Tunnel: a complex system

• 1. Complexity of the system
• 2. Subset "Civil Engineering"
• 3. Subset "Ventilation"
• 4. Subset "Operation equipment"
• 5. Subset "Safety"
• 6. Synthesis

1. Complexity of the system

A tunnel constitutes a "complex system" which is the result of the interaction of very many parameters. These parameters can be gathered by subsets, the principal ones of which are represented in the diagram below (fig. 1).

All these parameters are variable and interactive, within each subset, and between the subsets themselves.

The relative weighting of the parameters and their character varies according to the nature of each tunnel. For example:

• the determining criteria and the weighting of parameters are not the same for an urban tunnel and a mountain tunnel;
• the parameters differ for short and long tunnels, for tunnels passed through by vehicles transporting dangerous goods and for those transporting passenger vehicles only;
• the criteria are not the same for a new-build tunnel or a tunnel to be refurbished or upgraded to put it in conformity with new standards concerning safety.

Note 1: the links are multiple and often reversible - the general concept of the tunnel and the functional section are placed in the centre of the figure. Similar diagrams could be drawn up while placing other factors in the centre of the figure.

Note 2 : the first circle represents "technical fields". Some fields represent multiple aspects:

• safety: regulation - risk analysis - intervention means - requirement of availability,
• geology: geology - geotechnics - structural dimensioning,
• civil works: methods - construction schedule - risks and hazards,
• operation: operation and maintenance (technical aspects),
• costs: construction - operation - daily maintenance - major repairs,
• environment: regulation - diagnosis - impact assessment - treatment and mitigation.

Note 3: the second circle represents the "context" in which the project is to be developed. Some elements represent multiple aspects:

• human environment: sensitivity - urbanisation - presence of buildings or infrastructure,
• natural environment: sensitivity - water - fauna - flora - air quality - landscape,
• nature of the transport: nature and volume of the traffic - typology - types of goods that are transported - etc.
• various external constraints: accesses and particular constraints - climatic conditions - avalanches - stability of the ground - socioeconomic context - etc.
• level of profitability: economic acceptability - capacity of financing - control of the financial costs - general economic and political context in case of concession or Public Private Partnership (PPP).

The design of a new tunnel (or the refurbishment and upgrading of an old tunnel) requires these numerous parameters to be taken into account. The decision tree relating to these parameters is complex, and requires the input of experienced multidisciplinary parties. They must intervene as early as possible, for the following reasons:

• to enable all relevant parameters to be considered from project commencement, and to avoid numerous potential pitfalls noted in projects in progress or in recently completed tunnels. Such errors include the late consideration of the required equipment for operation and safety, and the development of a supervision system without integrating the results of the risks analyses, the emergency response plan or the operation procedures. As a consequence, the tunnel and its systems and equipment for operation and supervision may be inappropriate for safe and reliable operation.
• an early intervention contributes to a better optimisation of the project, both from the perspective of safety as well as for construction and operation costs. Recent examples indicate that transverse optimisations (civil engineering - ventilation - safety evacuation) made at early project stages can contribute about 20% towards cost savings.

Each tunnel is unique and a specific analysis has to be developed, while taking into account all the specific and particular conditions. This analysis is essential to bring suitable answers and to allow:

• optimisation of the project from a technical and financial aspect;
• reduction of the technical, financial and environmental risks;
• guaranteeing the required level of safety for tunnel users.

There is no "magic key solution", and a simple "copy and paste" process is almost always unsuitable.

The design and optimisation of a tunnel require:

• an exhaustive and detailed inventory of all the parameters,
• an analysis of the interactions between parameters,
• the evaluation of the degree of flexibility of each parameter, and if necessary of the sensitivity of each one of them with respect to the required objectives,
• a holistic approach to achieving success, because:
  • a purely mathematical approach is not possible, owing to the fact that the "system" is too complex, and there is no single answer;
  • too many parameters are still unspecified or variable during the early stages of a project, but essential choices still have to be made;
  • the evaluation of the risks, their gravity and their likelihood of occurrence must be taken into account;
  • many parameters are interdependent and many interactions are circular.

Several examples are given below showing how it is possible to clarify the complexity, the interactivity, as well as the iterative and "circular" character of the analysis.

These examples are not exhaustive. Their aim is simply to make the reader aware of the issues and make it possible to focus considerations on each specific tunnel.

2. Subset "Civil Engineering"

2.1. Parameters

Table 1 below gives an example of the principal parameters concerning the aspects relating to civil engineering:

TABLE 1 : MAIN PARAMETERS ACCORDING TO CIVIL ENGINEERING

The first column of the table indicates the principal sets of parameters,

• The second column of the table indicates the principal subsets of parameters relating to a principal set,
• The third column lists a certain number of elementary parameters relating to a subset. The list is not exhaustive,
• The fourth column of the table indicates by set, or subset, the principal outcomes related to the subset.

2.2. Interactions between parameters

The interactions between parameters are numerous and often connected by circular links taking into account the overlaps between the various parameters.

The example below (Table 2) relates to the interactions between ventilation, the cross section, and safety:

• The first column concerns ventilation. The parameters listed in this column are the elementary parameters resulting from table 1 above for the subset "ventilation",
• The second column concerns the cross section. The parameters result from table 1,
• The third column concerns safety.

TABLE 2 : INTERACTIONS BETWEEN PARAMETERS

The table reveals a certain number of parameters common to several columns (see line connectors), which create circular interactions between the various subsets of parameters.

These interactions are linked by complex functions, which make a purely mathematical resolution of the problem nearly impossible. The resolution of the problem requires the definition of a hierarchy between the various parameters, followed by taking into account assumptions for the parameters of higher hierarchy. This hierarchy differs from one project to another, such as for example:

• For a short bored tunnel or a medium-length bored tunnel with one-way traffic, the most probable ventilation system is "longitudinal ventilation". The jet fans fixed in the crown have indeed usually a very low impact on the dimension of the cross-section. This one could thus be dimensioned initially before designing the ventilation, but by taking into account the other determining parameters. The impact of the ventilation on the cross-section will then be checked afterwards,
• Conversely, if the tunnel is very long or the cross-section is rectangular (cut and cover), the ventilation system and its components (section, number and nature of the possible air ducts - dimension of the jet fans if required - etc.) have an essential impact on the cross-section size. The ventilation system will have to be pre-dimensioned at the beginning of the analysis by making preliminary assumptions of the dimension of the cross section. The geometry of the cross-section will then be checked.

The process of resolution is then iterative and based on a first set of assumptions, as the previous examples show. This process requires a large transverse multi-technical experience of the engineers, making it possible to take into account the relevant parameters for the project, to better target the successive iterations, and to guarantee the best optimisation of the project, with the required level of service and safety.

3. Subset "Ventilation"

Table 3 below gives an example of the principal parameters concerning the aspects relating to ventilation. This table is not exhaustive.

As for "civil engineering", the interactions between parameters are numerous. They also are subject to circular relations.

The process to solve the problems is similar to the one outlined above for "civil engineering".

Table 3 : Main parameters influencing ventilation

4. Subset "Operation equipment"

They do not constitute fundamental parameters for the definition of the functional section, with the exception of:

• box-outs and sleeves for the passage of cables, pipes for water supply to the fire fighting system,
• signalling, signage for information, safety or police instructions. Signalling may have sometimes (rectangular cut and cover) a very important impact on the geometry (distance between roadway and soffit with a possible impact on the vertical alignment and the tunnel length). This may eventually require a more global optimisation, which may concern the position and/or the design of the interchanges outside the tunnel close to the portals.

"Operation equipment" constitutes on the other hand essential parameters for the dimensioning of the technical buildings at the portals, of underground mechanical and electrical sub-stations, and of all underground technical spaces, or various provisions, recesses and niches. They often require particular arrangements concerning temperature, air conditioning, and air quality.

They also are important parameters in terms of cost: construction, operation and maintenance.

"Operation equipment" constitutes essential parameters regarding tunnel safety. It must be designed, built and maintained in this objective:

• availability and reliability, in particular power supply and distribution, as well as all the communication networks,
• protection against fire of all equipment, in particular of the main power supply cables and the cables of the transmission networks,
• hardiness of the equipment and its components in order to guarantee its life-span, reliability and optimisation of costs: operation and maintenance,
• to facilitate maintenance interventions, their low impact on the traffic conditions, as well as on the safety of the maintenance teams and the users, which requires particular arrangements concerning the design and the accessibility of these facilities,
• integration of the procedures for operation, and the emergency response plan in the design of the supervision system (SCADA), the ergonomics of the man/machine interfaces, and assistance to the operator in particular during an incident.

5. Subset "Safety"

5.1. ASPECTS RELATING TO "SAFETY" 

The statistics available in many countries show, quite generally, that the rate of tunnel accidents is notoriously lower than that for the road network in the open air.

Apart from disasters, almost all the accidents recorded and documented in tunnels are mainly due to the following causes:

• Poor design for the geometry: layout (too windy or with reduced characteristics), vertical alignment (significant gradient) and bad coordination between the horizontal and the vertical alignments,
• Too short distances of visibility,
• The behaviour of drivers, an excessive velocity and denials of priority in the exit or merging areas etc.
• An insufficient illumination level and poor identification of the curbs, and thus of the roadway width,
• For tunnels with underground interchanges or connections:
  • a bad design of the geometry of the exits and the merging areas, insufficient visibility and legibility – poorly sized exit and merging provisions,
  • a bad design of the signalling of the exits and entrances: signalling insufficient, or mispositioned, or illegible,
  • collisions at the rear of a traffic jam particularly in the vicinity or on the exit ramps: due to lack of visibility – poor identification of the fluctuating plug-tail – Insufficient information – poor traffic management by the operator – insufficient coordination between the tunnel operator and the surface network operator,
• For the tunnels with bidirectional traffic, additional risks of frontal collisions,
• For the tunnels in mountainous area additional causes due to the formation of stalactites of Ice from the vault or on the walls, of stalagmites or ice formations on the pavement,

 Safety aspects are to be broken down into:

• Preventative provisions: These are the ones that day to day allow to reduce the causes of accidents mentioned above. These causes and the resulting provisions are rarely analysed in the "risk and hazard analyses",
• Curative arrangements: These are the ones that are indispensable in the event of disasters or fire (emergency routes - emergency ventilation - organization and access of the emergency teams - etc.). These provisions are necessary and essential to ensure the safety of users in the event of fire, but they have low impact for improving the daily safety inside the tunnel.

Note: Additional information on tunnel accidents is available by following hyperlinks: 

• 2016R19EN : Road tunnels: complex underground road networks (§ 4.4.2 for accidents & § 4.4.3 for fires)
• 2017R35EN : Experience with significant incidents in road tunnels (Chapter 3 for accidents – Chapter 4 for fires)

5.2. Concept "Safety”

The conditions of safety in a tunnel result from many factors as presented in the Safety book contained in this Manual. It is necessary to take into account all the aspects of the system formed by the infrastructure itself to ensure safety as well as its operation, interventions, vehicles and users (Fig. 2).

The infrastructure is an essential parameter concerning the safety inside the tunnel (preventative and curative provisions), as well as the construction cost. However, one can invest highly in infrastructure without improving conditions of safety if essential provisions are not considered in parallel concerning:

• organisation, human and material means, the procedures of operation and intervention,
• training of operating staff,
• the emergency services' equipment with efficient material and training of their staff,
• communication with users.

5.3. How do these parameters affect a tunnel project?

These parameters relating to safety may affect in a more or less important way a tunnel project. The tables below give some examples.

Note: The four tables below refer to the four principal fields represented in Fig. 2.

• Column 1 indicates the principal infrastructure or actions concerned,
• Column 2 indicates the degree of influence on the tunnel project (civil engineering - ventilation - operating and safety equipment):
         Green: no impact,
         Yellow: medium impact,
         Red: important or major impact.
• Column 3 specifies the main reasons or causes of influence.

Table 4 : Main impacts on the project due to infrastructure

INFRASTRUCTURE	IMPACT	COMMENTS
Visibility – Legibility – Exit and merging conditions on the interchanges and connections with other underground infrastructures		Horizontal and vertical alignment - Coordination layout/vertical profile - Design of the interchanges and connections - Design of the ramps - Exit and merging areas
Escape route		Inside the tunnel - Parallel gallery - Direct external access - Connection between two tubes
Emergency team accesses		From the other tube - Dedicated access - Common with escape route
Volume of people to escape		Size of escape route - Spacing of the connections to the tunnel
Ventilation		Ventilation concept - Inadequacy of pure longitudinal system under certain operating and traffic conditions

Table 5 : Main impacts on the project due to intervention conditions and the organisation of the operation

OPERATION	IMPACT	COMMENTS
Response plan procedure		Signalling - SCADA - Communication with the users
Intervention rescue team		Size of the portal building - Eventual underground facilities - Specific tool - Size of water tanks
Team training		Particular external facilities - Special software

Table 6 : Main impacts on the project due to vehicles

VEHICLES	IMPACT	COMMENTS
Traffic flow average and peak hour		Number of lanes - Ventilation concept and sizing
Transport of dangerous goods		Ventilation impact - Particular drainage for hazardous goods spillage - Operating procedures with particular convoy with fire brigade accompanying --> parking facilities and staff
State of the vehicle		In particular condition, size control and overheat control before entering --> gantry heat control + parking + staff
Restriction of particular vehicle categories		Example: urban tunnel dedicated to light vehicles - Tunnel size, ventilation escape routes

Table 7 : Main impacts on the project due to the tunnel users

ROAD USERS	IMPACT	COMMENTS
Information		Leaflet distributed before entering - TV information campaign
"Live" communication		Signalling, VMS, radio broadcast, traffic lights, impact on cross section, mechanical and electrical equipment, SCADA, sometimes remote barriers
Teaching		Driving school (in several European countries)
Guidance to escape routes		Signalling - Handrail - Flash - Noise - Impact on mechanical and electrical equipment and SCADA
Speed and spacing between vehicles control		Radar and spacing detectors - Impact on mechanical and electrical equipment and SCADA

6. Synthesis

A tunnel is a "complex system" which means in particular that:

• approaching the design of a tunnel from the point of view of only the alignment, the geology or the civil engineering, leads to serious design deficiencies, which are likely to make the tunnel less safe (possibly even dangerous) and difficult to operate (perhaps impossible to be operated under reasonable conditions).
• in the same way, to approach the design of a tunnel from the point of view of only the operating equipment without integrating an upstream analysis of risks and safety, intervention and operation, will also lead to deficiencies that will very quickly appear as soon as the tunnel is open to traffic,
• not taking into account, from the preliminary design stage, all the objectives and constraints relating to the operation and to the maintenance, will inevitably lead to increased operational costs and to reduced overall reliability.

Partial treatment of problems is unfortunately still rather frequent, due to lack of sufficient "tunnel culture" of the various actors involved in the design.

Control of this complex system is difficult but essential in order to:

• find the appropriate solution to each problem,
• ensure the users have an essential level of safety, and to offer them a service of quality and good comfort.

In a parallel way the control of this complex system very often contributes to the technical and economical optimisation of the project, by a clear and early definition of the functions to be ensured and by using a value engineering process.

Taking into account, from the start of the project, the major issues relative to:

• horizontal and vertical alignments, geology, civil engineering construction provisions and methods,
• ventilation,
• safety (by a preliminary analysis of risks and danger and a preliminary emergency plan),
• operation and maintenance conditions,

constitutes an effective approach to solving this complex equation.

The definition of the “tunnel function", as well as the "preliminary risks and dangers analysis" are often neglected or superficially treated. They are, however, an essential and indispensable "tool" for the technical, economical and safe optimization of a tunnel. 

The " preliminary risks and dangers analysis " should not be confined to tunnel fires and constructive and operational provisions to minimize risks. It must also consider (which is rarely the case) the daily safety conditions to reduce the likelihood of incidents and their severity. This implies an analysis of the horizontal and vertical alignments, of the geometry of the ramps of the underground connections, of the visibility, of the likelihood of traffic congestion. This analysis must be done during the design of the alignment, while it is then still possible to improve the project in order to reduce the risk of incidents.

Steps of tunnel life

• 1. Design
• 2. Construction
• 3. Commissioning
• 4. Operation

The key items to consider during each stage of the tunnel life are presented below.

1. Design

This is the most important stage of the life of a new tunnel. It is has significant influence on construction and operation costs, safety, as well as management of the technical and financial risks.

This stage requires a transverse integration of all interfaces of the “complex system" that constitutes a tunnel. This integration has to start from the earliest stage of the design.

Experience testifies to the fact that this is unfortunately rarely the case and that often the design of a tunnel results from a succession of stages considered as independent.  We note that :

• the function is not always clearly defined,
• the alignment is designed without any integration of the tunnel, of its constraints, or of the whole set of optimisation possibilities,
• the civil engineering “makes do with” the set horizontal and vertical alignments, with all the consequences that can affect the construction costs and risks,
• the equipment, safety level and operation fit in somehow and not always harmoniously or optimally with the arrangements chosen during the preliminary steps.

2. Construction

With regard to civil engineering, the most important aspect is the management of technical risks (in particular geological) and of all the resulting consequences concerning construction costs and duration.

Considerations relating to risk management for construction have to be taken into account from the design stage. These considerations must be detailed and shared with the owner of the tunnel. Decisions concerning the risks must be developed and clearly documented.

The decision to take some risks does not necessarily constitute a mistake and must not necessarily be forbidden. For example  working to a tight schedule does not allow the implementation of all the investigations that would be required to eliminate all uncertainties.

However, the decision to take a risk must result from a very detailed and soundly argued consideration of:

• consequences that may result, which must be clearly identified, analysed and consigned: delays - costs - human and environmental impacts – safety – schedule – etc.,
• the real issues of this decision, its probability of success and its real interest.

Taking  a risk must not be the result of carelessness or incompetence of the various parties.

With regard to operational facilities, the reader’s attention is drawn to:

• all aspects likely to optimise the life span of the equipment, its reliability and ease of maintenance,
• the need for a rigorous process and continuous control of the functionality, performances and quality of the equipment throughout the manufacture of the components, their assembly, their installation on the site, then at the time of partial and global testing after integration,
• the added benefit to quality concerning the choice of the equipment and the contractor, even though the construction costs may increase as a result. Possible savings due to reduced initial costs are often quickly compensated for by higher maintenance costs, difficulties of intervention under traffic, and the additional constraints that would be suffered by the users.

3. Commissioning

This stage of the "tunnel life" is often under-estimated and taken into account tardily. It requires taking time that is not often granted, and leads to the commissioning of the tunnel under unsatisfactory conditions, or even under conditions that highly expose safety risks.

This stage includes:

• the organisation of the operation and maintenance,
• development and adjusting of all operation, maintenance, intervention and safety procedures under the normal conditions of tunnel operation, as well as under MOC (Minimal Operation Conditions),
• recruitment and training of the staff that will operate the tunnel,
• the “dry run" of all the facilities, that cannot take place before the equipment has been fully completed, tested and delivered (possibly with provisions requiring only minor corrective interventions),
• the practice, training and manoeuvres involving all the intervention teams and services before commissioning the tunnel.

4. Operation

The main mission is to ensure:

• the management of all facilities, their maintenance, their repair,
• the safety and the comfort of the users.

It is also necessary to be able to step back and look objectively at daily routines in order to:

• establish feedback from experience, adapt the procedures, the intervention conditions, the training and the safety manoeuvres,
• optimise operation costs without damaging the level of service and safety,
• identify, analyse, plan and implement heavy repairs, and renovation and upgrading works.

General design of the tunnel (new tunnel)

• 1. Horizontal and vertical alignment
• 2. The functional transverse profile
• 3. Safety and Operation
• 4. The operating equipment

This page relates to the design of new tunnels. The design concerning the refurbishment and the safety upgrading of tunnels under operation is presented in page Renovation - Upgrading of existing tunnels.

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 page 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.

The definition of the horizontal and the vertical alignments and of the geometry of the underground interchanges or connections (in particular exit or merging areas) are an important stage in road safety. Many accidents are due to design faults as outlined in section 5.1 of page “Tunnel: a complex system” above. 

The "Preliminary risk and Hazard analysis” should cover all aspects relating to geometry, legibility, visibility and the presence of any underground connections (see also section 6 of page “Tunnel: a complex system” above).

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. Countries having a tradition of construction and operation of tunnels

The concept of a "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 set of constraints and tunnel components.

It is however essential to take into account from this stage all the parameters and interfaces described in page "Tunnel: a complex system" 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.

2. The functional transverse profile

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 page Tunnel: 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 section 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 section 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.

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 chapter "Ventilation concepts"),
  • the space required for the ventilation ducts, for the installation of axial fans, jet fans, secondary ducts, and all the other ventilation equipment,
• The areas of separation or insertion of the branches of underground connections, in particular,
  • the length of the parallel lanes - good legibility and visibility on the points of disconnection and convergence,
  • the position and legibility of pre-signalling and signalling,
• Evacuation of the users and the access of the emergency and rescue teams which depend on the numerous factors detailed in chapter Construction and Geometry,
• 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 section1.2 above) 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 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.

3. Safety and Operation

3.1. RISK AND HAZARD ANALYSIS - EMERGENCY RESPONSE PLAN

Safety must be a permanent concern to the contracting authority, the designers and the operators. 

Safety must be considered from the early stage of the preliminary studies, using tools adapted to each of the design stages, of the tenders, of the preparation for the operation, and then during the operation period.

In a very schematic way:

• During the preliminary studies and the definition of geometry, the analysis will focus:
  • In very detailed way, on the current risks of road traffic (see section 5.1 of page “Tunnel: a complex system“ above) : horizontal and vertical alignments, visibility, traffic congestion, etc.
  • In a preliminary approach, on the hazards in case of fire,
• During the development of the detailed design, the analysis will focus:
  • on the validation of provisions to minimize daily incident risks,
  • on detailed evaluation of the risks in the event of fire, as well as on the conditions of escape and safety.
  • on a preliminary outline of the emergency response plan,
• During the preparation for the operation, the analysis will focus on:
  • the validation of the provisions defined during the previous stages,
  • the development of all operational operating and intervention procedures,
  • on the education and the training for all stakeholders,
• During the operation period, the analyses will be based on the collection of experience, and will focus on the adaptations for existing procedures, or on the implementation of additional procedures, as well as on further education, training, and on communication with users.

The "Risk and hazard analysis" provisions, as well as the "Emergency response plan" are specified in the Book Safety.

3.2. 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 book Safety.

This present chapter primarily treats safety and operation interfaces within the "complex system". The tables of section 5.2 of page "Tunnel: a complex system" 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

3.3. Parameters relating to the traffic and its nature

These parameters have an impact mainly on the functional cross-sectional profile (see section 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.

Another fundamental traffic parameter is often neglected or deliberately evaded when designing a tunnel. It concerns the traffic congestion and the formation of "traffic jams" in tunnel. This parameter is particularly sensitive for tunnels which incorporate ramps and underground connections.

To postulate, as is often the case, that traffic management provisions will be taken to avoid the formation of "traffic jam" is fallacious and unrealistic as shown by the daily reality in urban areas. These provisions further lead to drastically reducing the volume of traffic entering the tunnel, reducing the capacity of the route and degrading the function and economic profitability of the infrastructure. 

In most cases, this severe neglect inevitably leads to increased exposure of users to an unacceptable level of risk and danger.

The presence of "traffic jam" has significant impact on:

• The design and sizing of the ventilation systems. A "pure" longitudinal ventilation without a smoke extraction duct, or without mass extraction, is not acceptable because it puts the users in significant danger in the event of a fire during blocked traffic,
• The design and sizing of emergency exits. The number of users to be evacuated is more concentrated and the volume much more important in case of traffic blockage,
• The risk of collision is high at the tail of traffic jam, and the signalling for the position of a fluctuating rear end is difficult to implement inside a tunnel.

3.4. 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.

3.5. 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.

3.6. 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.

3.7. 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).

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 page "Regulations - Recommendations" hereafter.

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.

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.

4.2. Key recommendations concerning the main facilities

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.

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 the above, the reader should refer to the chapter on Ventilation concepts.

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 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.

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.

Page 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).

Page Fixed Fire Suppression 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.

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.

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.

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

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.

4.2.g. Signalling

Signalling refers to page Evacuation route signs.

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.

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 section 4.2.c above).

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.

Renovation – upgrading of existing tunnels

• 1. Diagnosis
• 2. Renovation or upgrading programme
• 3. Design implementation and construction

The upgrading (in particular for safety improvement) and refurbishment of existing tunnels in operation gives rise to specific problems of analysis and method. The degree of freedom is less than for new tunnels, because it is necessary to take into account the existing space and constraints. The technologies peculiar to each type of equipment and their integration are however identical.

The renovation and upgrading of a tunnel under operation quite often result in an increase of the construction schedule and costs, in much lower safety conditions during the works, and with badly controlled impacts on the traffic volume and conditions. These disadvantages are often the result of an incomplete analysis of the existing situation, the real condition of the tunnel, its facilities and its environment, as well as of a lack of strategy and procedures that would mitigate the effects on the traffic.

The page on Assessing and improving safety in existing tunnels proposes a methodology for the safety diagnosis of existing tunnels and the development of an upgrading programme. In addition, the page on Operation during maintenance and refurbishment works presents specific issues related to works carried out on tunnels in operation. Their dispositions help mitigate the problems mentioned above.

It is appropriate to draw the reader's attention to the key points of the following sections.

1. Diagnosis

Detailed and rigorous diagnosis of a tunnel is an essential stage in the process of its upgrading or renovation. Unfortunately this stage is often neglected.

The physical diagnosis of a tunnel is required in order to:

• establish in detail and to describe in a precise manner the functions and the geometry of the structure,
• establish a detailed condition statement of the structure. To evaluate in particular fire resistance, uncertainties and potential risks, and to list the tests that would be needed in order to provide a solid basis for the detailed design,
• list all existing equipment, their functions, their condition, their technology, their actual features (tests or measurements will be required) and the stock of spare parts that might be available,
• evaluate the remaining life span of the aforementioned equipment before their replacement, and to identify the availability or not of spare parts on the market (notably because of the technological obsolescence),
• identify maintenance and inspection reports, equipment malfunctions and the rate of breakdowns.

This physical diagnosis must be supplemented by a diagnosis concerning the organisation, maintenance and operation procedures, as well as by a specific diagnosis concerning all documents relating to the organisation of safety and rescue interventions. This stage of diagnosis may eventually lead to the setting up of actions for the training of the various intervention parties in order to improve the global conditions of safety of the tunnel in its initial state prior to renovation.

The diagnosis must be followed by a risk analysis of the tunnel based on its actual state. This analysis has a double objective:

• to assess if the tunnel can continue to be operated in its present state prior to renovation, or if it is necessary to take temporary transitional arrangements: restriction of access to some vehicles only - strengthening of the arrangements for surveillance and intervention - additional equipment - etc.,
• to constitute a referential of the existing state from the point of view of safety in order to refine the definition of the renovation programme.

The diagnosis has to identify (without running the risk of late discoveries during the works period) if the existing facilities, supposedly in working condition, can be modified, be added to or integrated in the future updated facilities (technological compatibility - performance in particular for data collection and transmission, automatic functioning devices and SCADA).

2. Renovation or upgrading programme

The renovation or upgrading programme proceeds from two stages.

2.1. First stage: programme development

The development of the programme results from:

• the detailed diagnosis as described above,
• the risk analysis developed considering the initial state of the tunnel,
• the gaps noted concerning safety,
• the analysis of what it is possible to achieve in the existing spaces and their potential enlargements in order to enable the upgrading of the tunnel.

Depending on the physical environment of the tunnel and the spaces available, the optimum upgrading programme for the infrastructure or equipment may not be feasible under acceptable conditions, and that it is necessary to define a more restricted programme. This restricted programme may require the implementation of mitigating measures ensuring that the required level of safety is achieved in a global sense, after completion of the works.

2.2. Second stage: validation of the programme

The validation of the programme requires:

• development of a risk analysis based on the final state of the tunnel after upgrading in order to test the new arrangements introduced by the programme. This analysis has to be established with the same methodology used for the prior analysis based on the initial state. It also enables a search for optimisations,
• detailed examination of the feasibility of the works to be carried out for the improvement or the renovation under the requisite conditions of operation: for example, banning of tunnel closure or of temporary traffic restrictions. In case of incompatibility between the objectives of the programme and the works required for its application, iteration is necessary. This iteration may concern :
  • the programme itself, insofar as adaptation of the programme is compatible on the one hand with the safety objectives, and on the other hand with its implementation in the required conditions of operation,
  • the required conditions of operation that may be necessary to modify in order to be physically able to carry out the works resulting from the upgrading programme.

The upgrading or improvement programme does not necessarily require physical works. It may only require modification of the functions of the tunnel, or of the operating arrangements, for example:

• modification of the category of the vehicles authorised to access the tunnel: no access to trucks – no access to vehicles carrying dangerous goods,
• setting up of specific procedures for traffic restriction: in a permanent way or only during peak traffic,
• tunnel operated initially in bi-directional traffic, transformed for the implementation of unidirectional traffic,
• modification of the means for supervision or intervention.

3. Design implementation and construction

The stage of design implementation and construction involves translating the renovation or upgrading programme into technical and contractual specifications and implementing it.

This stage requires a very detailed analysis of:

• the successive stages of construction, the content of each of these stages, the logical and priority sequences of the works,
• safety conditions inside the tunnel at each construction stage. This requires partial risk analyses and the implementation if necessary of mitigation arrangements: traffic regulation – traffic restrictions - patrol – strengthening of the intervention means - etc.
• traffic conditions inside the tunnel and on its approaches, with partial and temporary restrictions according to the various stages of works (different arrangements for daytime and night-time, for normal periods and vacation periods), of the potential diversions, of the global impact on the traffic and safety conditions in the areas concerned by the works,
• the constraints and subjections, the partial and global contractual deadlines for the works, in order to be able on the one hand to define the contractual specifications for the contactor, and on the other hand to implement all necessary temporary arrangements, and to proceed to an information campaign for the users and residents.

Costs of construction, operation, upgrading - financial aspects

• 1. Foreword
• 2. Construction costs
• 3. Operation costs
• 4. Costs of renovation and upgrading
• 5. Aspects relating to financing

1. Foreword

Tunnels are relatively expensive civil engineering structures with respect to their construction and operation. Particular attention must be paid from the beginning of the project in order to spot any possible technical and financial optimisations.

It is recommended from the first stages of the design to implement a process including:

• the detailed definition of the "function" of the tunnel,
• an iterative process of “value engineering analysis" achieved at all strategic stages of the project, to which must be integrated into the various stages of the risk analysis,
• detailed analysis and monitoring of the potential risks in the design and construction stages. These potential risks are related to:
  • technical uncertainties relating in particular to the complexity of the ground (geological and geotechnical uncertainties),
  • uncertainties of traffic volume forecasts, that constitute an important risk concerning earnings in the case of construction and financing by “concession",
  • uncertainties and risks concerning the financial environment, in particular changes in interest rates and conditions of financing and refinancing. This aspect constitutes an important risk in the case of construction and financing by "concession" or by PPP (Private Public Partnership) with a financial contribution.

This process will enable the optimisation of the project (construction and operation costs) and an improved management of the technical and financial risks, as well as the schedule.

2. Construction costs

2.1. Cost ratios per kilometre

The construction costs of tunnels are very variable and it is impossible to give representative ratios of costs per kilometre, because these ratios may vary in important proportions (average of 1 to 5) according in particular to:

• the geological conditions,
• difficulties concerning the access roads and the tunnel portals,
• the geographical location of the tunnel: urban or non-urban,
• the length of the tunnel: in particular the "weight of the ventilation facilities and safety arrangements is more significant for a long tunnel; on the other hand all the works concerning the access roads and portals have a more important impact for a short tunnel,
• the traffic volume which is a determining factor for the dimensioning of the number of lanes, as well as for the ventilation facilities,
• the nature of the traffic: in particular a tunnel used by vehicles carrying dangerous goods will require expensive arrangements for ventilation, safety and possibly the resistance of the structure to fire; conversely, a tunnel dedicated to the passage of only light vehicles may enable significant savings because of the possible reduction of the width of the lanes, headroom and reduced requirements for the ventilation facilities,
• the tunnel environment that may lead to expensive protection arrangements for the mitigation of its impact,
• arrangements taken for the management or the sharing of construction risks,
• the socioeconomic environment of the country in which the tunnel is to be constructed. The impact can reach about 20% of the costs,
• chosen construction methods as found to be the most technical and economically viable. However, tunnelling works must also comply with all external requirements imposed from  authorities, stakeholders and neighbours with regard to environmental protection and health and safety aspects during construction.

At most it is possible to indicate that the average cost of a usual tunnel, built under average geotechnical conditions is about ten times the cost of the equivalent infrastructure built in open air (outside of urban areas).

2.2. Breakdown of the construction costs

The construction cost of a tunnel may be broken down into three types of cost:

• the cost of the civil engineering structures,
• the cost of the operation facilities, including the supervision centre and the energy supply from public networks,
• various costs including in particular: owner’s costs for the development of the project – project management – design and site supervision – survey and ground investigations - environmental studies and mitigation measures – land acquisitions - various procedures - etc.

The two diagrams below show examples of the breakdown of construction costs, on the one hand for tunnels for which the conditions of the civil engineering works are not complex, and on the other hand for tunnels for which the conditions of the civil engineering works are less favourable.

Note: these two diagrams show how important the costs are of the civil works and illustrate the consequences of an almost doubling the costs of civil works (-hand diagram).

3. Operation costs

The operation costs of a tunnel may be broken down into three types of cost:

• the operation costs as such, which essentially include staffing, energy, as well as the management and expendable equipment. These are recurrent costs;
• the recurrent yearly costs of maintenance;
• the costs of heavy repairs, as well as the replacement costs of the equipment according to its life span and its state during the tunnel life. These costs are not recurrent and depend on the equipment, its quality and the conditions of maintenance, from the tenth or twelfth year after the start of the operation period.

The two diagrams below represent examples of breakdown (with constant economic conditions) of the construction costs (civil works, operation facilities, various costs) and of the global operation costs (accumulated over a duration of thirty years after the start of the operation period).

Note: these diagrams show how important the operation and maintenance costs are and how it is necessary to choose from the first stages of the tunnel design the arrangements that enable the optimisation of the recurrent operation and maintenance costs.

4. Costs of renovation and upgrading

This section concerns the renovation or upgrading works that are required for “upgrading” to new regulations. The works concern the arrangements for evacuation, the resistance of the structure to fire, the operation and safety facilities, and all the requirements to satisfy the new safety regulations.

It is not possible to give statistical prices due to the diversity of existing tunnels, their condition, their traffic and the more or less important requirements of new safety regulations that may vary from one country to another.

The observations made in France for this nature of upgrading works for complying with the new regulations, which have been carried out since the year 2000, show a large variation of the corresponding budgets with a range of costs between about ten million Euros and several hundred million Euros (there have been several upgrading programmes with a budget of more than 200 million Euros).

5. Aspects relating to financing

Tunnels constitute costly infrastructure in terms of construction and operation, but this is offset by economic benefits including regional development, traffic fluidity, comfort, safety, reliable routes (mountain crossings) as well as protection of the environment.

Financing of these works is ensured either by:

• the “traditional mode”: financing and maintenance by a public authority, the financial resources coming then from public taxation or fuel taxes,
• a "concession" to a private or semi-public body, which is charged with the construction and the operation of the tunnel during a contractual period of time. This body is in charge of the financing (often partly by loan), which is offset by a toll paid by the users, that reimburses the costs of the construction and the operation, as well as the risks and the financial expenses. This type of "concession" can be granted by the financial involvement of the grantor or by particular guarantees (example: guarantee of a minimal traffic volume with the payment of a financial compensation if this minimal traffic volume is not reached),
• “mixed mode” of PPP (Public Private Partnership) or similar, that may concern:
  • only the construction or the construction and the operation,
  • construction under a “turnkey” scheme in the case of a “design and build” process,
  • partial or whole financing.

The present manual does not intend detailing these various modes of financing, or presenting their mechanisms, their advantages or disadvantages. However, it is interesting to present some main guidelines found from experience, which give a preliminary illustration.

a) Financing by a public authority

This mode of financing is employed widely. It allows the development of an infrastructure project, whose financing could not be achieved by a “concession” (by lack of sufficient income from toll collection), or when there is political will to avoid a toll.

It requires however, that the public authority has the financial capacity to ensure this financing, or that it has the capacity to borrow money and to support a debt. The financial resources essentially come from public taxation or fuel taxes and sometimes partially from toll collection.

b) Financing by “concession” – tunnel part of a global infrastructure 

The financing of a “non-freestanding tunnel” by a “concession” (with or without financial involvement of the grantor) is the general case for a tunnel that is part of a new interurban highway with toll collection. The costs (construction and operation) of the tunnel are shared out among the tunnel and the linear infrastructure above ground. Experience shows that the over-cost of the average toll ratio per kilometre is accepted by the users as long as the new infrastructure brings sufficient added value concerning time savings, better or more reliable service, comfort and safety.

c) Financing by "concession" - isolated tunnel 

Two main categories of isolated tunnels exist.

• Tunnels corresponding to a major improvement of the traffic conditions. This is in particular the case of urban tunnels aiming to alleviate traffic and to reduce travel times. Experience shows that financing by “concession” is only really foreseeable when the following conditions are met:
  • high traffic volumes,
  • country with high standard of living and revenues, enabling substantial toll rates, which are essential to ensure the financial balance,
  • Significant time gains for the users so that they will accept in return a relatively high toll rate,
  • duration of the concession of about fifty years at least.
• “Regional development” tunnels, intended to cross a major natural obstacle (chain of mountains - estuary). These obstacles constitute an important handicap for trade. The initial traffic volume is usually relatively low. The new link with the tunnel will enable the growth of traffic, but such a development is often very difficult to predict in advance, and it constitutes an essential parameter of financial risk for the funding of the concession. Experience shows that financing by "concession" is then only realistic when the following conditions are met:
  • The natural obstacle is significant and the tunnel is sufficiently attractive (gain of time, level of service, delivered service, reliability of the link) in order to attract all existing traffic in spite of the toll,
  • Financial involvement of the grantor (possibly also the stakeholders), either with a financial contribution or direct involvement in the construction and the financing of a part of the works (for example construction of the access roads),
  • Guarantee of a minimal traffic volume by the grantor, with the payment of a contractual financial contribution if the minimal traffic volume is not reached,
  • Contractual arrangements for sharing major risks can put the financial model at risk if they over-run limits or conditions defined by the contract,
  • Very long concession duration: often 70 years or more,
  • Financial guarantee brought by the grantor, in order to enable the concession body to benefit from more favourable conditions of loans on the financial market, which may better ensure the feasibility of the financial plan.

d) Financing by PPP or similar

• The range of contents of a PPP mode is very wide, and it is difficult to establish guidelines because of the scope of possibilities.
• This mode of financing commits public authorities to financial contribution in the long term. Detailed analysis is necessary to evaluate the real advantage of this mode of financing compared to traditional financing. Indeed, this mode of financing very often contributes to increasing the global cost of the project (with equal functionality and quality) because of the compensation of the risk assumed by the developer.
• Public authorities have to carefully define the required functions of the tunnel, as well as objectives concerning quality, comfort, safety, level of service, life span, rate of availability, penalties etc. in order to prevent any ambiguity that may result in important misunderstandings and financial overruns in the development of the project.

Complex underground road networks   

• 1. Introduction
• 2. Case studies
• 3. Particular strategic challenges
• Multimedia Kit

1. INTRODUCTION

The technical report  2016R19EN Road Tunnels: Complex Underground Road Networks reflects investigations carried out on case studies of complex underground road networks. A summary of this report is presented in section 2 below.

Specific recommendations will be published in a second report very soon.

The terminology “Complex Underground Road Tunnels” covers the following infrastructure:

• A sequence of successive tunnels: examples include the analysis undertaken for Prague, The Hague, Oslo and Tromsø;
• Multimodal tunnels: examples include the analysis undertaken for the Hague and Lyon with shared usage between buses, pedestrians, bicycles and trams;
• Tunnels giving access to business and commercial centres (for public access and freight delivery): examples include the analysis undertaken for Helsinki and Paris-La-Défense. These structures usually comprise a multitude of interfaces between numerous operators which represents a significant part of their complexity;
• Tunnels with a dual function as transit and access to underground car parks: examples include the analysis undertaken for Annecy, Brussels and Tromsø;
• Tunnels with reduced vertical clearance: examples include the analysis undertaken for Duplex A 86 in the Parisian region;
• Underground infrastructure with numerous entrances and exits, as well as underground interchanges. This category of tunnels network is identified as the key example of “complex underground road tunnels”  and is the most important in the panel of analysis. 

All the structures share several similar characteristics:

• Complexity,
• Location - essentially in urban and suburban areas,
• Numerous interfaces with other infrastructure or neighbouring networks to which they are connected, thus creating  many interactions between the operators of various infrastructure and networks.

2. Case StudIES

2.1. Objectives and Methodology

The objective of the case studies was to identify structures of this type around the world, to summarise collected information, to analyse it and to establish a number of preliminary recommendations for owners, designers and operators. 

While this collection of information is not exhaustive and the summaries do not constitute a scientific database, it nevertheless contains pertinent and interesting findings. The collection of information was limited to the countries of origin of the Working Group 5 members, wherein the working group had active correspondents available to them. 

The general methodology has been the following:

• Drawing up a detailed questionnaire,
• Surveying through interviews with operators, owners and designers,
• Analysis of the information gathered during the investigation,
• Establishment of summaries,
• Writing up of preliminary recommendations.

At more than 600 pages, a significant volume of information was collected.  Therefore a direct publication of all information has been deemed unsuitable.  The working group decided to:

• Present an overview of the information,
• Establish a monographic sheet for each of the analysed structures (see section 2.5).

2.2. Tunnels investigated

Twenty-seven (27) “tunnel complexes” were analysed. The list is provided in section 2.5 below. Several “complexes” consist of two to four tunnels and the actual analysis reflects a total of 41 individual tunnels. 

The geographic distribution of structures analysed is shown in the graph below :

The European tunnels seem over-represented in the sample analysis. This is due to, 

• a greater precedence of structural planning of this nature in European territories, from a large necessary investment cost (limiting the number of countries that are able to bear the expense); 
• the difficulty of collecting complete information from several countries (outside of Europe) that were initially identified. 

2.3. Summary of Key Information

The key information outlined in the analysis focuses on the following aspects:

• The ‘nominal length’: these lengths span from 400m to 16.4km;
• The overall length of each underground network: these lengths span from 1.1km to 32.8km;
• The year of commissioning: the oldest tunnel of the sample was opened in 1952; the most recent tunnels were put into service in 2014. Of the tunnels investigated, 73% have been put into operation during the last thirty years;
• Traffic volume: the three busiest tunnels have a traffic volume between 150 000 and 160 000 vehicles per day;
• The geographic location of the structures with regard to the number of inhabitants populating the urban area serviced by the tunnel(s);
• Methods of construction: 44% were constructed by cut and cover, 44% by drill and blast, and 12% by TBM or shielding or immersed tube;
• Minimum geometrical characteristics including horizontal and vertical alignment;
• Maximum gradients for ramps on an incline and slopes on a decline;
• The number of underground interchanges or entry and exit ramps: for example, two tunnel complexes consist of more than 40 entrances and exits;
• The lane width: these are in the range of 3.0m and 4.5m with two thirds of the structures having a lane width equal to 3.5m;
• The vertical clearance (free height): these are in the range of 2.0m and 4.8m;
• The lateral elements: emergency stopping bays, sidewalks;
• The speed limit, which is limited to 70 km/h in the majority of structures investigated;
• The nature of traffic: the majority of tunnels investigated prohibit heavy vehicle usage;
• Breakdown and accident rates;
• Annual number of fire incidents;
• The emergency exits and safety equipment;
• The ventilation system;
• The organisation of operations and maintenance.

2.4. Preliminary recommendations

From the analysis of information, the working group established a number of preliminary recommendations. These recommendations will be the subject of detailed additional developments which will be published in Part B of the report at the end of the 2016-2019 cycle.

These preliminary recommendations, presented in Chapter 11 - Present Situation, Comments and Preliminary Recommendations of the report, deal with the following aspects:

Underground road networks are located mainly in urban areas, and their design (in particular their alignment) has several constraints.

Geometric conditions which often contribute to traffic incidents, include: meandering curved alignment, insufficient visibility near the access and exit areas, insufficiently defined characteristics of merging or diverging lanes and, poorly designed exit ramp connections towards the surface road network leading to congestion in the main tunnel, etc. 

It is recommended that in preparing the alignment, the following be considered:

• Not to be limited by a simple geometric approach, linked only to underground and surface land constraints, 
• To implement an overall vision, particularly taking into account the land constraints, the initial traffic conditions, the envisaged evolution of traffic conditions, the operation and safety conditions, the geological, geotechnical and environmental context, as well as the construction methodology and all the other parameters that are specific to the project concerned (see section 3 below).

b - Cross-section

The investigations mentioned above show that 80% of analysed tunnels prohibit the transit of vehicles that weigh over 3.5 tonnes (or 12 tonnes, in some instances). However, the tunnel design does not take into account this restriction, and does not reconsider optimisation of the lane width as well as vertical height clearance. 

Investigations carried out on recent projects show that substantial savings (from 20% to 30% depending on the final design characteristics) can be obtained by choosing a reduced vertical height for tunnels that prohibit heavy vehicle usage. 

It is recommended that at the earliest stage for developing tunnel projects detailed studies be undertaken to consider and analyse the “function” of the tunnel, traffic conditions (volume and nature of vehicles), as well as the financial feasibility and financing methods. This should be done in such a way as to analyse the advantages of a cross-section with reduced geometric characteristics. This may facilitate the financial optimisation of the project without reducing the level of service or affecting the safety conditions.

c - Ventilation

Underground road networks are usually subjected to large traffic volumes. Traffic congestion is frequent, and the probability of a bottleneck developing within the network is high and recurring. As a result, the ventilation system has to be developed with a detailed analysis of the risks and dangers, taking into account the existence of bottlenecks.

A “pure” longitudinal ventilation system is rarely the appropriate sole response to all the safety requirements, especially in the scenario of a fire located upstream of congested traffic. A longitudinal ventilation system will cause smoke de-stratification downstream of the incident location.  This constitutes a danger for any tunnel user blocked or in slow moving downstream traffic. 

The addition of a smoke extraction gallery or the choice of a transverse or semi-transverse ventilation system is often vital if no other realistic or feasible safety improvement measures can be put into place, and considered as efficient.

It is also necessary to implement equipment allowing the different network branches to operate inde-pendently of each other.  This will facilitate the control and the management of smoke propagation during a fire incident. 

The risks associated with dangerous goods vehicles travelling through a tunnel with a high urban traffic density must be carefully analysed. There are no ventilation systems capable of significantly reducing the effects of a dangerous goods large fire in such traffic conditions.

d - Firefighting

The necessary timeframe for response teams to arrive on site must be subjected to a detailed analysis under normal and peak hour traffic conditions. The objective is to determine whether or not it is necessary to install first line intervention facilities and resources in proximity of the tunnel portals.

The turnover of fire brigade staff is relatively high in urban areas and their interventions in tunnels are relatively rare. The high rate of turnover may lead to loss of specialist skills in tunnel intervention. Thus, it is essential to implement tools which allow continuous professional education and training of the teams. A virtual 3D model of the network, associated with simulation software, can provide pertinent, user-friendly and effective tools. 

e - Signage

It is fundamental to ensure clear visibility of the exit ramps and a clear legibility of signage, in order to reduce the risk of accidents where exit ramps diverge from the main carriageway. 

The locations of interchanges, entry and exit ramps, as well as the concept for signage should be analysed from the conceptual alignment studies. 

f - Environment

In order to reduce atmospheric pollution, communities, stakeholders and residents often demand the installation of filtration devices for in-tunnel air before it is released into the atmosphere. 

This results in a decision to install filtration equipment which is rarely rational or technical, but an ad-hoc response to public pressure. Before any decision-making on this issue, it is, however, essential to:

• Carry out an analysis to provide an assessment of the expected actual efficiency with regard to air quality, and compare this to the estimation of investment costs and operational costs (especially energy and maintenance costs) in order to establish a rational and balanced projected report of the technical and financial situation;
• Take into account the progress of the car industry in effecting a reduction in emissions and vehicle pollution and thus limiting the concentration of pollutants. This reduction in pollutant concentration would, over time, lead to the decline in the effectiveness of installed air filtration devices;
• Analyse international experience and identify the reasons why many existing air treatment installations have been removed from service. 

g – Traffic conditions – Traffic management

The connections between exit ramps and the surface network must be equipped in a way which allows supervision and management of traffic in real time. This arrangement allows traffic congestion to be reduced inside the tunnel, and an improvement of safety should tunnel incidents require quick evacuation of users. 

h - Operation

The coordination between operators of physically connected infrastructure is in general adequate. However, it is often essential to improve this coordination by clarifying the situation and role of each operator (particularly in the event of traffic congestion and fire incident) by defining common procedures and determining priorities between the different infrastructure parts and their traffic. 

2.5. Monographs

Monographs have been established for each of the structures listed in the table below. They are accessible in the Multimedia Kit at the bottom of the page. The monographs of the structures highlighted in amber are in the process of being updated and will be online shortly. 

Multimedia Kit

regulations - Recommendations

Countries that have many tunnels are endowed with regulations and have developed recommendations and guidelines for the design, construction, operation, maintenance, safety and the intervention of the rescue services.

Concerning safety conditions in road tunnels, countries belonging to the European Union are subjected to Directive 2004/54/CE that prescribes a minimum level of arrangements to be implemented in order to ensure the safety of users in tunnels longer than 500 m that are part of the trans-European road network. A wider group of European countries are also bound by an international convention, The European Agreement concerning the International Carriage of Dangerous Goods by Road (ADR) and includes specific arrangements for tunnels. Every member country has transposed these European regulations to its own national legislation. Some member countries have implemented additional regulations that are more demanding than the one that results from the transposition of the European regulation.

A list of the regulations and recommendations concerning the operation and the safety of road tunnels has been established in cooperation between the PIARC and the ITA Committee on Operational Safety of Underground Facilities (ITA-COSUF) of the international tunnelling and underground space association (ITA - AITES). This document can be consulted on the ITA-COSUF (Publications) web page. This list is not exhaustive but presents an international panel of twenty-seven countries and three international organisations.

Many countries do not have any regulations relating to tunnels and to tunnel safety, because they do not have road tunnels within their territory. It is recommended that these countries choose a complete and coherent package of the existing regulations of a country with lengthy experience in the field of tunnels, and not to multiply the origins of the documents by dipping into different sources. The recommendations of PIARC, as summarised in the present manual, as well as those of European directive 2004/54/EC also constitute international references that are being applied increasingly often.

 

Under CONSTRUCTION

 

 

 

            THIS PAGE IS CURRENTLY BEING UPDATED

Tunnel traffic capacity

 

 

 

 

 

            THIS PAGE IS CURRENTLY BEING UPDATED

General alignment OF ROADS and national examples

 

 

 

            THIS PAGE IS CURRENTLY BEING UPDATED

Carriageway geometry

 

 

 

            THIS PAGE IS CURRENTLY BEING UPDATED

Emergency exits

 

 

            THIS PAGE IS CURRENTLY BEING UPDATED

Facilities for vehicles

 

 

 

            THIS PAGE IS CURRENTLY BEING UPDATED

Other facilities

 

 

            THIS PAGE IS CURRENTLY BEING UPDATED

Ventilation principles

• 1. Types of ventilation systems
• 2. Ventilation during normal operation
• 3. Fire Scenarios
• 4. Other hazards

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".

Ventilation design and dimensioning

• 1. Choice and design of ventilation systems
• 2. Ventilation capacity for normal operation
• 3. Ventilation capacity for fire scenarios
• 4. Dimensioning of road tunnel ventilation
• 5. Other issues. Complex underground and urban tunnels

The ventilation design process basically includes the choice of the type of ventilation system, the computation of the minimum acceptable capacity of the system in terms of thrust and/or flow rates, the design of the ventilation network and the selection of appropriate ventilation equipment, which should meet a number of specifications, including resistance to fire and acoustic performance.

1. Choice and design of ventilation systems

The choice and design of a ventilation system depends on these main factors:

• Tunnel length, number of tubes, urban or rural
• Fresh air requirement under normal and special traffic situations
• Admissible air pollution around tunnel portals
• Fire safety considerations

The page on "Ventilation principles" provides general information on the different types of ventilation systems that can be found in road tunnels.

 A natural ventilation system can be very effective for the dilution of pollutants (especially for unidirectional tunnels), but it is not possible to rely upon natural ventilation for safety purposes for tunnels over a few hundred meters in length. Because of the number of contradictory design parameters, it is not possible to express universal recommendations about the limits of the natural ventilation.

In most countries, the need for a mechanical ventilation system during normal operation is assessed considering the length of the tunnel and the traffic type (bidirectional or unidirectional) and conditions (possibility of congestion). The same factors determine the requirements for ventilation in emergency situations, especially fire. The presence of other equipment or facilities, emergency exits for example, should also be taken into account.

Generally speaking, two types of ventilation strategies can be found:

• Longitudinal ventilation (LV): With the help of an air-jet placed in the tunnel air column, the flow resistance can be overcome by converting the jet momentum into static pressure. The air jets can be placed at the tunnel entrance (Saccardo), blowing outside air into the tunnel or as ducted fans along the tunnel, each one accelerating part of the tunnel air flow. Jet fans can work in both tunnel directions. (see section IV.2 “Longitudinal ventilation” of the PIARC report 1996 05.02.B “Road Tunnels: Emissions, Environment, Ventilation”)
• Semi-transverse (STV) or transverse ventilation (TV): is mainly applied in bidirectional tunnels or unidirectional tunnels with traffic congestion risk. By distributing the fresh air equally along the tunnel out of a separate fresh air duct, the car emissions are diluted locally along the tunnel. If there a fire in the tunnel, the smoke can be extracted by using a dedicated exhaust duct or reversing the fresh air supply system, thus the need for emergency exits can be reduced. (see section IV.3 “Semi-transverse ventilation” of the PIARC report 1996 05.02.B “Road Tunnels: Emissions, Environment, Ventilation”)

In order to decide between longitudinal ventilation (LV) and transverse ventilation (STV/TV) for a given tunnel situation, these are some of the arguments:

• Initial costs: STV/TV installation with its separate fresh air and exhaust distribution duct and fan station has higher construction costs than a LV, whereas the electromechanical installation costs may be about the same. Depending on the assessment of fire safety, there may be additional costs for emergency exits or smoke extraction duct work with a LV system.
• Ventilation energy: in a bidirectional tunnel the ventilation energy consumption for longer tunnels is less for a STV/TV than for a LV. In a unidirectional tunnel the piston effect of the fluid traffic creates sufficient self-ventilation also in long tunnels, but with congested or bidirectional traffic in such a tunnel the energy consumption is usually larger for LV relative to STV/TV. When pollution requirements lead to tunnel air stacks to control the amount of tunnel air issuing out of a portal, the energy consumption of the stack becomes dominant unless a system is chosen where the ventilation for diluting the exhaust gases in the tunnel can be run independently from the stacks which control the tunnel air outflow.

Further information on the impact of tunnel ventilation in the overall operational costs and energy consumption can be found in the PIARC report 1999 05.06 Road Tunnels: Reduction of operating costs

• Fire and smoke control: a TV, or a STV running in reverse mode, can extract smoke out of the traffic tube, creating a smoke-free space just above the road surface but it might show less capacity to prevent the smoke propagation the spreading of the smoke if longitudinal air flows cannot be controlled. A LV can push the smoke to one side of the fire, with little back-layering taking place, but it might not prevent filling up with smoke the whole tunnel section downstream of the fire until an extraction shaft or the portal is reached.
• Environmental protection: with a STV/TV in a bidirectionally used tunnel, there is tunnel air pollution at both portals, whereas with a LV this can be limited to one portal only.
• Long Tunnels: Maximum admissible longitudinal velocities have to be observed, which limits the use of LV and also STV for very long tunnels.

Nowadays, the choice between the different ventilation system choices is mostly guided by fire safety considerations although environmental aspects are gaining importance during the decision taking process. In sections V.7 “Recommendations on longitudinal ventilation” and V.8 “Recommendations on transverse and semi-transverse ventilation” of the 1999 PIARC report 1999 05.05 "Fire and Smoke Control", relevant design criteria and description of the main limitations can be found.   

PIARC report 2007 05.16 "Systems and equipment for fire and smoke control in road tunnels" analysed additional aspects related to this type of systems. Thus, section "4.4 Longitudinal ventilation" includes criteria and guidelines for the design and testing of this type of system, including considerations regarding the sound levels to be reached in the tunnel environment ("4.4.2 Sound Impact of Jet Fans in a Tunnel").

Furthermore, the design of ventilation systems has to cover other project-wide topics including availability, durability, maintainability or reliability. Some guidance and considerations on life cycle cost for ventilation systems can be found in the PIARC report 2012 R14 "Life Cycle Aspects of Electrical Road Tunnel Equipment".   

2. Ventilation capacity for normal operation

The section on "Ventilation during normal operation" of the page on "Ventilation principles" presents some general background information of interest in relation to normal operation in road tunnels.  

The design of a road tunnel ventilation system must consider fresh-air demand for maintaining in-tunnel air quality during normal and congested traffic operations and the control of smoke and hot gases in case of fire. The ventilation capacity to manage a fire incident frequently drives the ventilation sizing in highway and non-urban tunnels. Nevertheless, the fresh-air requirement for dilution during normal and congested operation, or special environmental constraints, can be dominant in tunnels with high traffic volumes and frequent congested traffic.

The ventilation capacity for normal operation is defined by the air demand required to dilute vehicle emissions to maintain allowable in-tunnel air quality values.

The fresh-air demand (airflow rate) is determined by the allowable increase of emission concentrations within the airflow. Air enters the tunnel environment with ambient pollutant concentrations. As this air moves through the tunnel environment, tailpipe emissions increase the pollutant concentrations. The vitiated air must then be diluted within the tunnel environment prior to reaching admissible pollutant limits. Emission concentrations within the tunnel are the product of the emission rates and the inverse of the airflow.

The required amount of fresh air for a given traffic situation in the tunnel depends on the number of cars in the tunnel, the average emission per car in this traffic and the admissible concentration for this particular emission.

Several PIARC reports dealing with the topic of design and dimensioning of tunnel ventilation for normal operation have been published during the last decades. The PIARC report 1996 05.02 “Road Tunnels: Emissions, Environment, Ventilation" defined a method for the calculation of the fresh-air demand and to provide emission rates for tunnel ventilation design, and in addition some general and specific information which may be useful when designing a longitudinal or semi-transverse ventilation system were provided.

However, due to the continual renewal of the vehicle fleet, a steady tightening of emission laws and the introduction of alternative propulsion systems (hybrid vehicles, electric cars, etc.) design emissions data required constant updating. As a result, new versions of the 1996 report were published in 2004, 2012 and the latest in 2019 (see PIARC report 2019 R02 “Vehicle emissions and air demand for ventilation”), which provide new methods for the calculation of the fresh-air demand and emission rates on an international basis.  

The emission rate is a function of several factors including:

• the number and type of vehicles (PC, LCV, HGV),
• the emission standard the vehicle was registered under (e.g. Euro 4),
• vehicle speed which includes congested or fluid traffic,
• road gradient,
• other parameters influencing the power needed to propel the vehicles (e.g. weight).

A detailed description of the method for the calculation of the emission generation rate (section 5) and the latest version of the database can be found in the 2019 emissions report. For practical purposes an electronic format excel sheet containing the emissions data can be downloaded from the PIARC virtual library. 

3. Ventilation capacity for fire scenarios

The section on fire scenarios of the page on "Ventilation principles" presents some general background information of interest in relation to emergency scenarios in road tunnels.

The ventilation capacity required by normal operation may not be sufficient to fulfil the requirements of smoke handling. In addition, the airflow required to achieve a sufficient control of the smoke depends on the fire size.

With the objective of defining the ventilation capacity, the design fire (defined as a Heat Release Rate or HRR, in MW, as a function of time) provides the fire characteristics that are used to establish the sizing of equipment in tunnels and the scenarios to consider when developing emergency response plans.  

The choice of a design fire depends on the type of traffic allowed in the tunnel. Following the 30 MW design fire peak value recommended in the PIARC report 1999 05.05 "Fire and Smoke Control", the PIARC report 2007 05.16 "Systems and equipment for fire and smoke control”  restated the previously established peak heat release rates for different types of vehicles and also discussed the fire growth in the context of emergency exits.

Lately, the PIARC report 2017 R01 "Design fire characteristics for road tunnels" gave some direction to the choice of a design fire from a life-safety perspective. The report provides information on tests and the events that have brought the topic into question and also summarises the criteria for the design fire that are currently adopted in different countries.

In addition, the report presents the different approaches currently adopted in most countries for the choice of design fires in road tunnels.

Prescriptive design involves the application of a design fire given by a code or standards which may vary depending on the traffic type, density and tunnel length and location, and the designer or authority would choose the appropriate value for a given case. Section 2.1 "Summary of practices adopted in different countries" of the PIARC Report 2017 R01 "Design fire characteristics for road tunnels" summarizes the typical design-fire assumptions used in different countries.

In a performance-based approach a process of design assessment will establish levels of risk that are acceptable. The starting point may be the prescriptive values adopted, modified in the light of mitigation measures and acceptable risk levels.

Between these two approaches, there are intermediate options that allow a degree of performance-based design on the basis of prescriptive guidance.

The influence of Fixed Fire fighting systems in the definition and choice of a design fire is discussed in the PIARC report 2016 R03 "Fixed Fire Fighting systems in road tunnels: Current practices and recommendations".

As a conclusion, an exact universal design fire cannot be specified; indeed to do so would be inconsistent with the known variability and probability of differing fire sizes in tunnels. However, once it is defined for each specific case, the ventilation capacity for fire scenarios can be accordingly evaluated, with different considerations depending on the type of ventilation system.

Longitudinal ventilation

Longitudinal ventilation systems induce a longitudinal flow along the axis of the tunnel, and this provides an efficient smoke-management system as long as the tunnel is occupied only on one side of the fire, thus assuming that traffic downstream can proceed out of the tunnel. Smoke is blown toward the unoccupied side, so that egress can be carried out in the upwind direction.

This is achieved when the longitudinal ventilation is conducted at a velocity of at least the critical velocity. Too low a velocity would result in smoke propagation upstream of the fire (i.e. back-layering).

Transverse ventilation

Recent developments in semi-transverse smoke extraction, aim at limiting the smoke spread on both sides of the fire. This enables egress away from the fire on either side. This method is essential in case of rescue from a fire in a tunnel with bi-directional traffic or traffic congestion. Ideally, the extraction system should cause air to flow in the traffic space, towards the fire on both sides, as shown in the figure. This confines the smoke region and increases the efficiency of extraction.

Some systems employ remotely controlled dampers that enable point extraction of smoke near to the fire. The construction costs for an extract system are higher than for longitudinal systems and since the required duct size increases with the heat-release rate, a larger design fire has an impact on the resulting investment costs.

Additional information on the implications on the design fire of the smoke-management can be found in chapter 3 of the PIARC report 2017 R01 "Design fire characteristics for road tunnels".

4. Dimensioning of road tunnel ventilation

Once the type of ventilation system has been chosen and the ventilation capacity (in terms of airflow required) is determined, dimensioning of tunnel ventilation must allow the definition of the equipment precisely to deliver the required ventilation capacity both for normal operation and fire scenarios.

In general, ventilation sizing for longitudinal ventilation consist of the calculation of the required thrust for the jet fans or Saccardo nozzles, while for transverse ventilation the size of the exhaust and/or supply ducts and the air flow, pressure and electrical power of the associated axial or centrifugal fans must be estimated.

More detailed information on the ventilation equipment characteristics can be found on the page "Tunnel Ventilation System" included on the general page on "Equipment and systems".

The following general and specific information may be useful when dimensioning a longitudinal or semi-transverse ventilation system.

Longitudinal ventilation

With the help of an air-jet placed in the tunnel air column, the air flow resistance can be overcome by converting the jet momentum into static pressure. The air jets can be placed at the tunnel entrance (Saccardo), blowing outside air into the tunnel or as ducted fans along the tunnel, each one accelerating part of the tunnel air flow.

Basic formulae were given in section IV.2 "Longitudinal ventilation" of the PIARC report 1996 05.02 “Road Tunnels: Emissions, Environment, Ventilation” for sizing jet fan systems (thrust and number)  used in longitudinal ventilation for normal operation, considering many factors such as the vehicle piston effect, meteorological counterpressure, resistance of the walls, etc. Considerations in jet fan design included the influence of fire on the jet fan system, the meteorological effects at tunnel portals (mainly wind), the optimal placement of the fans, the efficiency of the fans, and the sound levels created by the fan system.

PIARC report 2007 05.16 "Systems and equipment for fire and smoke control" included additional considerations to evaluate the influence of fire on a jet fan system, providing further recommendations for fan distribution along the tunnel. Of particular interest for the dimensioning of longitudinal ventilation systems is the example calculation in section 12.3.3. "Jet Fan calculation procedure", which provides detailed information on the procedure to size longitudinal ventilation systems.

Transverse ventilation

To size a transverse ventilation system for fire scenarios, two aspects must be considered:

• the smoke flow rate
• the requirements to control the longitudinal air flow.

Basic formulae are given in section IV.3 "Semi-transverse ventilation" of the PIARC report 1996 05.02 “Road Tunnels: Emissions, Environment, Ventilation” for estimating the total pressure loss along the distribution duct and ventilation stations during normal operation.

The PIARC report 1999 05.05 "Fire and Smoke Control", provides a table (see table 2.4.3) and the relevant relationship between heat release and smoke flow rates. In addition, in section V.8 “Recommendations on transverse and semi-transverse ventilation" criteria are provided for the sizing of ventilation systems of this type, including extraction capacity, the differences between distributed or concentrated extraction systems and the influence of the fresh air supply on the behaviour of the fumes.

PIARC report 2007 05.16 "Systems and equipment for fire and smoke control" included additional considerations for the choice of the ventilation equipment (see section 12.4 “smoke dampers”).

5. Other issues: Complex underground and urban tunnels

In comparison with conventional tunnels, specific design criteria that might have a significant impact on the design of the ventilation systems have to be considered in the case of urban and complex network tunnels.

The design of urban and complex tunnel ventilation systems strongly depends, among others, on the following factors:

• Changes in the tunnel cross section
• Interchanges with other tunnels, including with other transport means
• Low clearance
• Unavailability of ground spare space in urban areas
• Environmental impact
• High traffic volume

Further information on this topic can be found in chapter 6 of the PIARC report 2016 R19 “Road Tunnels: Complex underground road networks” and in the PIARC report 2008 R15 “Urban road tunnels - Recommendations to managers and operating bodies for design, management, operation and maintenance”.

Ventilation control and monitoring

There are two principal aims of a well-designed ventilation control system:

• in routine operations, to provide fresh air at a rate that is consistent both with the comfort of the tunnel users, and with economic operations i.e. at the minimum rate for an acceptable level of air quality.
• in exceptional circumstances or emergency cases (equipment breakdowns, accidents or fire in the tunnel), the ventilation system must be capable of responding quickly and reliably to each specific ventilation demand.

It is therefore important to bear in mind that the objectives of the ventilation system and the associated control system are different depending on the situation of the tunnel.

Normal operation

During a normal operating situation, the levels of pollutants in the tunnel may increase (depending on the traffic conditions and the natural ventilation of the tunnel) until it becomes necessary to activate the mechanical ventilation, in which case it can be done either automatically or manually.

In order to obtain information on the levels of pollutants, carbon monoxide and visibility sensors are usually available in the tunnel, although the installation of other types of sensors such as nitrogen oxides is becoming increasingly common.

Originally, the measurements of contaminants in the tunnel were used to perform the actions on ventilation manually. In some tunnels with a low level of supervision, these actions are implemented in the tunnel monitoring and control system that carries them out automatically through a series of predefined algorithms or sequences.

However, nowadays, most tunnels have automatic ventilation control systems whose objective is not only to guarantee the required air quality levels, but also to achieve this efficiently by minimising energy consumption and equipment maintenance needs (see section 4.3 "Actions to reduce tunnel operating costs" of the PIARC report 2017 R02 "Road Tunnel Operations: First Steps towards a sustainable approach").

Where routine ventilation is concerned, an optimal air flow rate is one that satisfies two conflicting requirements; the rate of ventilation must be sufficient to dilute the pollutants generated by the vehicles, while at the same time the air flow quantities should be as small as possible in order to reduce the energy consumption at the fans and therefore reduce running costs.

The optimisation of ventilation control for air quality considerations during normal operation is crucial to reduce energy consumption. This is an important issue since this consumption represents a significant part of the operational cost of a tunnel.

The constant adjustment of air flow to cope with the needs is a difficult problem especially in the case of long and complex tunnels where the control of the ventilating airflows can be difficult to maintain.

Chapter IV "Ventilation control" of the 2000 05.09.B. PIARC report “Pollution by nitrogen dioxide in road tunnels” report describes some of the criteria usually considered and the most common criteria.

Emergency ventilation

Emergency ventilation, on the other hand, needs fast and well-targeted interventions, short response times, and a well-defined sequence of all the operations. The objectives of incident ventilation are therefore quite different from those of normal ventilation, and economic considerations are no longer the principal concern.

In a fire situation, the actions on the ventilation are normally associated with automatic detection systems or with supervision from the control centre, which allow the triggering of the action sequences for the pre-defined ventilation. Thus, ventilation control systems are one more tool in the set of tools available to mitigate the consequences of a fire situation. 

For this reason, ventilation control must be considered with a global approach that takes into account the strong link between the emergency services' action procedures, the actions of the control centre operator and the tunnel results in terms of smoke behaviour. A general discussion of the importance of taking ventilation into account in the definition of fire response plans can be found in section VIII.4.1 "Fire response planning" of the 1999 PIARC report 05.05 "Fire and Smoke Control in Road Tunnels".

The design of appropriate ventilation control scenarios for each possible fire situation is a very important part of the process: see PIARC technical report 2011 R02 "Road tunnels: Operational strategies for emergency ventilation". These scenarios can be simple, especially when the purely longitudinal strategy is applied, or involve a large number of measurement and ventilation devices in complex, transverse-ventilated tunnels or in tunnels with massive point extraction for which the control of the longitudinal airflow can be necessary.

There are numerous ways and strategies to approach the design of a fire ventilation control system which depend on multiple factors such as the degree of supervision, means of detection, ventilation strategies and the type of system.

Firstly, its design must take into account the expected evolution of the incident, the different stages that occur throughout the emergency and the influence of ventilation on the behaviour of the fumes. Although in some cases the actions on the ventilation equipment are simple and do not require complex control systems, in many cases it is necessary to take into account sophisticated criteria.

Today, the development and implementation of ventilation control systems in case of fire represents an important part of ventilation design, in which the definition of the necessary control algorithms is particularly important and it is crucial to be able to guarantee the quality and reliability of field measurements (speed and smoke detection sensors).

The chapter "Response of ventilation control systems to fire" of the PIARC technical report 2011 R02 "Road tunnels: Operational strategies for emergency ventilation" provides detailed information about the challenges and needs of control systems for ventilation management and current experiences in this field.

Economic issues

A tunnel creates wealth. On a national level, the main part of this wealth creation is due to reduced travelling time, notably for goods transport. It should be noted that the economic benefits of investment in road tunnels are a very complex matter and mostly related to subjective evaluation by decision makers.  However, the economic arguments behind the decision to build a tunnel will largely consider the benefits of doing so against the costs.

The potential benefits from constructing a tunnel may consider the following range of issues:

• Road user benefits – due to change in travel time and vehicle operating costs
• Journey time reliability benefits – changes in the journey time reliability of the network.
• Wider economic benefits – job and housing support, potential for regeneration, agglomeration economics.

The decision-making evaluation behind the costs for installing a new tunnel will need to consider three main elements: investment, operating and maintenance costs. Construction and maintenance costs will need to consider the impacts on road user travel time and vehicle operating costs during scheme construction and for maintenance.

During the construction phase, it is necessary to ensure that all the planned technical specifications are implemented and that the identified objectives are attained. Finally, it is important to be particularly vigilant with regards to financial aspects, so as to ensure that the cost of works remains within the overall allocated budget.

Environmental issues

The primary aim of environmental protection is to reduce the impact on air, water and ground to a long-term acceptable level.

The benefits of the construction of a tunnel should consider the greenhouse gas benefits arising from reduction in travel time and vehicle operating costs.

The preservation of the species living around the tunnel may require the set-up of special measures. These measures may be aimed at restoring passageways for certain species or preserving reproduction areas. In some cases, the position of certain technical facilities may be modified (ventilation units, extraction shafts, etc.). 

In order to avoid the exceptional presence of these species on the roadway or in the tunnel (which can present significant risks for drivers and for the species in question) specific measures like fencing or enclosures may be necessary.

The use of natural resources should be carefully examined in the design phase, if possible by favouring materials which have the lowest carbon footprint, or by recycling materials already used elsewhere. Also, to minimize the use of energy resources, the design should take into account the energy consumption during both the construction phase (adapt the design to less energy-intensive construction methods) and the operational phase of the tunnel.

With regard to the three components of the environmental pillar (conservation of species, resources and energy), the construction phase has considerable impacts. A tunnel should be built in such a way that the impact on the environment is as low as reasonably practicable. Selected suppliers must:

• apply a sustainable process for building material production and means of transport,
• reduce the distance required to transport building materials to the site,
• use energy-friendly equipment, etc.

All the actions planned for the conservation of animal species present in the vicinity of the works or in the vicinity of the finished tunnel must be undertaken in strict compliance with the environmental specifications outlined during the study phase.

Societal issues

The impact of a tunnel on housing conditions and health may be positive for the residents who will no longer suffer from sound nuisances as they will disappear with the construction of the tunnel. It may sometimes be negative for people living near the portals of the tunnel (if the tunnel was badly designed) who will be subjected to increased sound nuisances or higher pollution levels. The installation of noise barriers could therefore be considered a measure for sustainable road tunnel operations. 

In addition, we need to examine the impact that the road tunnel may have on the economic attractiveness of areas which previously had poor access conditions.

The points mentioned above on economic appraisal should also be viewed through the lens of social benefits or costs. For instance, by creating new links with a tunnel the journey time for someone commuting to work could be greatly improved and this would yield not just economic but social benefits – for instance with greater time with family and friends through a reduced commute time.

Other aspects for consideration for sustainable options could be feasibility for walking and cycle ways with measures such as dedicated cycle ways that could be used by pedestrians also being built into the design at the outset.

From a social point of view, the construction phase can have quite different effects: either positive or negative.

For inhabitants in the vicinity of the works site, it is clear that the works can pose a nuisance (traffic disruptions, noise, dust, etc.). Suitable measures must therefore be taken to reduce such forms of nuisance, so that residents are disrupted as little as possible.

On the positive side, it is during the construction phase that the impact on employment is the greatest. Tunnel construction requires considerable manpower over a long duration, especially if the size of the tunnel is substantial. This manpower is not necessarily local, but more often than not a considerable part of this manpower is hired near to the works site. Moreover, non-local manpower has an indirect impact on the local economy (hotels, restaurants, etc.).

Energy consumption

The main energy-consuming devices in a tunnel during normal operation are: 

• Lighting;
• Sanitary ventilation;
• Safety devices (signalling, closed circuit television CCTV, etc.);
• Pumping (in subsea tunnels or when there is water seepage).

The respective share of each of these energy-consuming systems varies greatly, depending on the specific characteristics of the tunnel: length, gradient, water ingress, etc. Let us consider, for example, the case of a short tunnel: it will not be ventilated (therefore the ventilation element is removed), but it will be lit with entrance zones covering almost its entire length and, due to its shortness; lighting will be a major energy-consumption factor. In contrast, for very long tunnels, the energy-consumption of lighting will be low compared with the energy-consumption of the ventilation system.

In relation to energy expenditure, the first thing that an operator can do, for any given energy requirement is to play competitors off against each other by consulting several suppliers that provide the kind of electricity to be used (renewable energy). This approach assumes that the installation is optimized in terms of the power installed and the operating times of the various pieces of equipment. 

In effect, we have seen that energy expenditure is closely linked to two factors: the power installed per family of equipment and each family of equipment’s operating time.

For each family of equipment, the installed power is assessed during the study phase and is fixed during the implementation phase. Once the structure is operational, the power can mainly be changed during renovation. At this time, it may be decreased if the regulations haven’t changed and if the energy performance of the replacement equipment has improved. It may be increased if regulations have become stricter (for example: greater smoke extraction capacities). 

Basically, outside of renovations, if an operator wants to reduce its expenditure on electricity, it can only do so by optimizing the operating times of the installed equipment and by monitoring peak hours.