fire engineering in tunnels

David Charters, Bre global

Digest DG 509

All these advances have made tunnels safer, but if tunnels are to remain cost-effective and safe, there is a need to select the best combinations of fire precautions as part of a fire strategy. Fire safety engineering provides a structured and formal way of developing a fire strategy. Because multiple fatalities are the events of concern for tunnel fire safety, most fire engineering involves a quantitative fire risk analysis.

As tunnels are not all the same, it is important to consider a wide range of tunnels and tunnel types when deciding on a fire strategy. For example, tunnels variations include different:

tunnel functionstunnel contentstunnel occupantstunnel layout and arrangements.

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Pressure on the use of land and environmental protection increasingly mean that new road and rail transport infrastructure is being placed underground in tunnels, with all the inherent fire challenges that this presents. the purpose of this Digest is to provide background information and good practice guidance on the application of fire safety engineering principles to tunnels. it briefly covers a range of issues related to tunnel fire safety, fire safety engineering and consideration of some tunnel-specific issues related to the sub-systems of fire safety engineering.[1–3] it also lists references for further reading and where to obtain further information.

1 intrODuCtiOn AnD BACKgrOunDAccording to international accident statistics, rail tunnels have a good safety record and road tunnels are safer than the surface roads to which they connect.[4] However, with a long enclosed space like a tunnel, fire statistics indicate that wherever there are people and a sufficient fire load present, there is always the potential for a large multi-fatality fire incident.[5-7]

In response to these documented multi-fatality fires, improvements to tunnel fire safety have been made:

Vehicle and rolling stock engineers have investigated crash-worthiness.Reaction to fire of materials and first-aid fire fighting. Mechanical and electrical engineers have specified ventilation and smoke extraction systems.New forms of automatic fire detection and emergency lighting/way-findingCivil engineers have developed twin bore tunnels, evacuation shafts, escape tunnel and cross-passages.

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tunnel functionsFor example:

road tunnelsrail tunnels:

metroheavy rail.

services:cablepipelinesconveyor belt eg raw materials in mines or suitcases at airports.

for pedestriansa combination of all or some of these functions.

tunnel contentsTunnel contents tend to follow their function and include a wide range of sources of fuel and ignition sources, eg:

vehiclesroad:

passenger vehicles: cars and people carriers, and buses and coachesgoods vehicles: light and heavy goods vehicles (with or without ‘hazardous goods’).

rail:metro (normally electrically powered)heavy rail (sometimes diesel powered): passenger carriages and freight.

servicescablespipelines

raw materialspersonal possessions.

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tunnel occupantsTunnel occupants (staff and emergency services personnel) can vary in terms of their:

number: eg high numbers in rush-hour urban road tunnels or a handful of staff in rail freight tunnelslocation: eg distributed in different vehicles along a road tunnel or contained in a set of train carriagesproportion with a disability: estimates vary between 6% and 30% of the population depending on the definition of disabilityfamiliarity with the tunnel: eg road tunnel operators or rail passengerstheir commitment to their journey or their vehicle: eg commuters on their way to work or touristsdegree of autonomy: eg road drivers or rail passengerstraining: eg in first-aid fire fighting, emergency response, leadership role: staff or passenger.

tunnel layout and arrangementsEqually, tunnel arrangements can vary significantly and these variables can include its:

location: eg urban or rural (such as under a mountain or lake)length: a few hundred metres or tens of kilometresnumber of bores: single or twin boredimensions: shape (circular, square and rectangular) and cross-sectional area (eg circular or square, 10 m2 to 150 m2 in diameter)depth: a few metres to tens or hundreds of metresentrance/portal locations: urban or remote ruralinterconnectivity: many doors in a dividing wall or a few widely spread cross-passagesconstruction: cut and cover or bored, rock, cast-iron or concrete liningoperating philosophy: manned or unmanned.

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Distance from fire

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Figure 1: Spacial/temporal map of the development of a high consequence tunnel fire.[8]

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Figure 1 shows a spacial/temporal map of the potential development of a high consequence tunnel fire (eg involving casualties and tunnel spalling). The vertical axis shows the distance from the fire and the horizontal axis shows the time, where the intersection of the axes is the time and location of ignition. Figure 1 shows typical post-ignition events, such as detection and evacuation, to the right of the vertical axis. It also indicates the potential role of fire protection measures in reducing these consequences through fire engineering.

2 fire engineering PrOCessFire safety engineering can be defined as the: “application of scientific and engineering principles to the protection of people, property and the environment from fire”.[1]

Fire engineering has been applied in the design of buildings since the 1970s and there are now international and national codes of practice on the application of fire engineering, such as ISO TR 13387[2] and BS 7974.[1] These codes of practice have largely been focused on the design of buildings, so although the general principles and process are applicable to the design of tunnels, there are several specific aspects of tunnels that merit detailed consideration and many of these are identified in this Digest. Figure 2 shows the general fire engineering process.

At the start of the fire engineering process, a qualitative design review (QDR) is undertaken to characterise the nature of the tunnel, to identify design challenges, and potential fire safety issues and possible design solutions (see section 3 of this Digest).

One of the functions of the QDR is to set and agree the acceptance criteria and what quantitative analysis (QA) should be undertaken. Generally, there are two kinds of life safety acceptance criteria:

Comparative: where the level of risk for a comparable activity eg travel by similar surface transport (or a design to a prescriptive standard) is compared to the level of risk of the proposed tunnel design.Absolute: where some countries have set levels of risk, for example 1 x 10-4 per year for workers or and 1 x 10-6 per year for members of the public.

Most countries fall into the former of these two categories and so national standards and international good practice for fire safety in tunnels can be important in

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setting benchmark risk or design standards. National and international standards for tunnel fire safety include:

road:PIARC[9]

BD 78/99[10]

NFPA 502.[11]

rail:EU[12]

NFPA 130.[13]

There are individual company standards for fire safety in tunnels produced by operators such as London Underground and MTRC (Mass Transit Rail Corporation in Hong Kong). It should also be noted that each of the prescriptive standards listed above contains an implicit fire strategy. For example, some of the standards (where urban tunnels are the norm) contain greater provisions for the intervention of fire and rescue services than others where remote tunnels are most common.

Once the QDR has been completed, the QA can be undertaken. This can take various forms, including QA of fire growth, smoke movement, detection and activation of systems, response and movement of people, fire and rescue service intervention and behaviour of structure. Tunnel-specific aspects of this analysis are discussed in section 4 of this Digest.

Once the QA has been completed, the output can be compared with the acceptance criteria and a judgement made on the adequacy or otherwise, of the proposed fire safety solution. Where complex fire-engineering analysis is used, there may be a need for an independent third party review to provide an expert view on the appropriateness of the analysis.[14]

3 QuAlitAtiVe Design reVieWThe QDR (also known as the fire engineering design brief or fire engineering brief) can be described as a process for ensuring that all the individual aspects of the tunnel design can be considered in the context of the fire safety objectives (such as life safety, property protection and business continuity) and that the impact of proposed design solutions on other aspects of the tunnel design are fully appreciated.

The QDR should include a meeting with the tunnel project fire safety stakeholders. Tunnel projects tend to

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Figure 2: Fire engineering process.

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have different stakeholders from buildings. For example, there is generally no architect and the lack of a central coordinating role can make it difficult to integrate fire safety engineering in the design process. Instead of a developer, the clients for many tunnel projects are local or national governments and they have specific governance and due diligence needs; depending on their location they may not have previous tunnel project experience.

The QDR should follow the general procedures set out in one of the codes of practice on fire safety engineering, eg:

review of the design of the tunnelestablish tunnel fire safety design objectivesidentify fire hazards and possible consequencesestablish trial fire safety designsidentify acceptance criteria and methods of analysisestablish scenarios for analysis.

The QDR should also take into account the variables listed in section 1 of this Digest.

4 QuAntitAtiVe AnAlYsisThis section discusses some of the challenges in undertaking a QA for fire engineering in a tunnel. There is also useful information and data contained in a number European tunnel fire research projects, such as UPTUN, FIT and SAFE-T.[15-21]

initiation and growth of fireThe QDR will have identified the fire hazards relevant to the tunnel system under consideration, eg:

road tunnel: largely uncontrolled and uncontrollable, with the possible exception of hazardous materials which can be controlled. It should be noted that many of the multi-fatality road tunnel fires involved HGVs that were carrying loads that are not normally considered major hazards, such as flour, margarine, tyres and furniture.rail tunnel: more controlled (eg reaction to fire testing of vehicle materials) and staff action (eg first-aid fire fighting).

The operating mode of the tunnel may have an impact on fire hazards. For example, some tunnels only allow transport of hazardous materials to take place overnight when the tunnel is closed to passengers.

The most important consideration around the initiation and growth of fire in tunnel fire safety engineering is the definition of one or more design fires. Design fire sizes play a significant role in the fire safety strategy and the required performance of fire protection systems used in railway systems. For example, the sizing of jet fans in tunnels or the evacuation strategy from stations. Therefore, there may be a need to define design fires, for example:

a growing internal carriage fire for evacuation from the train a steady state fully developed carriage fire for the design of ventilation systems and protection to the tunnel structure.

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Design fires can also be challenging to define. There are only a limited amount of full-scale experimental data. Data that do exist are often for a different type of rail carriage built to different standards. However, there are sources of information that may be useful in defining a design fire.

Design firesThere is no definitive method of defining a design fire for a tunnel system, however, the following sources of information are relevant:

Review of literature relating to fires in similar tunnel systems.Cone calorimetry (eg ISO 5660-1[22]).Furniture calorimetry of sub-assemblies such as a typical seating and wall/ceiling corner arrangement.

The review of literature relating to fires in similar tunnel systems will provide insight into the experience of other tunnels with similar fire hazards. Cone calorimetry can provide a bench-scale measurement of the potential rate of heat release which can be useful in providing bounding values of peak rate of heat release. Furniture calorimetry can provide information on potential fire growth rates and the likely interaction of different material in a realistic arrangement/orientation.

Each approach has its limitations and if the additional information from tasks 1 to 3 does not provide a robust basis for consensus on the design fire(s), then consideration can be given to computer fire modelling and full-scale fire experiments.

Computer fire modelling (using data from the cone and furniture calorimetry) can provide insight into the interaction between the growing fire and the carriage/ventilation openings. Full-scale fire experiments can provide the evidence of the performance of the whole carriage assembly or typical road vehicles.Throughout the approach outlined above, consideration should be given to the potential for more than one design fire because this:

reflects the likely experience of a tunnel systemresolves the design fire dilemma by allowing explicit consideration of more frequent as well as less likely (larger) fire sizesensures that the design is robust against a range of fire scenarios (bigger design fires do not always err on the side of safety)will naturally support a probabilistic safety case, if this is required for the new tunnel system.

One of the myths of design fires is that bigger is always safer. There may situations where this assumption may break down illustrated in the following scenarios.

scenario 1In one situation, a very large and fast-growing fire was selected for the design of a tunnel rail system. The designers identified a base design and alternative designs that included additional fire precautions. They then undertook a quantitative risk analysis to compare the base and alternative designs. However, this analysis found

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that there was no safety benefit from the additional fire precautions (because of the severity of the design fire). Whereas, against a range of more realistic fires, the analysis is more likely to have shown some reduction in risk for the designs incorporating additional fire precautions.

scenario 2A large design fire indicated that longitudinal ventilation would have been beneficial in reducing risk. However, when a range of fire sizes and their likelihoods were analysed, this showed that an alternative natural ventilation strategy was safer because of the risk associated with smaller, more frequent fires.

Other design considerationsOther tunnel design fire points should be considered:

There is some evidence that longitudinal ventilation may increase the rate of heat release of a tunnel fire.[8] Some very high multiples are presented in the literature, but the true extent of this increase is not clear given the current scientific evidence.There is some evidence that thermal instabilities determine fire spread between packages of fuel such as vehicles.[8]

Tunnel safety management can have a large impact on tunnel fires. For example, the Channel tunnel fire in 1996[23] only apparently stopped spreading when it encountered an HGV trailer load containing tinned pineapples. Although tinned pineapples are not yet a standard fire precaution, Channel tunnel freight trains now stagger combustible and non-combustible loads, when possible, to reduce the potential for fire spread between trailers.

smoke movementOne of the main forms of QA for fire safety engineering in tunnels is the modelling of smoke movement. Smoke is the main hazard to life in most fires and the geometry of most tunnels can lead to very rapid uni- or bi-directional flow of smoke.

There are two main approaches to the modelling of smoke movement in tunnels: zone modelling, such as FASIT, and computational fire dynamics (CFD), such as JASMINE.

Zone and CFD modelling of tunnel smoke movement are described in the literature.[8, 21, 24] Zone modelling is usually used where there are a large number of cases to be analysed in the early design stages of a tunnel project.

Variations that need to be considered include:fire scenario (size and location)ventilation conditions (natural, normal, forced and emergency)emergency evacuation scenariofire strategy.

These variations can lead to a significant number of combinations of variables. CFD modelling is usually used later in the design process when there are fewer variables and cases, and where details of the flow and visualisation of the results are much more important.

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Computational fluid dynamicsFor CFD modelling, it is worth noting that not all models are the same and two types of turbulence model are currently used:

Reynolds averaging numerical simulation (RANS) eg k-ε which provides a prediction of the average flows resulting from turbulent eddy formation and break-up.Large eddy simulations (LES), which attempt to predict the flow of large eddies.

Competent CFD users can make an informed choice as to the type of turbulence CFD model to use. For example, LES CFD models have been developed more recently and provide a strong visual image of turbulent mixing, whereas RANS CFD models have been applied to a wide range of fire applications and are typically more widely validated against full-scale experiments.

When presented with CFD simulations (some of the animations can be quite seductive), it is also worth asking:

to what extent has the modelling been validated for tunnels?are the solutions independent of the CFD grid?are the boundary conditions realistic and conservative?

Additional guidance on good practice in CFD modelling is given by Kumar S.[21]

tunnel ventilation and fire plume interactionsAnother important consideration in the movement of smoke in tunnels is the speed with which the smoke might start to affect occupants. Recent research indicates that there may be a dynamic interaction between the tunnel ventilation conditions and the fire plume, which can cause the smoke layer to descend very rapidly.[25]

There is strong eye-witness and other circumstantial evidence that one of the factors that contributed to the loss of life in the Mont Blanc and Tauern tunnel fires was the rapid loss of a clear layer once rapid fire growth of a high fire load HGV occurred. Computer modelling and experiments have shown how a rapid reduction of clear layer height and a reverse of the clear layer flow downwind of the fire can occur. The tunnel section may become rapidly filled with hot toxic smoke and processes such as diffusion, turbulent mixing or loss of buoyancy have traditionally been identified as the cause. A previous study proposed that plume entrainment and continuity may also contribute to the effect.[26]

A numerical study, using the CFD fire model JASMINE, covered a range of parametric simulations of various fire scenarios in longitudinally ventilated road tunnels with rectangular and arch cross-sections. A tunnel section extending 700 m downstream was modelled. The fire modelled was indicative of an HGV fire of varying intensity and was based on previous published information[8] and the Runehamar tunnel fire tests.[27, 28] Peak heat release rates were in the range 30 MW to 150 MW. Figure 3 shows the smoke visibility for a tunnel fire growing to 70 MW and the resultant rapid loss in visibility (note the rapid loss between 5 and 7 minutes along 400 m of the tunnel).

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Fire engineering analysis that takes account of a range of ventilation conditions (ambient and mechanical) and fire scenarios and included tenability analysis[29] should address this effect.

smoke extraction systemsWith longitudinal smoke control systems in tunnels, there is always the dilemma of when to activate it and which way to blow the smoke.[29] Quantitative fire risk analysis can inform these decisions, but the challenge for the operator to make the right decision on the day, with limited and perhaps conflicting information, and under time pressure, remains.

Therefore, some road tunnels in Europe are now being fitted with smoke extraction systems. Some tunnels use semi- or fully transverse ventilation systems which are operated in a smoke extraction mode during a fire. If configured correctly, these should improve tunnel conditions, but most of them only extract a small fraction of a m3/s per metre length of tunnel from the tunnel and so their effectiveness, against for example an HGV fire, should be analysed carefully.

Other tunnels are incorporating dedicated smoke extraction systems that are intended to restrict the presence of smoke to, for example a 100 m section of the tunnel covered by typically three extraction points. Some systems also use jet fans on either side of the fire to increase the efficiency of the extraction system and balance the flow. Many road tunnel extraction systems are being designed to deal with a 30 MW fire. Although a 30 MW fire may equate to a bus fire, its effectiveness against an HGV fire (which may be up to 100 MW[8]) should be analysed.

fire resistanceThe structural stability of the tunnel lining is a key safety factor in the event of a fire. In many modern tunnels, high-strength concrete is used as the tunnel lining material. Concrete contains a high percentage of water chemically bound into its structure and when exposed to high temperatures (greater than 400°C), the water is released and turns to steam. Without a route to atmosphere, the pressure of the steam can build up within the concrete and lead to spalling. With high-strength concrete this spalling can be explosive.

The use of high-strength concrete has increased worldwide in recent years. The reduced section sizes resulting from the increase in material strength provide the potential for significant savings in construction costs. There is a very real concern, however, regarding the performance of such concretes in fire. Research has shown that high-strength concrete is prone to explosive spalling failure on heating.[30, 31] Such failure could lead to catastrophic collapse and represents a significant threat to the life safety of those in and around the tunnel or to the tunnel itself.

Previous research and commercial testing has shown the potential for polypropylene fibres to be used in conjunction with high-strength concrete to achieve the necessary performance requirements in relation to fire resistance. Tests on high-strength concrete columns with compressive strengths up to 100 N/mm², as well as elevated temperature testing of tunnel lining segments, have been undertaken for different projects including the Channel tunnel rail link and Heathrow terminal 5. It is important that any testing undertaken incorporates transient temperature tests including a range of nominal fire curves in conjunction with steady-state loading conditions to demonstrate the suitability of specific design solutions. It is also important that the residual strength of

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Figure 3: Loss of visibility in a tunnel fire.

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cores from the test specimens is tested. Figure 4 shows concrete specimens (without and with polypropylene fibres) after they have been fire tested.

Activation of fire detection and suppression fire detectionOne of the main factors in ensuring a safe outcome to a tunnel fire is early detection. Many fires are detected by tunnel occupants, who raise the alarm at emergency extinguisher/call points in road tunnels or emergency communication systems in railway carriages. Other fires may be hidden from tunnel occupants during their early stages. There are a range of technologies that can be applied to automatically detect fires in tunnels.[8] There are, however, some aspects of the tunnel environment that make the detection of fires more challenging and these include:

The fire may be moving. This makes detection more difficult because the products of combustion (heat and smoke) are diluted, which makes the fire harder to detect. It also means that if the fire is detected, it is no longer in the same location as the products of combustion that have been detected.Strong and fluctuating air movements due to rolling stock/vehicle movements or atmospheric differences between the tunnel portals will also dilute and move the products of combustion.Mechanical ventilation in road tunnels to dilute carbon monoxide from vehicle exhausts may also dilute and move the products of combustion. Feedback loops in these systems may, in effect, hide the fire by trying to maintain a low concentration of carbon monoxide with increased ventilation velocities.The atmosphere may be corrosive, hot, dirty and dusty.There may be electrical interference of electronic devices, cables and boards.

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Temperatures can change through external conditions in the portal area.The fire detection system must be resistant to cleaning machines.There may be hot exhaust fumes from trucks (with high exhaust pipes) held up in traffic jams.There may be mechanical forces from high speed trains, goods lost from HGVs or a sailing boat mast touching the tunnel ceiling.

Added to these challenges is the fact that there is no fail-safe operation for a tunnel fire detection system. There are risks to tunnel occupants if a fire detection system operates when there is no fire, and if there is a delay in detection or the system fails to detect a real fire.

fire suppressionFire suppression in most tunnels is by first-aid fire fighting with fire extinguishers by tunnel occupants and by the fire and rescue service response to a fire. Some heavy rail power cars also have a suppression system built in.

Recently there has been increased interest in automatic fire suppression systems in tunnels, particularly those road tunnels that are used by HGVs. Some road tunnels incorporate manually operated water deluge systems and these have been reported to have operated effectively in some fire events.[32] Again, these systems are not without their challenges, as:

many tunnel fires are covered (eg they may be under the roof of a road vehicle or rail carriage)they introduce water to the road surfacethey reduce visibility.

One rail tunnel system, dedicated to the transport of freight, also incorporates a sprinkler system to deal with releases of flammable liquids.[33]

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Figure 4: Concrete specimens (without and with polypropylene fibres) after they have been fire tested.

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Other more innovative suppression systems have been proposed and some have been tested.[15] These include:

oxygen depletionwater mistinflatable tunnel plugs.

There is some design analysis that indicates that suppression systems can be highly effective in ensuring business continuity in strategically important tunnel links. The fire in the Channel tunnel in 1996 resulted in an extensive outage of the incident bore and so resulted in significantly reduced operation and subsequent loss of revenue.[23]

Some research and project design life safety fire risk analysis have indicated that the cost of an automatic suppression system usually far outweighs the reduction in risk, ie the level of risk to life without the suppression system is ‘as low as reasonably practicable’ (ALARP). This outcome of the risk/cost-benefit analysis for life safety is generally due to the low level of risk in most tunnels. The result may be different if the frequency and/or consequence of fires increase or the analysis takes business continuity and/or sustainability into account.

fire service interventionWhen considering access and facilities for the fire and rescue service, consideration should be given to the potential extent and duration of tunnel fire incidents. The 1996 Channel tunnel and Mont Blanc tunnel fires burned for many hours before being suppressed.

In safety philosophy, there can be two strategic options: a safe place or safe person. An example of a safe place philosophy in tunnel fire safety is maintaining a tenable atmosphere through an emergency ventilation system, whereas the safe person alternative might be breathing apparatus. Although a safe place philosophy is usually preferred in tunnel fire safety, it is not always possible or practicable.

Another aspect of tunnels that can have a significant impact on fire service intervention is the tunnel’s remoteness. The tunnel portals may be in a remote rural part of a country or the tunnel may be so long that much of its length is remote from the portals. In some tunnels, specially designed emergency access vehicles or trolleys have been provided. It is also worth considering that not all prescriptive codes provide for the same access and facilities for the fire and rescue service. Some tunnel fire codes, written in countries with many rural tunnels and a lower provision of emergency intervention, may prescribe very little in the way of access and facilities for the fire and rescue service.

response of people and evacuationOver 700 people have perished in the last 10 years in serious tunnel fires across the world. Following these disasters, a major research effort has reviewed the behaviour of people in tunnel fire incidents, large and small.[34, 35] This knowledge is useful when considering the design and operation of the tunnel and it has also been used to adapt a generic human behaviour and evacuation simulation in order to model tunnel emergencies.[36]

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The behaviour of people in tunnel fires shows many similarities to that in other types of buildings:

Recognition, response/pre-egress activities and evacuation stages apply in both cases; a person’s role has a major effect on the behaviour they will exhibit, and people cluster in family, social or ad-hoc groups.Communication between people occurs throughout the incident, in order to raise the alarm, and give instructions/directions to the exits.Pre-egress activity includes similar actions to those performed in building fires, eg investigation, fire-fighting, searching for, warning and rescuing others. Rapidly-worsening conditions in tunnel fires may, however, reduce the options available to people.

During evacuation, people usually head away from the fire, although they may often find themselves moving through smoke. Tunnel portals (‘familiar’ routes) will be preferred to side exits unless directions to the contrary are given, or smoke/darkness leaves no choice.

Some differences from other building fires are that drivers are very reluctant to abandon their vehicles, and in rail tunnels, passengers are reluctant to abandon their luggage. Another difference is the consideration of which language(s) tunnel will understand. Depending on the location and use of the tunnel, one or two languages may be understood sufficiently well to follow simple emergency instructions. Pictograms can also be very useful in communicating emergency messages.

fire risk analysis Quantitative fire risk analysisQuantitative fire risk analysis is widely used in the design and operation of tunnels for a range of reasons,[4, 37] including:

Tunnel fire experience indicates that it is highly probabilistic in nature, eg the London Underground operated relatively safely for nearly 100 years until the King’s Cross fire killed 30 people in 1987.Tunnel fire experience also shows that tunnel fire deaths are relatively rare, but that fatal fire events are likely to involve large numbers of fatalities.[37] Society naturally has a greater aversion to hazards that cause multiple deaths in a single incident.Many of the factors important in tunnel fire safety are highly variable, including:

location of the firefire growth ratepeak rate of heat releaseair flow velocity and directionnumber, location and mobility of people presentresponse of occupantsreliability of systems.

The above factors mean that to avoid designing and operating an unsafe tunnel, or an overly-expensive tunnel, it is necessary to gain a broad understanding of the level of safety (or risk) in a tunnel. Deterministic analysis alone cannot provide insight into the combinations of factors that can give rise to a range of fire outcomes. Only quantitative fire risk analysis can provide a robust insight

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to aid informed decision making.[4, 24, 37] Figure 5 shows the process for a quantitative fire risk analysis.

fire risk assessment using points schemes or risk indicesSome organisations use points schemes or risk indices to assess the level of fire risk in a tunnel.[38] The main advantage of points schemes and risk indices is that they are generally quick and easy to use. However, they often have significant limitations including:

they give a fixed value to each type of fire precaution and so have an inherent fire strategy. This will mean that a safe tunnel using a different fire strategy will be unfairly penalised.these fixed values are based on arbitrary judgements of contribution not physical measures.these arbitrary values are combined in a way that is not physically meaningful.

For example, a points score for emergency lighting might be added, among other things, to a score for traffic management. What are the units of these scores? What is the meaning of their addition (other than more is better)? Even so, in terms of risk ranking, this approach masks the real value of each individual fire safety measure in the context of each tunnel and so may be highly misleading.

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5 COnClusiOnsThe number and variety of tunnels has increased and is continuing to increase. It is increasingly difficult to apply prescriptive tunnel fire safety guidance to these tunnels and ensure a safe and cost-effective solution. Fire engineering offers a rational approach to identify safe and cost-effective design solutions for tunnels.Design fires are a crucial part of a tunnel fire engineering design process or tunnel safety case. A range of complementary techniques should be used to identify robust and appropriate design fires. Design fires should consider fire growth as well as a peak rate of heat release. A range of design fires should be identified for different scenarios (with different likelihoods of occurrence).The modelling of smoke movement should be undertaken using models that are appropriate for tunnels and well validated. The analysis should be undertaken by people who are competent in the application of the model to tunnel fire situations. Multiple fire and ventilation scenarios, including the fire growth’s stage, should be modelled to gain the necessary insight into tunnel fire dynamics and the resultant tunnel fire hazard development.The potential effectiveness of smoke extraction systems (and semi- and fully transverse ventilation systems)

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Figure 5: General process for a quantitative fire risk analysis.

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should be assessed for credible fire scenarios for tunnels.The structural response of the tunnel lining should be considered for the safety of fire fighters and business continuity and for the safety of tunnel occupants in tunnels under water. For concrete linings consideration should be given to improving the lining’s resistance to spalling.The detection of fires and information on the fire’s location are complex and require careful consideration.This consideration should include the specification and design of systems and their interface with the operational procedures and training of tunnel operators and staff.There is some evidence that tunnel fire suppression systems can be very effective in suppressing a tunnel fire. Tunnel fire suppression systems may also have unwanted side effects that should be considered in their specification and operation. A range of suppression systems have been proposed for application in tunnels and experimental evidence indicates that they have a wide range of performance. Therefore, evidence of effectiveness should be sought and thoroughly reviewed before a suppression system is specified. [39]

Consideration should be given to the logistical challenges of emergency intervention. Alternative approaches to the design of access and facilities for fire and rescue services can be very cost-effective, and for some long remote tunnels they are essential.The likely behaviour of tunnel occupants in a fire should also be considered. Evidence indicates that people in tunnels behave in a similar way to people in buildings, eg they take time to recognise an emergency and their first response is unlikely to be evacuation. One major difference in human behaviour in tunnel fires is that people tend to be very reluctant to leave their vehicles and possessions. Tunnel emergency communication systems have an essential role to play in enabling occupants to make good and timely decisions in a fire situation.Quantitative fire risk analysis is essential in the development of safe and cost-effective design solutions for tunnels. They may also be a regulatory requirement in tunnels that are only allowed to operate under a safety case regime. Quantitative fire risk analyses allow rational informed decisions to be made concerning small frequent fires and larger more rare fires in a way that can be communicated to stakeholders. Conversely, fire risk assessments based on simple points schemes have many limitations and drawbacks that can mean that their results are at best misleading.

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referenCes[1] BSI, 2001. BS 7974: 2001. Code of practice on the application

of fire engineering principles to the design of buildings.[2] ISO, 1999. ISO TR 13387. Fire safety engineering – Part 1: The

application of fire performance concepts to design objectives.[3] Chitty R, Fraser-Mitchell J, 2003. BR 459 Fire safety

engineering – a reference guide. BRE Bookshop, Watford.[4] Beard A, Cope D, 2007. Assessment of the safety of tunnels

study. IP/A/STOA/FWC/2005-28/SC22/29. European Parliament.

[5] Fennell D, 1988. Investigation into the King’s Cross underground fire. Department of Transport, HMSO, London.

[6] Lacroix D, 2001. The Mont Blanc tunnel fire: What happened and what has been learned. In: Proceedings of 4th International Conference on Safety in Road and Rail Tunnels, Madrid, April 2001, pp3–16. A E Vardy (ed.). Independent Technical Conferences Ltd and University of Dundee.

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[35] Fraser-Mitchell J, 2005. Human behaviour in tunnel fire incidents. In: 8th International Symposium on Fire Safety Science, Beijing, 2005. D Gottuck and B Lattimer (eds). Interscience Ltd, Greenwich.

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[39] The Loss Prevention Certification Board Red Book. Volume 1: List of approved fire and security products and services: A specifier’s guide. Vol. 2: Directory of listed companies, construction products, environmental profiles and assessments.www.redbooklive.com.

1� fire engineering in tunnels – DG 509

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