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Design Fires in Road Tunnels (2011)

Chapter: Chapter Eleven - Design Fire Scenario for Fire Modeling

« Previous: Chapter Ten - Compilation of Design Guidance, Standards, and Regulations
Page 96
Suggested Citation:"Chapter Eleven - Design Fire Scenario for Fire Modeling." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Suggested Citation:"Chapter Eleven - Design Fire Scenario for Fire Modeling." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Page 97
Page 98
Suggested Citation:"Chapter Eleven - Design Fire Scenario for Fire Modeling." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Page 98
Page 99
Suggested Citation:"Chapter Eleven - Design Fire Scenario for Fire Modeling." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 99
Page 100
Suggested Citation:"Chapter Eleven - Design Fire Scenario for Fire Modeling." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 100
Page 101
Suggested Citation:"Chapter Eleven - Design Fire Scenario for Fire Modeling." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 101
Page 102
Suggested Citation:"Chapter Eleven - Design Fire Scenario for Fire Modeling." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 102
Page 103
Suggested Citation:"Chapter Eleven - Design Fire Scenario for Fire Modeling." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Page 103

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97 The preceding chapter summarized information about fire dynamics and the release of heat and toxic gases based on the literature review. Design fire scenarios are discussed in chapter nine. A fire scenario is designed to provide an opti- mum fire life safety strategy for road tunnels. Design fire scenario discussion found in the literature is summarized in this chapter. Fire scenarios are used for the following: • Design of emergency exits, • Choice of a fire-detection system, • Choice of ventilation and fire suppression systems, • Tunnel structural engineering, • Specification requirements for tunnel structures and equipment, • Operation of the tunnel, and • Training of operators and first responders dealing with tunnel fires. Fire scenarios usually include: • Governing standards and guidelines; • Description of the scenario; • Thorough definition of the fire parameters (e.g., HRR/ temperature versus time); • Traffic scenario operation during fire emergency and tunnel ventilation operation; • Guidelines for structural protection; and • Specifications for materials, equipment, and structure. In general, a broad spectrum of design fire scenarios is pos- sible regarding their different goals (tunnel construction, equip- ment, tunnel operation). Therefore, the intention is to select the most important design fires and to prepare a short descrip- tion of the fire scenarios, as shown in Table 30. The fire HRR of a vehicle is one of the most important parameters. It is a main parameter in the calculation of criti- cal velocity required to prevent backlayering of smoke and heat resulting from a fire, which in turn determines the air- flow required to be delivered by a longitudinal system of ven- tilation. Among the possible fire loads the following vehicle fires are considered: • Incidents with one vehicle (car, bus, truck, or gasoline tanker), and • Collision incidents (a collision of two to three passen- ger cars, of a passenger car with a truck or bus, or of a bus with a truck). The consequences of fire incidents in the following traffic situations are investigated according to the characteristics of the tunnel, such as an urban tunnel: • Congested traffic (e.g., rush hours) • Traffic jam (e.g., as a result of another accident) • Flowing dense traffic (e.g., increased probability of multiple vehicle incidents). The worst conditions may not be considered in the design or may not be correctly identified in design. For example, an assumption is usually made based on one incident at a time. In rear situation collisions, one incident may lead to another, such as when a blackout leads to a collision and then a fire event. TIME–TEMPERATURE AND TIME-OF-TENABILITY CURVES Time–Temperature Curve If the specific fire scenario is known, such as with a truck with a specific load, it is recommended that a predetermined time–temperature curve be used when designing the tunnel structure and equipment. Ideally, for a given fire scenario such as a single burning car, fire curves are used together with different exposure times. There are a number of known time–temperature curves used worldwide and these are presented in Figure 27. The Dutch RWS–temperature curve includes the most strin- gent temperature requirements and is referenced in NFPA 502 for structural design, as shown in Figure 28. The RWS curve was developed by the Rijkswaterstaat, the Dutch Ministry of Transport, Public Works, and Water Management and applies to tunnels that are open to the transport of hazardous sub- stances. This curve is based on the assumption that, in a worst case scenario, a 50 m3 (1,765 ft3) fuel, oil, or gasoline tanker fire with a fire load of 300 MW (1024 MBtu/hr) occurs, lasting up to 120 min (65). The RWS curve was based on the results of testing carried out by TNO (the Netherlands Organization for Applied Scientific Research) in 1979. Recently, the accu- CHAPTER ELEVEN DESIGN FIRE SCENARIO FOR FIRE MODELING

racy of the RWS fire curve as a design fire curve for road tun- nels was reconfirmed in the full-scale tests in the Runehamar Tunnel in Norway. The RWS curve and the temperature devel- opment table of the RWS fire curve is presented in the Annex (Explanatory material to Protection of Structural Elements) of NFPA 502 (see Figure 28). The RWS curve is based on the level of temperature found when a fire occurs in an enclosed area, such as a tunnel, where there is little or no chance of heat dissipating into the surroundings. The RWS curve simulates the initial rapid growth of a fire using a fuel tanker as the source and the grad- ual drop in temperatures to be expected as the fuel load is burned off. In reality, the construction may not be exposed to these time–temperature curves over the entire tunnel length. In a 98 tunnel with a single vehicle fire, the tunnel lining is exposed locally to heat fluxes from the flame volume and the hot smoky gases. In a tunnel accident with multiple vehicles, the fire spreads from one vehicle to the next resulting in different heat expo- sures to the tunnel lining depending on time, location, fuel load, and oxygen available. The fire moves within the tunnel in a dynamic manner and the heat fluxes to the linings vary depending on the origin of the fire, the ventilation rate, the type and amount of fuel (HRR), and the size of the cross section. The gas temperature, the surrounding wall temperatures, the emissivity of the hot gases in the vicinity of the fire, and the surface temperature of the linings govern the net heat flux at the surface of the linings. The net heat flux to the linings will in turn govern the temperature rise inside the lining material. TABLE 30 EXAMPLES OF DESIGN FIRE SCENARIOS BASED ON INTERNATIONAL STANDARDS Fire Scenarios Important Requirements That Have to Be Met Description of the Design Fire Examples of Related Standards No. Purpose 1 Test of construction material for immersed reinforced concrete tunnel structures, when passing of dangerous goods such as gasoline tankers is allowed - Temperature at the interface of heat insulation panels and the concrete of the tunnel structure may not exceed 380°C (716°F). - Temperature at the steel reinforcement of the tunnel structure may not exceed 250°C (482°F). - Time dependence of the temperature in the test oven according to the RWS curve. - Maximum temperature 1350°C (2462°F)—duration of the test burning: 2 h. Dutch K.I.V.I. and Rijkswater- staat guidelines 2 Test of construction material for reinforced concrete tunnel structures when: - Dangerous good are allowed and - An immediate tunnel collapse or water intake is not anticipated Temperature at the steel reinforcement of the tunnel structure may not exceed 300°C (572°F). - Time dependence of the temperature in the test oven according to the ZTV Tunnel. - Maximum temperature 1200°C (2192°F)—duration of the test burning: 1 h 50 min (decline phase included). ZTV-Tunnel, Germany 3 Test of jet fans for longitudinal ventilation systems The jet fans and their related equipment for the electrical power supply must work at least 90 min, when hot air and smoke (temperature 250°C or 482°F) is flowing through them and surrounding them. The test equipment must be able to deliver hot soot-enriched air at a temperature of 250°C (482°F) for at least 90 min. RABT 1994, Germany 4 Designing of a longitudinal ventilation system with jet fans capable of controling a truck fire event with a calorific heat output of approximately 20 MW (68 MBtu/hr) - Enough power to push the smoke in one direction of the tunnel (e.g., account for thrust loss of fans in hot air). - Choice of fan distribution along the tunnel for retaining enough fans for smoke control when some fans are damaged by the fire. - Availability of a fan operation mode which keeps emergency paths free from smoke. - Fire data: see no. 2 - Smoke generation: approx. 60 m3/s (2,119 ft3/s) at a reference temperature of 300°C or 572°F. RABT 1994, Germany Source: PIARC (21).

99 The net heat flux q″s to the lining can be estimated by the following equation: where: q″s is the net heat flux to the linings, εg is the emissivity of the hot gas, hs is the convective heat transfer coefficient, Tg is the gas temperature, Twall is the surrounding wall and floor temperatures, and Tlin is the lining temperature where q″s is determined. ′′ = + −( ) − + −(q T T T hs T Ts g g g wall lin g linε σ ε σ σ4 4 41 ) ( )27 The incident thermal radiation from the fire to the tunnel lining is highly dependent on the geometry of the flame vol- ume and its smokiness. The flame volume and its geometry are dependent on the HRR and ventilation conditions within the tunnel. The fraction of the flame radiant heat flux of the total heat release varies for most fuels and is between 0.25 and 0.4. For large tunnel fires, the tunnel linings in the vicinity of the fire are primarily affected by this incident flame radiant heat flux. The project shall develop a time-of-tenability criteria based on the design maximum HRR. This maximum HRR may dif- fer from 300 MW (1024 MBtu/hr) and gasoline tankers may FIGURE 27 Time temperature curves (65). FIGURE 28 RWS curve (65).

not be allowed to travel through the tunnel (conditions at which the RWS curve was obtained). Simple heat transfer equations do not allow for the mak- ing of a direct correlation between the time–temperature curve and the time–heat release curve. It appears that the known fire growth rates follow the super fast (highest increasing rate measured) temperature rise in the time–temperature curves. However, the use of HRR curves for the design is often allowed. When using HRRs instead of time–temperature curves for calculating structural stresses resulting from a fire, a super fast increase of the HRR is to be used as it was observed with the Runehamar tests in Norway in late 2003. This phase is fol- lowed by a maximum design HRR according to the type of vehicle investigated. The HRR within this scenario will be determined by the type of load that is allowed to pass through the tunnel as well as by the ventilation and fire suppression conditions, if applicable. Following the decay of the fire, a linear or steeper decrease is used. The duration of the maximum HRR can be deter- mined by using the burning load and type of fire suppression. At the very least, the equipment must be able to function for the duration of the anticipated escape and rescue time. It must be considered that equipment in the direct fire zone may not withstand the fire for an extended amount of time. Time-of-Tenability For fire life safety an integrated approach is to be taken. Time- of-tenability can be understood by analyzing the entire system with all components working together. To develop a time-of-tenability final curve the project must develop: • A fire HRR curve as a function of time. • A design evacuation (egress) curve as a function of time. • A design systems response curve as a function of time. This time line is illustrated in Figure 29. The development of a fire, or the fire heat release curve, was discussed in the previous chapters and is a function of: • Maximum FHRR, • Fire growth rate (quadratic curve for either super fast, fast, medium, slow fire growth rate), and • Fire decay rate. 100 EMERGENCY EGRESS TIMELINE The egress timeline depends heavily on human behavior. Human behavior in a tunnel fire emergency can be a compli- cated. Unfortunately, in general, people tend to do the wrong thing in the event of a tunnel fire, such as staying inside their cars instead of heading for the emergency exits. Intelligent Transportation Systems (ITS), warning motorists of any impending danger and providing them with valuable early directions, could be the extremely helpful. Significant research has attempted to address such issues as to why people in vehicles in tunnels do not leave their cars and escape, but instead end up dying? Why do some people leave their vehicles and then return to them when the fire grows? Educating people and notifying them of danger is a separate subject. For design purposes, there is a need to assume that people will realize the danger, be notified to evacuate, make the correct decision on the direction for evacuation, and go to the point of safety. However, this may not happen immediately and some reaction time will be needed in realizing the danger of the situation. It could be assumed that occupants of vehicles will have noticed the fire event within 30 to 60 s of ignition if the fire is rapidly developed. After that, there is some reaction time needed to make a decision. The project may consider that people will not move until they hear an alarm and get direc- tion from the operator to evacuate. In addition, it is necessary to add times for detecting and alerting, reaction and leaving the vehicles, and walking to a safe place, to know if people can escape the fire safely. The sum of detection and alerting times depends on the type of fire detection and how the infor- mation is given to people in their vehicles. Therefore, this can take 2 to 5 min in manned tunnels. The sum of the reaction times and leaving the vehicle is also difficult to estimate. For example, it takes longer for pas- sengers to escape a bus than a car. Therefore, the sum of these times may vary between 30 s and 5 min. FIGURE 29 Fire emergency timetable (6, 66).

101 It is especially important when considering evacuation from a bus. A German study, Fire Protection in Vehicles and Tunnels for Public Transport (59) cites 2 min as the maximum period of time acceptable for evacuating a bus. Other studies report that 3 min is the expected time to fully empty a loaded transit bus. Walking speeds can also vary. Depending on age and state of health, people can walk at a speed from 1 to 1.6 m/s (197 to 315 fpm). A series of experiments exploring the relationship between visibility in smoke and evacuation movement were conducted in a smoke-filled corridor 20 m (65.6 ft) long. The experi- mental population consisted of 17 females and 14 males, ranging from 20 to 51 years in age. Experiments were con- ducted using both nonirritant and irritant smoke. People were asked to travel from one end of the corridor to the other, iden- tifying when they could see a fire exit sign. Both the irritancy and the density of the smoke affected the volunteer’s walking speed. Figure 30 shows the gradual decline of the recorded walking speed through nonirritant smoke as the density of the smoke is increased, whereas in irritant smoke the gradient is far steeper. This was explained as being caused by the erratic movement of the volunteers owing to their inability to keep their eyes open. The volunteers attempted to compensate for this lack of orientation by using the walls for guidance. Results suggest that in nonirritant smoke with an OD of 0.43 m (1.4 ft) (extinction coefficient of 1.0) walking speeds are reduced to 0.5 m/s (98.4 fpm). However, in irritant smoke at an OD of 0.22 m (0.72 ft) (extinction coefficient of 0.5), the walking speed is reduced to 0.4 m/s (78.7 fpm) (see Figure 30). PIARC suggests that the walking speed in a smoky environment (with some level of visibility) is from 0.5 m/s (98.4 fpm) to 1.5 m/s (295.3 fpm) (21). Consideration needs to be made for people with mobility impairments. The speed of movement for those who are mobility impaired was tested in Leipzig on the station’s platform (60) and is presented in Table 31. This table shows that a walking speed of 0.5 m/s (98.4 fpm) can be considered as a reasonably good estimate. Depending on the number of evacuees (occupant load), a bottleneck may form approaching the cross passages or egress stairs. It is not possible to take fire and smoke under control immediately. Therefore, for several minutes, fire and smoke will be driven by natural factors. This is the most important FIGURE 30 Walking speed in irritating and nonirritating smoke (9). Users Speed of Movement Movement Time Distance 110 m (360 ft) Wheelchair Users 0.7 m/s (138 fpm) 150 s People with Prams/Carriages 1.1 m/s (217 fpm) 95 s People with Walking Aids 0.6 m/s (118 fpm) 175 s People with Infants 0.55 m/s (108 fpm) 190 s Source: Fire Protection in Vehicles and Tunnels for Public Support (59). TABLE 31 SPEED OF MOVEMENT AND EVACUATION TIMES OF MOBILITY-IMPAIRED PEOPLE

102 noitavitcAsmetsySSLFeucseR-fleS A. Make a decision to evacuate B. Disembark the bus C. Walk away from the fire effected zone D. Reach cross passage 1. Detection Time 2. Operator Reaction Time (alarm) 3. Systems Activation 4. All Fans Activated 5. Ventilation Mode in Full Operation TABLE 32 EXAMPLE OF PROJECT ESTABLISHED TIME-OF-TENABILITY CURVE FIGURE 31 Example of project established time-of-tenability curve (67). time for evacuation. The sooner smoke and fire will be taken under control the sooner there will be a tenable environment for evacuation. The distance that people can safely travel to an exit depends on the fire development and system activa- tion. The primary role in system activation is fire detection. Thus, spacing between cross passages will largely depend on the fire-detection system. For example, if the fire is not detected, the smoke control systems are not activated and spacing between cross passages would be determined based on the speed of the loss of visibility and smoke growth in the path of evacuation. Application of the tenability criteria at the perimeter of a fire is impractical. The zone of tenability is defined by apply- ing it outside the boundary, away from the perimeter of the fire. This distance will depend on the FHRR. EQUIPMENT ACTIVATION TIMELINE It was discussed in previous chapters that it is not possible to achieve a fully operating mode for all fire fighting equipment instantaneously. Equipment activation time consists of the following phases for supervised tunnels: 1. Fire-detection time (from 2 to 3 min if reliable auto- matic fire-detection system is installed). 2. Fire alarm and operator reaction time (from 60 to 90 s). 3. Time to bring the first group of fans to full speed (60 s for unidirectional and 90 s for reverse mode— NFPA 502). 4. Activate fixed fire suppression system if desired (30 s– 60 s if wet). 5. Achieve a full operational mode for ventilation system (180 s). For the unmanned tunnels, the system is usually designed to be fully automatic or operated by the local fire department. In any case, the first and the most critical element of the system is fire detection. Although many tunnels still rely on manual fire detection, this needs to be revisited. Operators may require help in detecting a fire, which would allow them to take appropriate actions in a timely manner. COMBINED CURVE FOR EVACUATION AND SYSTEM ACTIVATION Based on fire development, emergency egress, and the equip- ment activation timeline, it is possible to create a combined heat–egress system activation time curve similar to the one presented in Table 32. This curve allows one to analyze the design HRR at every evacuation and system activation phase and to make the correct decisions. When the evacuation phase is concluded, fire fighting must be facilitated by proper smoke handling. A basic requirement is to provide maximum opportunity for the fire fighting access in minimum smoke. During evacuation, the direction of smoke flow must not change. With the arrival of the fire department, it can be decided on-site which fan control is the best to facil- itate the fire fighting. The time-of-tenability graph can be prepared as the result of fire life safety systems design and CFD analysis. A sam- ple of this graph is shown in Figure 31. This graph is called a tenability map and shows all time steps discussed earlier and the resulting impact on casualties and tunnel structure. It allows one to predict for how long the environment will be tenable

103 in the tunnel and helps to decide what needs to be done to achieve fire life safety goals. In this figure the pre-movement time is the time between discovery of a fire and the start of egress travel. Figure 32 illustrates how longer detection and pre- movement times with greater fire hazards can lead to casual- ties. Figure 33 illustrates the impact of longitudinal ventilation on fire life safety and structural protection. It shows no casu- alties and much safer fire fighting. Tunnel spalling danger is eliminated on the upstream side and significantly reduced on the downstream side. SUMMARY A fire scenario must be designed to get an optimum fire life safety strategy for road tunnels. Fire scenarios are used for the following: • Design of emergency exits • Choice of a fire-detection system • Choice of ventilation and fire suppression systems • Tunnel structural engineering • Specification requirements for tunnel structures and equipment • Operation of the tunnel • Training of operators and first responders dealing with tunnel fires. Fire scenarios usually include: • Governing standards and guidelines • Description of the scenario • Thorough definition of the fire parameters (e.g., HRR/ temperature versus time) • Traffic scenario operation during fire emergency and tunnel ventilation operation FIGURE 32 Time-of-tenability sample graph with no ventilation (67). FIGURE 33 Time-of-tenability sample graph with longitudinal ventilation (35, 68).

• Guidelines for structural protection • Specifications for materials, equipment, and structure. If the specific fire scenario is known, such as with a truck with a specific load, it is suggested that a predetermined time–temperature curve be used when designing the tunnel structure and equipment. There is a number of known time– temperature curves used worldwide. The Dutch RWS temper- ature curve includes the most stringent temperature require- ments and is referenced in NFPA 502 for structural design. This curve is based on the assumption that, in a worst-case scenario, a 50 m3 (1,765 ft3) fuel, oil, or gasoline tanker fire with a fire load of 300 MW (1024 MBtu/hr) occurs, lasting up to 120 min. Simple heat transfer equations do not allow for the making of a direct correlation between the time–temperature curve and the time–heat release curve. It appears that the known fire growth rates follow the super fast (highest increasing rate mea- sured) temperature rise in the time–temperature curves. How- ever, the use of HRR curves is often allowed for the design. For fire life safety an integrated approach is to be taken. Time-of-tenability can be understood by analyzing the entire system with all components working together. To develop a time-of-tenability final curve, the project must develop: 1. A fire heat release curve as a function of time. 2. A design evacuation (egress) curve as a function of time. 3. A design systems response curve as a function of time. 1. The development of fire or a fire heat release curve is a function of: • Maximum FHRR, • Fire growth rate (quadratic curve for either super fast, fast, medium, or slow fire growth rate), and • Fire decay rate. 2. The egress timeline depends greatly on human behavior. For design purposes, there is a need to: • Assume that people will realize the danger, be noti- fied to evacuate, make the right decision on the direc- tion for evacuation, and go to the point of safety. 104 • It is necessary to add times for detecting and alert- ing, reaction and leaving the vehicles, and walking to a safe place, to know if people can escape the fire safely. Spacing between emergency exits shall be justified by calculations. It is impossible to take fire and smoke under control immediately; therefore, for several min- utes, fire and smoke will be driven by natural factors. This is the most important time for evacuation. The sooner smoke and fire are under control, the sooner there will be a tenable environment for evacuation. The dis- tance that people can safely travel to an exit depends on the fire development and system activation. The primary role in system activation is fire detection. Thus, spacing between cross passages will largely depend on the fire- detection system. 3. Equipment activation time consists of the following phases for supervised tunnels: • Fire-detection time • Fire alarm and operator reaction time • Time to bring the first group of fans to full speed • Time to activate the fixed fire suppression system if desired • Achieve a full operational mode for ventilation system. For unmanned tunnels, the system is usually designed to be fully automatic or operated by the local fire department. Based on fire development, emergency egress, and equip- ment activation timeline, it is possible to create a combined heat–egress system activation time curve. This curve allows for the analysis of the design HRR at every evacuation and system activation phase and aids in making the correct decisions. A tenability map shows all time steps and the resulting impact on casualties and tunnel structure. It allows one to predict for how long the environment will be tenable in the tunnel and helps to decide what needs to be done to achieve fire life safety goals.

Next: Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size »
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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 415: Design Fires in Road Tunnels information on the state of the practice of design fires in road tunnels, focusing on tunnel fire dynamics and the means of fire management for design guidance.

Note: On September 20, 2011, the following errata was released related to NCHRP Synthesis 415. The electronic version of the publicaiton was changed to reflect the corrections.

On pages 106 and 107, an incorrect reference was cited. In the final paragraph on page 106, the last sentence should read: One study came to the conclusion that, although some minimum water application rates would achieve a certain objective, a marginally higher rate would not necessarily improve the situation (79). The figure caption for Figure 35 at the bottom of page 107 should read: FIGURE 35 NFPA 13, NFPA 15, and other International Water Application Rates (79).

The added reference is as follows:

79. Harris, K., “Water Application Rates for Fixed Fire Fighting Systems in Road Tunnels,” Proceedings from the Fourth International Symposium on Tunnel Safety and Security, A. Lönnermark and H. Ingason, Eds., Frankfurt am Main, Germany, Mar. 17–19, 2010.

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