitudinal" systems, "mixed" systems, or for ventilation systems including shafts or intermediate galleries permitting to draw or to discharge air outside other than at the tunnel portals. All these facilities have a very important impact on the "functional cross section", the "alignment" and all the additional underground structures.

The ventilation facilities of the traffic space are essentially designed in order to :

  • provide healthy conditions inside the tunnel by the dilution of air pollution in order to keep the concentrations to a level lower than those required by the recommendations of national regulations,
  • ensure the safety of the users in case of fire inside the tunnel, until their evacuation outside of the traffic space, by providing efficient smoke extraction,

The ventilation facilities may also provide additional functions:

  • limitation of air pollution at the tunnel portals, by improved dispersal of the polluted air, or by cleaning the air prior to its discharge outside the tunnel,
  • underground plants for cleaning the polluted air in order to reuse it within the tunnel. These facilities exist in urban tunnels or in very long non-urban tunnels. They are complex and expensive technologies, requiring a lot of space and considerable maintenance,
  • in case of fire, to contribute to limiting the temperature inside the tunnel in order to reduce the deterioration of the structure by thermal effects.

The ventilation facilities do not only concern the traffic space. They also concern:

  • the connection galleries between the tubes,
  • the evacuation galleries or the shelters used by the users in case of fire,
  • the technical rooms or plants situated inside the tunnel or outside near the tunnel portals that may require air renewal, or management and control of the temperature level (air heating or conditioning according to the geographical conditions).

The ventilation facilities have to be designed in order to be able to:

  • adapt in a dynamic and fast way to the numerous conditions and capacities in which they are operated in order to face :
    • climatic constraints, in particular significant and fluctuating differentials of pressure between the portals for long tunnels in mountainous areas,
    • variable operating rates for smoke management in case of fire, according in particular to the development of the fire, then its regression, as well as throughout the fire period in order to be suited to the evolution of the fire fighting strategies at each stage of evacuation, of fire fighting, of preservation of the structures, etc.
  • present enough development capacity in order to be able to adapt throughout the tunnel's life to the evolution of the traffic (volume - nature), lowering of the admissible pollution levels and various conditions of operation.

1.2.3.5 - Communication with the users – supervision

Communication with users has an important impact on the "functional transverse profile" through signalling.

The other major impacts do not relate to the whole of the "complex system". They relate to the subsystem concerning the operating equipment, in particular remote monitoring, detection, communications, traffic management, control and supervision, as well as the organisation of evacuation.

1.2.3.6 - Particular requirements for operation

The operation of a tunnel and the intervention of the maintenance teams may require particular arrangements in order to enable interventions under full safety conditions, and to reduce restrictions to the traffic.

These arrangements concern for example the provision of lay-bys in front of the underground facilities requiring regular maintenance interventions, accessibility to materials for their replacement and maintenance (in particular heavy or cumbersome material).

1.2.4 The operating equipment

The objective of this section is not to describe in detail operation facilities and equipment, their function or their design. These elements are defined in the recommendations of the current "Road Tunnels Manual", as well as in the handbooks or national recommendations listed in section 1.6 below.

The objective is to draw the attention of owners and designers to the particular issues peculiar to the equipment and the facilities of tunnel operation.

1.2.4.1 Strategic choices

The operating equipment must allow the tunnel to fill its function, which is to ensure the passage of traffic, and to satisfy the double mission of providing for the users a good level of comfort and safety when crossing the tunnel.

The operation facilities must be suited to the function of the tunnel, its geographical location, its intrinsic features, the nature of the traffic, the infrastructures downstream and upstream of the tunnel, the major issues relating to safety and to emergency organisation, as well as the regulation and the cultural and socioeconomic environment of the country in which the tunnel is situated.

A plethora of operation facilities does not automatically contribute to the improvement of the level of service, comfort and safety of a tunnel. It requires increased maintenance and human intervention, which, if not implemented, may lead to a reduction in the reliability of the tunnel and its level of safety. The juxtaposition or the abuse of gadgets is also useless. The facilities must be suited, complementary, sometimes redundant (for the essential functions of safety), and have to form a coherent whole.

The facilities of operation are "living":

  • They require a rigorous care and maintenance regime, recurrent and suited to their level of technology. This maintenance has a cost and requires skilled human resources, as well as recurrent financial investment throughout the tunnel's life. Lack of maintenance (or insufficient maintenance) leads to major dysfunctions, to the failing of the facilities, and as a consequence to the calling into question of the tunnel's function and the users' safety. Maintenance of the facilities under traffic conditions is often difficult and very restricted. Arrangements must be considered from the design of the facilities. For this reason the "architecture" of the systems, their design and their installation have to be thought out in order to limit the impact of the dysfunctions on the availability and the safety of the tunnel, as well as the impact of the maintenance interventions or the renovation of the facilities,
  • Their "life span" is variable: about ten to thirty years according to their nature, their hardiness, the conditions to which they are exposed, as well as the organisation and the quality of the maintenance. They must therefore be replaced regularly, which requires adequate financing (see technical reports 2012R14EN "Life cycle aspects of electrical road tunnel equipment" and 2016R01EN "Best practice for life cycle analysis for tunnel equipment"),
  • Technological evolution often makes essential the replacement of facilities that include advanced technologies, because of technological obsolescence and the impossibility of obtaining spare parts,
  • The facilities must show evidence of adaptability to take into account the evolution of the tunnel and its environment.

All these considerations lead to strategic choices of which the main ones are:

  • To define the necessary facilities according to the real needs of the tunnel, without yielding to the temptation of accumulating gadgets. Risk analysis combined with value engineering is a powerful tool allowing the rationality of the choice of the necessary facilities. This approach also allows to better master the complexity of the systems, that is often a source of delays, cost over-runs and major dysfunctions if this complexity has not been managed by a rigorous and competent organisation,
  • To give priority to the quality and the hardiness of the equipment in order to reduce the need and frequency of maintenance and the difficulties of intervention under traffic conditions. This can result in a higher investment cost but is compensated very extensively during the operation period,
  • To verify the quality and the performance of the facilities at each stage of the design, manufacture, factory acceptance tests, installation on site and then site acceptance tests. Experience shows that numerous facilities are deficient and do not satisfy the objectives because of lack of rigorous organisation and efficient controls,
  • To choose technologies suitable to the climatic and environmental conditions, which the facilities will have to face, as well as to the socio-cultural conditions (deficiency of the maintenance concept in some countries), and to technological and technical conditions, as well as to the organisation of the services,
  • To take into account, from the design of the facilities and the choice of the equipment, the operation costs and in particular energy costs. These costs are recurrent throughout the tunnel's life. Ventilation and lighting facilities are in general the highest consumers of energy. Particular attention must be drawn to this aspect from the preliminary design stages,
  • To take into account from the preliminary stages of design and financing analysis:
    • the necessity to implement, to organise, to learn and to train teams dedicated to operation and intervention on the one hand, and on the other hand to cleaning and maintenance,
    • the constraints of intervention under traffic conditions for maintenance,resulting operation, maintenance and refurbishment costs,
  • To take into account in the general organisation and scheduling of a new tunnel project, the time required to recruit the teams and to train them, for tests, as well as the "dry run" of all the facilities and systems (period of 2 to 3 months), for practices and manoeuvres on site with all the external intervening parties (in particular emergency services - fire brigade) in order to familiarise them with the particularities of the tunnel.

1.2.4.2 - Key recommendations concerning the main facilities

1.2.4.2.a Energy - sources of power - electric distribution

For the tunnel equipment to function there must be a power sources. Large tunnels can require a power of several MW (megawatts), which may not always be available on site. Particular arrangements must be taken from the first stages of the design in order to strengthen and make more reliable the existing networks, or often to create new networks. The power supply is essential for the operation of the tunnel. It is also essential for its construction.

The supply of electric energy and its distribution inside the tunnel must provide:

  • the required capacity,
  • a reliable supply,
  • a reliable, redundant and protected energy distribution system: redundancy and interconnectionof the distribution networks - transformers in parallel - cables located inside sleeves and in manholes protected against the fire.

Every tunnel is specific and has to be subjected to a specific analysis according to its geographical position, the context of the existing electrical networks, the energy supply conditions (priority or not priority), the possibility of increasing or not the power and the reliability of the existing public networks, the risks peculiar to the tunnel, as well as the conditions of intervention of the emergency services.

The facilities must be then designed consequently, and the operating procedures must be implemented according to the reliability of the system and the choices that have been taken during the design period.

The objectives concerning safety, in case of a power supply cut are:

  • immediate emergency supply without interruption of all of the following safety equipment during a period of about half to one hour (according to the tunnel and the evacuation conditions) :
    • minimal lighting level - signalling - CCTV monitoring - telecommunications - data transmission and SCADA - sensors and various detectors (pollution - fire - incidents - etc.),
    • power supplies to safety niches, evacuation routes and shelters,
    • this function is usually ensured by UPS systems, or diesel generators immediately able to supply energy,
  • varying from tunnel to tunnel, its urban or rural location and the risks incurred, additional objectives of MOC (Minimal Operation Conditions) can be set to assure the electrical supply of the following equipment, as long as specific procedures are implemented during the whole duration of the power cut. For example: emergency power supply of the ventilation system (by generators or a partial external supply) permitting the tackling of light vehicle fires, but not truck fires: the passage of trucks is then temporarily forbidden.

The arrangements usually implemented for the electrical power supply are as follows:

  • Emergency power supply from the public network:
    • 2 to possibly 3 supplies from the public network grid with connections to independent segments of the high voltage or middle voltage network. Automatic switching between "normal supply" and "emergency supply" inside the tunnel power substation with, if required, interruption of the power supply to some of the equipment, if the emergency external power supply is insufficient,
    • no diesel generators,
    • installation of a UPS emergency power supply.
  • No external emergency power supply:
    • a single external power supply from the public network,
    • diesel generators able to provide a part of the power in case of interruption of the main external power supply, and setting up of MOC and particular operating procedures,
    • installation of a UPS emergency power supply.
  • Full autonomy of the power supply - no external power supply available:
    • the public network is not able to provide the required power, or does not have the required reliability. The tunnel is then in complete autonomy. The energy is entirely provided by a set of diesel generators working simultaneously. An additional generator is installed as "back up" in case one of the generators should fail,
    • possible installation of a UPS emergency power supply, if the level of reliability of the generators is considered insufficient, or for safety reasons.

1.2.4.2.b Ventilation

PIARC recommendations are numerous in this field and constitute the essential international references for the conception and the design of ventilation facilities.In addition to section 1.2.3.4 above, the reader should refer to section Ventilation.

However, it must be remembered that even if the ventilation equipment constitutes one of the essential facilities in assuring the health, comfort and safety of the users in a tunnel, it is only one of the links of the system, in which the users, the operators and the emergency and rescue teams constitute the most important elements by their behaviour, their expertise and their capacity to act.

The ventilation facilities alone cannot deal with all scenarios, nor satisfy all the functions that might be assumed, especially concerning air cleaning and the protection of the environment.

The relevance of the choice of a ventilation system and its dimensioning requires lengthy experience, the understanding of the complex phenomena of fluid mechanics in an enclosed environment, associated with the successive stages of the development of a fire, the propagation, radiation and thermal exchanges, as well as the development and the propagation of toxic gases and smoke.

The ventilation facilities are in general energy-consuming and particular attention must be paid to the optimisation of their dimensioning and their operation, by using for example expert systems.

The ventilation facilities may be very complex, and their relevant management in case of fire may require the implementation of automated systems that allow to manage and master the situation more efficiently than an operator under stress.

As indicated in section 1.2.3.4 above, the ventilation facilities must above all satisfy the requirements for health and hygiene during normal conditions of operation, and to the objectives of safety in case of fire.

Hardiness, reliability, adaptability, longevity and optimisation of energy consumption constitute major quality criteria that the ventilation facilities must satisfy.

1.2.4.2.c Additional equipment to the ventilation facilities

Two types of additional equipment for ventilation are often the subject of pressing demands from stakeholders, resident associations or lobbies:

  • Air treatment or air cleaning facilities,
  • Fixed fire suppression systems.

A. Air cleaning facilities.

Section Tunnel impact on outside air quality deals with this question and the reader is invited to refer to it.

The implementation of air cleaning facilities is a recurrent demand of resident protection associations in urban areas. These facilities, usually installed underground, are very expensive to construct as well as to operate and maintain. They are also high consumers of energy.

Results to date are far from convincing, due in particular to important emission reductions from the vehicles, and to the difficulty for these systems to clean the very low concentrations of pollutants that are in the tunnel, contained in large volumes of air. Consequently, many systems installed in the last ten years are no longer operational.

The future of air cleaning facilities is very uncertain in countries where there is more coercive regulation, imposing more and more rigorous reductions of polluting emissions at the source.

B. Fixed fire suppression system (FFSS).

Section Fixed Fire Fighting Systems deals with this issue, and the reader is invited to refer to it.

The technologies are numerous and answer to varied criteria: fire fighting - containment of the fire - reduction of thermal radiation and temperature for the users situated in the vicinity of the fire - preservation of the tunnel structure against damage due to high temperature, etc.

These systems, even though presenting positive aspects, also present negative aspects related in particular to the deterioration of the conditions of visibility if they are activated from the start of the fire. The use of an FFSS requires a coherent approach to all aspects of the users' safety, as well as to ventilation and evacuation strategy.

The decision concerning the implementation or not of such systems is complex and has important consequences. It must be subject to a thorough reflection relating to the particular conditions of safety of the work concerned and to the added value obtained by the implementation of the system. It should not be taken under the influence of fashion or a lobby.

The FFSS requires the implementation of important maintenance measures, the carrying out of regular and frequent tests, without which its reliability cannot be assured.

1.2.4.2.d Lighting

The recommendations of the CIE (International Commission for Lighting) have been criticised by PIARC because of the high levels of lighting to which they often lead. The reader is invited to refer to the technical report published by the CEN (European Committee of Standardization) that presents several methods including the CIE's.

Lighting is a fundamental tool to ensure the comfort and safety of the users in a tunnel. The objectives of the lighting level must be adapted to the geographical location of the tunnel (urban or not), its features (short or very long), to the volume and nature of the traffic.

Lighting equipment consumes a lot of power and developments are in progress to optimise their features and performance.

1.2.4.2.e Data transmission - Supervision - SCADA

SCADA is the "nervous system" and the "brain" of the tunnel, permitting the compilation, transmission and treatment of information, and then the transmission of the equipment's operating instructions.

This system requires a meticulous analysis according to the specific conditions inside the tunnel, its facilities, the organisation and the mode of operation, the context of risks in which the tunnel is placed, as well as the arrangements and procedures implemented for interventions.

The organisation of the supervision and control centre has to be analysed very carefully, according to the specific context of the tunnel (or of the group of tunnels), the necessary human and material means, the missions to be assumed, the essential aid brought by the automatic devices or the expert systems to the operators in event of an incident, allowing the operators to reduce and simplify their tasks and to make them more efficient.

The detailed design of these systems is long, delicate and requires a very rigorous methodology of developing, of controlling by successive stages (in particular during factory tests), of testing, of globally controlling after integration of all the systems on site. Experience shows that the numerous errors noted on these systems come from the following gaps:

  • badly defined specifications, insufficient functional analysis, or ignorance of operational conditions and procedures,
  • late systems development, which does not allow the time necessary for detailed analyses, transverse integration, or to take into account the peculiar conditions of operation of the tunnel,
  • lack of rigour in the development, testing, control and integration of all of the systems,
  • lack of taking into account human behaviour and general ergonomics,
  • lack of experience in tunnel operation, in the hierarchy of the decisions that are to be integrated and the logical sequences of these decisions in the event of a serious incident.

Section Supervisory Control And Data Acquisition systems (SCADA) of the manual sums up these different aspects.

1.2.4.2.f Radio-communications - low voltage circuits

These facilities include:

  • emergency phone network,
  • radio network for the operation teams and the emergency services. Radio channels for tunnel users, through which it is possible to transmit information and instructions related to safety,
  • numerous sensors destined for taking measurements and detection,
  • CCTV network.
  • an AID system (Automatic Incident Detection) is usually associated with a CCTV system. The AID system requires an increased number of cameras in order to make detection more reliable and more relevant.

1.2.4.2.g Signalling

Signalling refers to section Signposting.

Even more than for the other facilities, an overabundance of signalling is detrimental to its relevance and objectives.

The legibility, the consistency, the homogeneity and the hierarchy of signalling (priority to evacuation signalling and information for users) have to be a priority of the signalling design inside the tunnel and on its approaches.

Fixed signage panels, traffic lanes signals, variable messages signals, traffic lights and stopping lights, signalling to emergency exits, the specific signalling of these exits, signalling of safety niches, physical devices for closing the lanes (removable barriers),horizontal markings and horizontal rumble strips are all part of the signalling devices. They assure a part of the communication with the users.

1.2.4.2.h Devices for fire fighting

The devices for fire detection are either localised (detection of fire in the underground substations or the technical rooms), or linear (thermal sensing cable) inside the traffic space.

There are various devices for fire fighting:

  • automatic facilities in the technical rooms and underground substations,
  • powder extinguishers for use by drivers,
  • facilities for firemen: water pipe and hydrants - foam pipe in some countries. The volume of the water tanks is variable. It depends on the local regulations and the particular conditions of the tunnel.
  • Some tunnels have an FFSS (see § 1.2.4.2.c above).

1.2.4.2.i Miscellaneous equipment

Other equipment may be installed according to the objectives and needs concerning safety, comfort and protection of the structure. Some examples are:

  • luminous beacons inserted in the side walls or walkway kerbs,
  • a hand rail or a "life-line" fixed on the side wall permitting the movement in safety of firemen in a smoke-filled atmosphere,
  • painting of the side walls or the installation of prefabricated panels on the side walls,
  • devices for the protection of the structures against damage resulting from a fire. Such protective arrangements have to be taken into account from the origin of the project. Thermal exchanges (with the concrete lining or with the ground) are indeed modified during a fire, as well as air characteristics, which must be designed for when dimensioning the ventilation facilities,
  • management and treatment of water collected on the road pavement inside the tunnel before discharge outside in the natural environment,
  • arrangements for the measurement of environmental conditions at the tunnel portals, associated with particular operational procedures if the limits defined by the regulations are exceeded.
Reference sources

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