ROAD TUNNELS MANUAL
STRATEGY AND GENERAL DESIGN
Strategic issues
Tunnel: a complex system
Steps of tunnel life
General design of the tunnel
Tunnel renovation & upgrading
Costs & financial aspects
Complex underground networks
Regulations - Recommendations
Construction and
Geometry
Construction
Tunnel traffic capacity
General alignment
Carriageway geometry
Emergency exits
Facilities for vehicles
Other facilities
Ventilation Concepts
Ventilation principles
Design and dimensioning
Control and monitoring
Sustainability Issues
Economic issues
Environmental issues
Societal issues
Energy consumption
Document written by the "Road Tunnel Operations" technical committee
Version 2.1 - 01/10/2022
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STRATEGY AND GENERAL DESIGN
This "Road Tunnels Manual" comprises 4 parts. The first part, Strategy and General Design, considers the
strategic and design aspects and comprises the following chapters.
The chapter on Strategic Issues presents the principal strategic elements which any decision maker must
take into account before making a decision concerning the selection or design of a tunnel. This chapter is
particularly aimed at the decision-makers and tunnel designers of countries that are starting to tackle the
construction or major refurbishment of a tunnel.
The chapter on Construction and Geometry reviews the main elements to be considered during the
general design of a tunnel.
The chapter on Ventilation Concepts presents the principles and design of tunnel ventilation, as well as its
control and monitoring systems.
The chapter on Sustainability Issues examines the main economic, environmental, societal and energy
issues that should be taken into account during the tunnel design and operation phases.
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STRATEGIC ISSUES
Tunnels, initially aimed at crossing an obstacle (in general a mountain), have become increasingly complex
during recent years, incorporating increasingly complex equipment (including ventilation systems) and
methods of operation. Such operation includes the deployment of control and supervision systems that
enable the handling of vast amounts of data, and which can accommodate increasingly sophisticated
management scenarios.
Figure 1: St Gotthard tunnel fire
Following the catastrophes in the Mont Blanc, Tauern and Gotthard (Figure 1) tunnels in the years 1999
and 2001, the need to adopt a holistic approach to tunnel safety was recognized. This resulted in stricter
provisions from the tunnel design stage onwards, which have an important impact on specific civil
engineering and tunnel equipment requirements.
In general, tunnels are considered as "expensive and risky" works, both with regard to their construction
as well as their operation. This "image" makes some countries very reluctant to embark upon the
construction of their first or railway tunnels. In order to address such concerns, increasing importance is
being given to risk management (including construction and operation costs), accident or fire mitigation
during operation and the optimisation of the tunnel facilities at the design, construction and operation
stages. This risk and cost management is reinforced through current procurement and financing models
for the construction of tunnels, which are increasingly being implemented as "Concession", "Design and
Build" or "Private Public Partnership" models.
The chapter entitled Strategic Issues aims to:
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make the reader aware that a tunnel is a "complex system";
highlight the importance of defining a facility’s "function" at both the upstream (design) and downstream
(operational) stages;
draw the attention of the tunnel owner to the need for multidisciplinary competencies with sufficient
skills and in-depth experience to ensure the success of the mission;
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make the reader aware that a tunnel is essentially designed to be used in conditions of comfort and
safety, and that it must undergo continuous and reliable maintenance by the operator. The concept of a
tunnel must take into account these safety and operational objectives and constraints;
make the reader understand that the facility itself constitutes only a part of the problems which the
owner will have to solve, and that very often it will be necessary to develop certain external elements in
parallel, which may be outside the tunnel owner’s authority: regulation, intervention and safety services,
procedures, etc.
This chapter is not designed to be a detailed handbook of the actions required by tunnel owners, or to
specify the technical provisions to be implemented by designers, or to determine the tasks to be
undertaken by operators to ensure a safe and comfortable tunnel environment. In particular, this chapter
does not aim to be a design handbook. Its main objective is to make the reader aware of certain issues, in
order to facilitate comprehension of this complex field, to raise awareness of the many possible errors in
tunnel operations, and to have a better understanding of optimisation possibilities.
The page entitled Tunnel: a complex system, explains why a tunnel constitutes a "complex system" and
lists the main interfaces between Civil Engineering, Ventilation and Safety aspects;
The page Steps of tunnel life analyses the various stages of a tunnel’s life cycle and underlines the key
actions of each of these phases;
The page General design of the tunnel (new tunnel) presents the major elements which have to be
considered when designing a new tunnel;
The page Renovation - Upgrading of existing tunnels concerns the upgrading and the refurbishment of
existing tunnels under operation;
The page Costs of construction - Operation - Upgrading - Financial aspects explains issues relating to
construction, operation and renovation costs, as well as the main issues specific to the modes of financing;
The page Complex underground road networks highlights the special features of complex underground
and interconnected structures and provides case studies of complex tunnels via numerous monographs.
The page Regulations - Recommendations gives a list of the main recommendations, directives and
regulations published by various countries in Europe and elsewhere in the world.
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TUNNEL: A COMPLEX SYSTEM
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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:
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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.
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Fig. 1 : Diagram of main subsets of the "complex tunnel system"
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:
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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:
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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.
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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:
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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:
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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:
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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.
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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
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engineering:
TABLE 1 : MAIN PARAMETERS ACCORDING TO CIVIL ENGINEERING
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The first column of the table indicates the principal sets of parameters,
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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:
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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:
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For a short bored tunnel or a medium-length bored tunnel with one-way traffic, the most probable
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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
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4. Subset "Operation equipment"
They do not constitute fundamental parameters for the definition of the functional section, with the
exception of:
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box-outs and sleeves for the passage of cables, pipes for water supply to the fire fighting system,
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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:
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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:
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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,
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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:
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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:
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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”
Fig. 2 : Factors affecting 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:
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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.
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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.
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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
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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:
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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.
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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:
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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:
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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.
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STEPS OF TUNNEL LIFE
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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 :
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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:
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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:
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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:
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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:
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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:
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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.
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GENERAL DESIGN OF THE TUNNEL (NEW
TUNNEL)
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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
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of tunnels:
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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:
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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
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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.
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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:
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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:
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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
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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.
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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:
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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.
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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:
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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:
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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 (crosspassages, 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:
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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: crosspassages - 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.
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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 :
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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:
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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:
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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:
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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.
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3.6. COMMUNICATION WITH THE USERS – SUPERVISION
Communication with users has an important impact on the "functional transverse profile" through
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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":
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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
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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:
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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.
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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.
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The supply of electric energy and its distribution inside the tunnel must provide:
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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.
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The arrangements usually implemented for the electrical power supply are as follows:
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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.
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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:
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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.
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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.
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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:
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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:
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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:
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automatic facilities in the technical rooms and underground substations,
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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:
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●
●
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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 smokefilled 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.
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RENOVATION – UPGRADING OF EXISTING
TUNNELS
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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:
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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
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a double objective:
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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:
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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:
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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.
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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:
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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:
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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.
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COSTS OF CONSTRUCTION, OPERATION,
UPGRADING - FINANCIAL ASPECTS
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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:
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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.
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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:
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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,
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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:
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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.
Fig. 1: Breakdown of construction costs
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).
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3. OPERATION COSTS
The operation costs of a tunnel may be broken down into three types of cost:
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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).
Fig. 2: Breakdown of the costs during a 30-year 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).
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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:
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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.
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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.
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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
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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.
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D) FINANCING BY PPP OR SIMILAR
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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.
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COMPLEX UNDERGROUND ROAD NETWORKS
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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:
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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:
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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
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correspondents available to them.
The general methodology has been the following:
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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:
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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 :
Fig 1 : Distribution of tunnel complexes within the case study and detailed distribution in Europe
The European tunnels seem over-represented in the sample analysis. This is due to,
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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:
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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
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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:
a - Geometry
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:
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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
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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 independently 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
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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:
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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.
TABLE 1 : LIST OF ANALYZED "COMPLEX TUNNELS "
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Continents
Countries
Cities
Names of the tunnels complex
Appendices
Changsha
Yingpan Tunnel
1-1
Chongqing
Underground Ring Road of Jiefangbei CBD
1-5
Chiyoda
1-2
Yamate
1-3
Shinlim-Bongchun and Shinlim-2
1-4
China (CHN)
Asia
Japan (J)
South Korea (ROK)
Tokyo
Seoul
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Continents
Countries
Austria (A)
Belgium (B)
Czech Republic (CZ)
Finland (FIN)
France (F)
Cities
Names of the tunnels complex
Appendices
Kaisermühlen
2-1
Leopold II
2-2
Belliard
2-3
Blanka Tunnel complex (3 tunnels)
2-4
Mrazovka and Strahov
2-5
Helsinki
KEHU - service tunnel
2-6
Annecy
Courier
2-7
Ile-de-France
Duplex A 86
2-8
Lyon
Croix-Rousse (road tunnel + multimodal tunnel)
2-9
A14 / A86 motorway interchange
2-10
Voie des Bâtisseurs
2-11
Vienna
Brussels
Prague
Paris La Défense
Germany (D)
Düsseldorf
Kö-Bogen Tunnel
2-21
Italy (I)
Valsassina
Valsassina tunnel
2-12
Monaco (MC)
Monaco
Sous le rocher tunnel
(2 interconnected tunnels with “Y” form layouts)
2-13
Oslo
Opera tunnel (chain of 4 tunnels)
2-14
Tromsø
3 interconnected tunnels with roundabouts
and access to parking lots
2-15
M30 By-pass
2-16
M30 Rio
2-17
AZCA Tunnel
2-22
Cuatro Torres Tunnel
2-23
Ring Road – Northern link
2-18
Ring Road – Southern link
2-19
Sijtwendetunnel (chain of 3 tunnels)
2-20
Europe
Norway (N)
Spain (E)
Sweden (S)
The Netherlands
(NL)
Madrid
Stockholm
The Hague
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Continents
Countries
Canada / Quebec
(CDN) / (QC)
North
America
Cities
Ville-Marie and Viger tunnels
3-1
Boston
Boston Central Artery
3-2
Seattle Interstate 90 Mt Baker Tunnel
3-3
SR 99 Alaskan Way Viaduct Tunnel through
Seattle
3-4
M7 Clem Jones Tunnel (CLEM7)
4-1
USA
Australia (AUS)
Appendices
Montreal
Seattle
Oceania
Names of the tunnels complex
Brisbane
3. PARTICULAR STRATEGIC CHALLENGES
“Underground Road networks” are “complex systems”. All the recommendations presented in the 5
first pages of Chapter "Strategic issues" are applicable to them. Nevertheless, certain “subsets” and
“parameters” mentioned in the page "Tunnel: a complex system" present a much more significant
potential impact on underground networks. The “interactions between parameters” (see its section 2.2)
are generally and much more extended and complex.
Several major strategic challenges presented in the above references, as well as their principal
interactions, and the additional parameters below, must be well considered in the process of developing
tunnel designs and for the construction and operation of tunnels.
3.1. GEOMETRY
This term is applicable to tunnel cross-section, vertical alignment, implementation of interchanges, access
and exit ramps. In addition to the recommendations from section 1 of page "General design of the tunnel
(new tunnel)" the following elements should be considered for:
a – Land occupation
Land occupation deals with the surface occupation in open air (roads, buildings and various structures,
parks and protected areas, etc.) and the volumetric occupation of the underground space (underground
infrastructures such as metro, car parks, various networks, building foundations, etc.)
The interfaces between the underground and surface spaces are numerous: ventilation stacks, access and
exit ramps, evacuation corridors and intermediate emergency access.
The underground and surface land occupation constraints are not always compatible with a given location
and it is often necessary to decouple surface structures from those underground. This relationship can be
implemented through inclined shafts or underground corridors that link any vertical shafts that are located
away from the tunnel alignment.
b - Geology, geotechnical, hydrogeology
The geological, geotechnical and hydrogeological conditions have a significant impact on the horizontal
and vertical alignment especially with regard to the risk of settlement, the possibility of construction
underneath existing structures and any required maintained distances to existing surface or underground
structures, in relationship with the construction methodology considered.
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These conditions can also influence the position of underground interchanges. For example, in the case of
loose soil below groundwater level a localised widening of the cross section to build ramp merge and
diverge areas could require construction works starting from the surface (large shafts, treatment and land
consolidation works). These works require setting up temporary occupation on the surface. Under such
conditions the location of underground interchanges should then also consider the type of land occupation
on the surface.
c - Functionality for traffic
The functionality of the alignment mainly deals with areas where connection to the road network at the
surface (or possibly with other underground structures) has to be built. The position and the design of the
main tunnel portals, the access and exit ramps, as well as the location of interchanges depend on these
functionalities.
The location of all these connections is also linked to the volume of traffic in the underground network, as
well as its multiple entrances and exits. The connections must take into account the absorption capacity of
traffic in the surface road network, adjustments to connections design in order to avoid underground traffic
congestion and thus reduce accidents and significant tunnel fire incident risks.
d - Safety – Risks of accidents
The analysis of existing networks demonstrates a concentration of accidents around areas with curved
geometry, overly steep slopes and insufficient visibility around the merge and diverge areas of ramps.
All these elements must be carefully taken into account from the early stage of the design of the horizontal
and vertical alignments of a new network.
e - Methods of construction – Time period
The construction methodology has a direct impact on the horizontal and vertical alignments (and viceversa). They are also strongly guided by the geological, geotechnical and hydrogeological conditions.
The methods of construction can have an important impact on the location of the tunnel portals. In
particular, the use of a shield (slurry shield or earth pressure balanced) requires significant site area not
only for the assembly of a tunnel-boring machine but also throughout the duration of the works
(particularly for the treatment of slurry and provisional storage). A conventionally bored tunnel (when soil
conditions permit it) requires fewer facilities close to the portal, and can be accommodated in a smaller
site area.
The analysis for the shortening of construction timeframes can have an impact on the horizontal and
vertical alignments, for example in order to make possible intermediate construction access sites.
f – Environmental conditions
During the operation period of the network, the main concerns are air quality and noise impacts. These
concerns have repercussions on the positioning of tunnel portals and ventilation shafts. These issues must
be analysed carefully, in particular the ventilation plants as well as the additional equipment likely to
reduce the environmental impact.
The position of portals, and the associated temporary work site plants, must also be analysed from an
environmental aspect in terms of construction methods and timeframes. For example, a conventional
method of construction will have a more significant noise impact as opposed to a TBM construction
method. If the tunnel portal is situated in a noise sensitive area, works will have to be suspended during
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quieter night periods, leading to a prolonged construction period and consequent inflation of costs. A
modification of the portal location or changes to the alignment can reduce these impacts.
3.2. CROSS-SECTION
In addition to the recommendations from section 2 of the page "General design of the tunnel (new tunnel)"
the following elements should be considered for:
a – Nature of traffic - Function
As mentioned in section 2.4.b above, the nature of traffic is a factor that must be carefully analysed
regarding their initial conditions as well as its evolution over time. Many urban underground networks
prohibit heavy vehicles (more than 3.5 t or 12 t depending on different conditions), even though they were
designed with standard vertical height clearance and lane width characteristics (defined for the allowance
of all types of vehicles).
Analysis of the “function” of the underground network and the evolution of that function is essential. It
allows the cross-section to be optimised by choice of geometrical characteristics (vertical height clearance
and lane width) to ensure adequacy for the present and future traffic that will use the network.
Savings made regarding construction costs are significant (from 20% to 30% depending on the chosen
characteristics). Where applicable, these savings may allow a project to be financed, and thus feasible,
where it may not have been with standard vertical clearances and lane width.
b - Volume of traffic
The volume of traffic is the determining factor in defining the number of lanes of the main tunnel, as well
as interchange or access and exit ramps.
The volume of traffic should be taken into account when defining the length of merging and diverging
lanes for entrances and exits. The risk of congestion, at the connection of exit ramps to the surface
network, must also be considered, as well as the consequences that this has on the main tunnel
(bottleneck queue) to determine whether or not it is necessary to design and lengthen a parallel lane
upstream from the divergence point of the exit ramp from the main road.
c - Ventilation
The ventilation galleries to be installed inside the structure contribute considerably to the spatial
requirement. Therefore, it is necessary to proceed to a preliminary “analysis of hazards and risks”, and an
initial sizing of ventilation installations before definitively setting the characteristics of the functional crosssection. This approach is often iterative.
d – Geology - Geotechnics - Hydrogeology - Methods of construction
The geological, hydrogeological and geotechnical conditions, as well as methods of construction (which are
often interlinked) have a vital impact on the shape and surface area of the cross-section. The following
example illustrates this interaction.
In loose soil below groundwater level, the use of a shield will be required for the construction of the main
tunnel. The main tunnel will be circular in shape. However, the cross-section will also depend on other
functions:
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For a tunnel consisting of two tubes, the emergency exits are usually provided by connecting passages
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between both tubes. The construction of such passageways in these ground conditions is extremely
costly since it requires significant ground consolidation works (grouting or freezing). Studies have shown
that it is more economical to integrate the emergency galleries inside the excavated section (usually
underneath the roadway) and to connect the escape gallery to vertical linkages along the carriageway.
A carriageway diverge for exit ramps or merge of on-ramps requires widening of the section over several
hundred metres. These works are extremely costly to build in these ground conditions. It is usually more
economical to develop a cross-section with a supplementary lane that will be used as an exit or merging
lane towards the ramps, and as an emergency stopping lane in the main tunnel. The area requiring costly
widening works is thus limited to approximately 50 metres. It can be constructed inside a temporary
shaft that can also be sized to allow the construction of technical rooms or a ventilation station.
3.3. SAFETY AND OPERATION
Recommendations in section 3 of page "General design of the tunnel (new tunnel)" are also applicable to
“underground road networks”. The analysis approach must, nevertheless, take into account the complexity
of underground networks and the aggravating influence of certain factors, in particular:
a - Traffic
The volume of traffic is generally more significant and in high traffic volume conditions traffic congestion is
much more frequent. It follows that the number of persons in the tunnel is much higher and in the event of
an incident, the number of users to evacuate will be more significant.
Ramp merge and diverge areas are important locations in terms of risk of accidents.
The assumption, which is sometimes prevalent from the start of projects, that there will never be a traffic
blockage must be analysed with much circumspection. It is indeed possible to regulate the volume of
traffic entering into an underground network in order to eliminate all risk of bottlenecks. Nevertheless, this
leads to a significant decrease in the capacity of the infrastructure (in terms of traffic volume) which often
goes against the reasoning that justifies its construction. Over time, measures of reducing entering traffic
must be relaxed, or even abandoned because of the need to increase traffic capacity. The probability and
recurrence of bottlenecks increase, disregarding the initial assumption upon which the network was based
(particularly in terms of safety and ventilation during incidents).
b - Emergency evacuation – emergency access
The analysis must take into account:
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The potentially higher volume of road users needing to evacuate, and the consequent necessity of
providing adequate information, communication and evacuation methods,
The complexity linked to the “network” and its numerous branches, the eventual multiplicity of operators
and the resulting interfaces, the precise location of incidents and users to secure and evacuate,
The delays in response times, taking into account the traffic and possible congestion of the surface
network, a correct identification of the incident locations, and adequate definition of access points and
incident engagement methods,
The necessity of response teams to have a good knowledge of the network, leading to a reinforcement of
training and practical sessions (see section 3.4. above).
c - Ventilation
The concept and design of ventilation systems must take into account:
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The volume and classification of traffic, as well as its evolution over time,
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The traffic congestion risks, generally making the construction of a smoke extraction system essential,
Environmental constraints especially discharge points for polluted air, release methods and their
acceptability. This may require:
The construction of discharge points that are remote from the main alignment and the construction of
ventilation galleries independent of the tunnel for connecting the tunnel to the shafts,
The implementation of in-tunnel air filtration systems before release into the atmosphere
The multitude of network branches and the necessity of making them operationally independent of each
other to prevent the spread of fumes throughout the network should there be a fire.
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d – Communication with users
Communication with tunnel users must be reinforced and adapted throughout the multitude of branches
within the network. Communication must be able to be differentiated between the different branches
according to operational needs, especially in the case of fires.
Users must be able to identify their position inside the network, which would require, for example, the
installation of specific signs, colour codes, etc.
Directional signs and prior information signs at interchanges or ramps must be subjected to careful
consideration, particularly the visibility distances with regard to signals and the clear legibility of the
signage.
e – Operational needs
Specific operational needs (see section 3.6 of page "General design of the tunnel (new tunnel)") must be
adapted to the complexity of a network, to the volume of traffic and to the resulting increased difficulties
of achieving interventions under traffic conditions.
3.4. OPERATIONAL AND SAFETY EQUIPMENT
Recommendations in section 4 of page "General design of the tunnel (new tunnel)" are also applicable to
“underground road networks”. Nevertheless, analyses must take into account the complexities of
underground road networks and the supplementary needs or conditions mentioned in section 3.
The interfaces between operators of associated or related network must be subjected to a specific
analysis, particularly for all aspects concerning, on the one hand, traffic management and, on the other
hand, safety (especially fire incidents), including evacuation of users and intervention of emergency
response agencies in response to fire incidents.
Control centres must take account of the interfaces within the network and between diverse operators.
They must allow the transmission of common information which is essential to each operator, and facilitate
the possible temporary hierarchy of one control centre over another. The architectural design of the
network of control centres, and of their performance and methods, must be subjected to an overall
analysis of organisations, responsibilities, challenges and risks. This analysis should reflect a range of
operational conditions such as during normal and emergency scenarios and should review the interaction
between the different subsections of the network and the respective responsibilities of each control centre.
MULTIMEDIA KIT
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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.
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CONSTRUCTION AND GEOMETRY
For a tunnel to operate sustainably and ensure good comfort and safety to users, it is imperative that the
entities which will be in charge of its operation are closely involved in the design phase. This chapter
reviews the main elements which ensure good design of a tunnel.
In 1974, the ITA (International Tunnelling and Underground Space Association) was created to address
construction aspects of all types of underground works, including road tunnels. In 2005, a Memorandum of
Understanding was signed between PIARC and the ITA to ensure that their actions are and remain
complementary and do not overlap. PIARC deals with tunnel geometry while the ITA takes care of purely
constructional aspects.
The page on Construction presents the main tunnel construction methods and their interfaces with
geometry and safety aspects. For more information concerning pure construction aspects, the reader is
invited to consult the ITA Website.
The page on Theoretical and practical tunnel traffic capacity gives a summary of the theoretical notions
related to traffic capacity.
The page on General alignment of roads and national examples recalls the main rules concerning the
general alignments of roads, including the main figures used in some countries, and insists on the need to
maintain the largest geometrical characteristics of the outside road in the tunnel itself (with the important
exception of the maximum slope, which has to be limited).
The page on Carriageway geometry deals specifically with the transverse profile of the carriageway of road
tunnels, for uni- as well as bidirectional types.
Emergency exits are provided in all except the shortest tunnels to allow tunnel users to evacuate on foot
from the traffic tube to a place of safety. The different types of emergency exits for pedestrians are
considered in the page on Emergency exits. These include cross-connections and cross-passages between
tubes, and safety galleries (passages) constructed alongside the traffic tubes or perhaps under the
carriageway and leading to the surface.
The page on Facilities for vehicles considers the facilities provided for vehicles. These include lay-bys,
turning bays and cross-connections between tubes for vehicles, which cater for situations such as vehicle
breakdowns or allow vehicles to turn around or cross into an adjacent tube, which could be useful for
maintenance, for manoeuvring emergency vehicles during an incident, or for traffic management following
an incident.
The page on Other facilities describes other facilities that may be provided within or at the portals of a
tunnel.
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CONSTRUCTION
THIS PAGE IS CURRENTLY BEING UPDATED
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TUNNEL TRAFFIC CAPACITY
THIS PAGE IS CURRENTLY BEING UPDATED
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GENERAL ALIGNMENT OF ROADS AND NATIONAL
EXAMPLES
THIS PAGE IS CURRENTLY BEING UPDATED
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CARRIAGEWAY GEOMETRY
THIS PAGE IS CURRENTLY BEING UPDATED
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EMERGENCY EXITS
THIS PAGE IS CURRENTLY BEING UPDATED
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FACILITIES FOR VEHICLES
THIS PAGE IS CURRENTLY BEING UPDATED
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OTHER FACILITIES
THIS PAGE IS CURRENTLY BEING UPDATED
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VENTILATION CONCEPTS
Ventilation in tunnels has two functions:
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In normal operation, it ensures sufficient air quality in the tunnel, generally by diluting pollutants;
In a fire situation, it should make the environment as safe as possible for the tunnel users and rescue
services by controlling the flow of smoke in an appropriate way.
Historically, the first reason for installing ventilation systems in tunnels was the reduction of pollution
levels. Although the emissions of pollutants by road vehicles have decreased dramatically over the last
decades, this function is still important and must be paid close attention at the design stage. In some
cases, natural ventilation due to the piston effect of moving vehicles may be sufficient to fulfil the air
quality requirements in normal operation. The need for a mechanical ventilation system is assessed
considering, among other factors, the length of the tunnel, traffic type (bidirectional or unidirectional) and
conditions (possibility of congestion) and type of vehicles.
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. Natural ventilation might be sufficient in some cases, but mechanical ventilation is often required
for tunnels over a few hundred meters in length.
In addition to these major aspects, environmental issues linked to ventilation, issues relating to energy
consumption and the related carbon footprint need to be considered. Those are linked to the localised,
concentrated discharge of polluted air from the portals and stacks. Reducing their impact on the tunnel
surroundings is part of good environmental design.
The following chapters describe key aspects to be taken into consideration for a proper design and
operation of ventilation systems in road tunnels.
The page on Ventilation principles presents the strategic elements that must be taken into account before
making a decision concerning the choice or the design of a tunnel 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, covering both normal and emergency ventilation.
The page on Control and monitoring examines some specific aspects related both to the tunnel ventilation
system and the control system and SCADA.
Other aspects related to tunnel ventilation equipment can be found in the pages Ventilation and Tunnel
Ventilation Systems within the page on Equipment and systems.
And last, but not least, additional considerations related to tunnel operation are given in the page
on ventilation strategies . These are essential to ensure that all actions required are handled in a
consistent and safe way, recognising that the level of safety provided for tunnel users is highly dependent
upon the specific characteristics of the tunnel, but also depends strongly on operational procedures which
need to be taken into account for each tunnel.
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VENTILATION PRINCIPLES
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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:
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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:
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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
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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:
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possibly reduced supply of oxygen to the fire site,
heat release,
heat convection,
longitudinal slope,
type of ventilation,
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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".
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VENTILATION DESIGN AND DIMENSIONING
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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:
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●
●
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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:
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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 “Semitransverse ventilation” of the PIARC report 1996 05.02.B “Road Tunnels: Emissions, Environment,
Ventilation”)
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In order to decide between longitudinal ventilation (LV) and transverse ventilation (STV/TV) for a given
tunnel situation, these are some of the arguments:
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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
●
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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
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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:
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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.
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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.
Fig. 1: Idealized HRR curve
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
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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).
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Fig. 2 : Example of the critical velocity as a function of the heat-release rate and tunnel height
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.
Fig.3. Principle of confinement
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.
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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 semitransverse 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:
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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
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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:
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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”.
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VENTILATION CONTROL AND MONITORING
There are two principal aims of a well-designed ventilation control system:
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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
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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 predefined 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, transverseventilated 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.
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SUSTAINABILITY ISSUES
The ‘’sustainable development’’ concept may have a slightly different meaning from one country to
another and its scope may vary. However the most widely accepted concept is based on three main pillars:
economic, environmental and societal.
The concept of sustainable development may be applied to different fields or sectors, including road
tunnels, in particular for the operation of this type of infrastructure.
As we know, the operation of a road tunnel is highly dependent on the design and construction phases
which precede its commissioning. More precisely it is necessary to take account of the effect of solutions
opted for during a project’s design phase on subsequent operating conditions. In other words, this means
that, if the chosen solution is not ideal, it will be very difficult to optimize it throughout the tunnel’ life
cycle. The technical Report 2017R02EN "Road Tunnel operations: First steps towards a sustainable
approach’’ is entirely devoted to sustainability issues and in particular, to optimizing the three pillars.
This chapter gives information about the three main pillars of the sustainable development concept. The
balance between these pillars should be estimated accurately for each project design. Nevertheless we
should have in mind that sustainable development represents a holistic approach of all involved sectors
and parameters, as well as an acceptable and balanced weighting of economic, environmental and societal
objectives.
The chapter is broken down into four pages:
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Economic issues
Environmental issues
Societal issues
Energy consumption.
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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:
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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.
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ENVIRONMENTAL ISSUES
The primary aim of environmental protection is to reduce the impact on air, water and ground to a longterm 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:
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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.
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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.).
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ENERGY CONSUMPTION
The main energy-consuming devices in a tunnel during normal operation are:
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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 energyconsumption 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.
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