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83 TABLE 25 (continued) Tunnels 05.04.B 42 PIARC Integrated Approach PIARC Recommendation PIARC (2007) C3.3 to Road Tunnel Safety R07 43 NVF Ventilation av Nordic Road Report of a NVF 1993 Vgtunnelar Technical Nordic working (Ventilation of Road Association group Tunnels) NVF Sub Committee 61: Tunnels 44 ASTRA Tunnel Task Force, Swiss Federal Recommenda- May 2000 Final Report Roads Office tions for improved safety 45 PWRI/Japan Road Tunnel Public Works Technical Ministry of Technology in Japan Research Institute Memorandum Construction, 1991 PWRI no. 3023 46 PWRI/Japan State of the Road Public Works Technical Note Ministry of Tunnel Equipment Research Institute Construction, 1993 Technology in Japan--Ventilation, Lighting, Safety Equipment Public Works PWRI Vol. 61 47 PWRI/Japan Report on Survey and Public Works Survey Report Technology Centre of Research on Tunnel Research Institute Metropolitan Expressway Ventilation Design (1993) Principles (Tunnel Ventilation Design Principles--Draft) 48 European Fire in Tunnels FIT Technical Report Thermal Network FIT Thermal supported by European Network Community G1RT-CT- 2001-05017 From numerous sources. UN = United Nations; EU = Europan Union; PIARC = World Road Association (l'Association mondiale de la route). The regulations and guidance need to provide better con- neath the smoke layer (applicable to bi-directional or sideration of the inter-activity of all systems that interact congested unidirectional tunnels) or in a tunnel. Integrated approaches shall be applied to tun- That smoke must be completely pushed to one side nel fire safety. of the fire (preferably applied to noncongested unidi- Better identification with regard to human behavior of rectional tunnels where there are normally no people both tunnel users and operators is important, as well as downstream of the fire). identification of the means to improve safety. 2. People must be able to reach a safe place in a reasonably Consideration shall be given for technical innovations short time and cover a reasonably short distance. Emer- that allow more ambitious safety objectives. gency exits are provided whenever necessary. 3. The ventilation system must prevent smoke from spread- TUNNEL VENTILATION AND INTERNATIONAL ing to uninvolved areas. STANDARDS REQUIREMENTS 4. The ventilation system must be able to produce good conditions for fire fighting. Ventilation for fire and smoke control requirements in the 5. In the event of a fuel fire, secondary explosions result- international standards are summarized based on the literature ing from incomplete combustion have to be avoided. review conducted for this effort. When there is a fire, the fol- Therefore, the ventilation system must be able to deliver lowing safety criteria have to be applied in the design: enough air for the complete combustion or dilution of explosive gases. A suitable drainage system is provided 1. The purpose of controlling the spread of smoke is to keep to minimize the surface area where fuel evaporation people in a smoke-free environment as long as possible. takes place. This can mean one or both of the following: That either the smoke stratification must be kept intact, There are two categories of ventilation used in most leaving more or less clean and breathable air under- tunnels: natural and mechanical. Appendix F1 (web-only)

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84 provides comparison tables on tunnel ventilation require- downstream of the fire. This phenomenon is more evident at ments in different national (including NFPA 502, 2008 edi- higher air velocities. The smoke stratification can also be dis- tion) and international standards. It covers requirements for turbed by the longitudinal slope of the tunnel (especially when natural ventilation, transverse ventilation, and emergency air flows downwards) and by vehicles. exits pressurization. Smoke from a fire in a tunnel with no slope will naturally Natural ventilation relies on natural phenomena and traf- tend to propagate in both directions owing to buoyancy effects. fic piston effect to renew the air in the tunnel. This ventila- If the ventilation is in operation, the smoke will tend to be tion system can be very effective for the dilution of pollutants driven in the direction of the ventilation airflow. At low tun- (especially for one-way tunnels); however, it is not possible nel airflow speeds, the buoyancy-induced flow is not entirely to rely on natural ventilation for safety purposes. Indeed, overcome and some smoke will flow upstream, which is often in the event of a fire in a tunnel, traffic will most likely stop, termed "backlayering." and the ventilation is only provided by natural phenomena that could be only partially deterministic (as the chimney The backlayering distance may be defined as the distance effect). However, the main component of the ventilation will from the fire where the upstream smoke velocity is eliminated be quite uncertain (as meteorological components) and there- by the tunnel ventilation flow. Hence, a backlayering distance fore unreliable. of zero would imply that no smoke flows upstream. The tun- nel air velocity required to achieve this condition is termed the Naturally, ventilated tunnels rely primarily on atmospheric "critical velocity." conditions to maintain airflow and provide a satisfactory envi- ronment. The main factor affecting the environment is the pres- Air velocity to prevent backlayering depends on the FHRR sure differential created by variations in elevation, the ambient Q, the tunnel area A, and height H. Air velocity increases with air temperature, or wind effects at the boundaries of the facility. the FHRR, but then levels off as the HRR increases. Unfortunately, most of these factors are highly variable with time and, therefore, the resultant natural ventilation is neither The design of the ventilation system and its operation must reliable nor consistent. take into consideration that, owing to the presence of the lon- gitudinal airflow, the zone downstream of the fire is exposed Because of the number of different parameters that interfere to smoke and hot combustion gases. This can lead to suffoca- in the choice to ventilate a tunnel or not (length, location, traf- tion or burns for users in this zone. Any possible design mea- fic, type of vehicles using the tunnel, and so forth), it is not pos- sure aiming for a safe escape from the dangerous section (fire sible at this moment to express universal recommendations area or downstream) must be taken. For this reason, the pres- about the limits of the natural ventilation, especially the allow- ent UPTUN recommendations take into consideration the fol- able length without mechanical ventilation. lowing cases. A tunnel that is long or experiences frequent adverse atmos- A tunnel with one-way traffic not designed for queues pheric conditions requires fan-based mechanical ventilation. (a nonurban area) has a ventilation design that can assume that Among the alternatives available for road tunnels are longitu- drivers downstream of the fire are free to escape by means of dinal and transverse ventilation. their own cars, whereas drivers upstream will not. Tunnels located in nonurban areas are generally not situated in fre- Longitudinal ventilation introduces or removes air from quent congestion situations. Therefore, the relevant ventila- the tunnel at a limited number of points, primarily creating tion systems are generally not designed for queues. Nonurban longitudinal airflow along its length, from one portal to the tunnels, which are frequently congested, have instead to be other. Longitudinal ventilation can be accomplished either designed for queues. The event of a fire ignited by vehicles by injection, using central fans, using jet fans mounted within involved in a secondary accident in the presence of other vehi- the tunnel, or a combination of injection and extraction at inter- cles trapped downstream is possible, but the relevant proba- mediate points. bility is low. This case is almost never taken into account in the design phase. If necessary, the risk of such an occurrence In longitudinal ventilation systems, using jet fans or por- can be reduced by automatic incident detection and a traffic tal nozzles (often called a Saccardo system), a longitudinal control system. airflow sweeps all exhaust gases from the entrance to the exit portal. The required longitudinal air velocities preventing smoke backlayering must be calculated by considering the following The only feasible way to evacuate smoke with longitudinal parameters. Meteorological parameters, especially longitudi- ventilation is by pushing it through the tunnel toward the por- nal, can influence the performance of the ventilation systems. tal. However, the airflow velocity necessary for such operation The ventilation system must have sufficient capacity to pro- is the cause of turbulence and affects the smoke stratification duce the required air velocity against a stated adverse wind

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85 pressure. The difference in pressure can be evaluated using The blockage effect of the fire on the longitudinal the following simplified equation of Bernoulli: airflow produces a supplementary local head loss. With a tunnel with a steep grade, the chimney effect 1 can be raised to significant values. = k 2 (26) 2 The decrease of air density results in the lowering of the driving force of the jet fans that work in the hot where: air. represents the pressure induced by wind, The reversibility of the system can be helpful during the fire the air density, fighting phase. the wind speed, and k a design parameter that depends on the configuration of When planning the reversing of the air, it must be taken the portals. into consideration that such operations can take a longer time, depending on the ventilation system, the tunnel geometry, the This effect was studied by Blendermann (54) (see Figures 23 fans used, and other conditions. and 24 and Table 26): The reversal of jet fans is generally not recommended The orientation of both tunnel portals with respect to the during the evacuation phase, even if the fire is located near prevailing winds is a very significant parameter. The effec- the entrance portal. In the period between the ignition of the tive wind resistance (or thrust) is a function of the angle fire and the reversal of the jet fans, the smoke already can between the direction of the wind and the direction of the air have traveled several hundred meters. When the smoke flow entering or exiting the tunnel. layer flow is reversed, it will be spread over the whole cross section, whereas during the people evacuation phases it is The traffic condition must also be evaluated. When eval- important to maintain good visibility conditions. Therefore, uating the necessary thrust in case of a fire, it must be only after everyone is out of the tunnel can the reversal of assumed that a certain number of vehicles can be trapped the air flow direction take place. The reversing can be eval- in the tunnel and their presence reduces the performance uated in the event of a traffic jam inside the tunnel, but it of the ventilation system. The number of vehicles trapped must be a human choice, not an automatic configuration. can be assessed according to the design mix of traffic Table 27 summarizes the recommended ventilation opera- (percentage of passenger cars and heavy vehicles), the tion in case of fire. level, and the performance of the current road operation and traffic control system available for the tunnel. In the case of twin tunnels, reversing the flow in the non- For the effects of fire on the air flow, several aspects incident tunnel can prevent the circulation of smoke evacu- must be taken into account: ated through the portal of the twin tunnel. Such circulation of In the event of a big fire, the high temperature induces smoke can also be prevented by civil engineering work (the an increase of air volume (resulting from expansion) distance between the twin portals, protection walls between and therefore of air speed, as a result of which the air portals, and so forth). friction losses increase. The density decreases, friction velocity increases, and The ventilation system needs to be designed to meet the the overall local losses increase. previous requirements in case of a fire. For the design and FIGURE 23 Mean Wind Pressure Coefficients (54).

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86 (a) (d) (b) (e) (c) (f) (g) FIGURE 24 Some configurations of tunnel portals tested by Blendermann (54). choice of all equipment it has to be taken into account that the A normal fan motor has to be cooled by outside air to hot smoke, traveling over the whole tunnel length, can seri- meet the cooling requirements. However, there are ously affect the installations (especially if the tunnel has a motors available that have a very high resistance with- thermal insulation). out external cooling. All the auxiliary equipment as well as the wiring of the Cables, junction boxes, and all other nonprotected parts of fan has to meet the air temperatures. the ventilation system have the same fire resistance as fans. For these reasons, fans must be designed and built to with- Special requirements shall be provided to jet fans operat- stand high temperatures. ing in fire emergency: There are several national standards for the heat resistance The strength of a normal aluminum blade falls quickly of fans, ranging from 250C (482F) for 1 h (Austria, the at high temperatures, although it depends on the type of Netherlands, United Kingdom, and the United States), 250C alloy. When high air temperatures cannot be avoided, it (482F) for 1.5 h (France), 300C (572F) for 1 h (Norway is important that steel blades be chosen. and Sweden), and 400C (752F) for 1.5 h (France and Owing to high temperatures, the length of the blades Switzerland). grows more quickly than the housing enlarges. The blade tips then tend to block the rotation. Abrasive tips Where fans are distributed along the tunnel, a limited fan may be introduced or a larger distance between blades redundancy is suggested and can avoid the use of fireproof and housing provided. fans. In case of a fire, the temperature decreases rapidly when

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87 TABLE 26 CONFIGURATION OF TUNNEL PORTALS TESTED BY BLENDERMANN Portal Below Ground Level Portal Above Additional Feature Ground Level With vertical side walls With sloping bounds -- Figure 24 (a) Figure 24 (d) * Dividing Wall Figure 24 (b) Figure 24 (e) * Light Adaptation Figure 24 (c) Figure 24 (f) Section Da m * * Figure 24 (g) Source: Blenderman (54). the distance from the fire sight increases. It may be cost- of the sidewalls. It is important to indicate that all supply air effective to envisage the destruction of a few jet fans. ducts and all extraction smoke ducts be very tight. For a tunnel with one-way traffic, designed for queues (an Continuous extraction into a return air duct is needed to urban area), the ventilation design must take into considera- remove a stratified smoke layer out of the tunnel without dis- tion that cars can likely stand to both sides of the fire because turbing the stratification. However, the following conditions of the traffic. In urban areas it is usual to find stop-and-go must be fulfilled: traffic situations. Therefore, this case generally applies to urban tunnels of sufficient length. The longitudinal velocity of the tunnel air must be below 2 m/s (394 fpm) in the vicinity of the fire incidence zone. For a tunnel with two-way traffic, where the vehicles run These were the observations in the Japanese full-scale in both directions, it must be taken into consideration that in tests. With higher velocities, the vertical turbulence the event of a fire vehicles will generally be trapped on both in the shear layer between smoke and fresh air quickly sides of the fire. cools the upper layer and the smoke then mixes over the entire cross section. Transverse ventilation uses both a supply duct system and With practically zero longitudinal air velocity, the smoke an exhaust duct system to uniformly distribute supply air and layer expands to both sides of the fire. The smoke spreads collect vitiated air throughout the length of the tunnel. The in a stratified way for up to 10 min, even without smoke supply and exhaust ducts are served by a series of fixed fans extraction (depending on the tunnel and fire conditions). usually housed in a ventilation building or structure. A vari- After this initial phase, smoke begins to mix over the ant of this type of ventilation is semi-transverse ventilation, entire cross section, unless by this time the extraction is where either a supply or exhaust duct is used, but not both. in full operation. The balance of airflow is made up within the tunnel portals. With an air velocity of around 2 m/s (394 fpm), most of The purpose of controlling the spread of smoke is to keep the smoke of a medium-size fire spreads to one side of the fire people as long as possible in a smoke-free environment. This (limited backlayering) and starts mixing over the whole cross means that the smoke stratification must be kept intact, leav- section at a distance of 400 to 600 m (1,312 to 1,968 ft) ing a more or less clear and breathable air underneath the downstream of the fire site. This mixing over the cross sec- smoke layer. The stratified smoke is taken out of the tunnel tion can also be prevented if the smoke extraction is activated through exhaust openings located in the ceiling or at the top early enough. TABLE 27 LONGITUDINAL VENTILATION OPERATION IN TUNNELS WITH ONE-WAY AND TWO-WAY TRAFFIC Longitudinal Ventilation Evacuation Phases Fire-fighting Phase One Tube with Two-way Traffic The smoke stratification must not Avoid backlayering of smoke: (not recommended in the U.S. be disturbed: - higher longitudinal velocity and many other countries) - longitudinal air velocity is - direction of airflow adaptable quite small - no jet fans working in smoke zone Two Tubes with One-way Traffic Normal free traffic: Avoid backlayering of smoke: sufficient longitudinal air velocity in the same direction as traffic flow. Congested traffic, or fire at the end of the queue behind an accident, or one tube used bi-directionally: Same as one tube with bi-directional traffic for the two phases.

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88 Vehicles standing in the longitudinal air flow increase operation design. Therefore, it is not sufficient to only open a strongly the vertical turbulence and encourage the ver- few exhaust openings near the fire, but a minimum exhaust tical mixing of the smoke. rate along the whole ventilation section is suggested as well. In a transverse ventilation system, the fresh air jets An extraction strategy needs to be developed depending on entering the tunnel at the floor level induce a rotation of the type of tunnel and its ventilation system. the longitudinal airflow, which tends to bring the smoke layer down to the road. This is the reason for the sugges- The extraction capacity over the tunnel length that is per- tion to throttle the fresh air rate from one-half to one-third missible for smoke to spread must exceed the smoke rate gen- of full capacity, depending on the initial fresh air jet erated by the fire, because the openings will not only exhaust momentum. No fresh air is to be injected from the ceiling smoke but inevitably some fresh air as well. in a zone with smoke because this increases the amount of smoke and tends to suppress the stratification. In reversible semi-transverse ventilation with the duct at Single-point Exhaust Opening the ceiling, the fresh air is added through ceiling openings in normal ventilation operation. If a fire occurs, as long as The spreading of smoke over the entire length of the tunnel fresh air is supplied through ceiling openings, the smoke can be prevented by a large extraction of tunnel air directly quantity increases by this amount and strong jets tend to above the traffic with suitable extraction ports. This system bring the smoke down to the road surface. The conver- works best in conjunction with jet fans (see Figure 25) or por- sion of the duct from supply to extraction must be done as tal (Saccardo) nozzles to localize smoke around openings and quickly as possible. to prevent smoke from being driven by natural factors (such as wind and tunnel grade) and spreading along the tunnel. It is usually part of longitudinal ventilation with one or several Continuous or Concentrated Smoke Extraction central exhaust shafts. (Single Point) The traditional way to extract smoke is to use small ceiling The exhaust capacity and the longitudinal velocity cre- openings distributed at short intervals throughout the tunnel. ated by the jet fans in the tunnel section filled with smoke Another efficient way to remove smoke quickly out of the traf- have to be matched and controlled under operation; it does fic space is to install large openings with remotely controlled not matter whether the smoke is stratified or spread over the dampers. They are normally in an open position where equal entire tunnel cross section. The recommended extraction extraction is taking place over the whole tunnel length. value is based on a cross-sectional area times longitudinal velocity. The system must be able to extract a longitudinal In case of a fire, the single-point extraction is achieved airflow of 3 to 4 m/s (591 to 787 fpm) and the small air veloc- in the fire location by remote control of the dampers. Recent ity in the following ventilation section toward the exhaust tests by CETU and the Memorial Tunnel fire tests have proven opening to prevent the spreading of smoke beyond the suc- the advantages of this system. To facilitate maintenance, there tion point. are systems in use where the large dampers are held by a mag- net in a closed position. In the fire zone, the magnets release Fresh Air Supply for Transverse Ventilation the damper mechanism automatically by command from fire detectors and the dampers then open by gravity force. How- During fires, it is suggested that the fresh air jets enter the ever, this system does not allow the openings to close if a tunnel near the road surface. Their exit velocity and the dis- smoke plume moves to another place in the tunnel. tances between the individual jets are small in order to obtain a uniform fresh air layer above the road. Extraction Capacity A large tunnel fire creates strong longitudinal airflows to Once a design fire and its amount of smoke production have supply the oxygen to the fire. With a continuous transverse been chosen, a permissible length over which the smoke may fresh air supply along the tunnel this longitudinal velocity is spread has to be fixed. Depending on the type of exhaust reduced, which minimizes the air mixing with the smoke layer. openings (fixed or remote-controlled), the extraction capacity per unit tunnel length in the fire zone is derived. In general, Fresh air jets entering from ceiling openings are unfavor- an extraction system needs less total exhaust volume when able. When they enter the tunnel vertically, they destroy the remote single-point extraction dampers are installed than with smoke layer, induce smoke into the jet, and thus suppress fixed openings. However, it also needs to be considered that smoke into the fresh air layer. The exit velocity of these ceil- in the first phase of the fire between the start of the smoke ing supply air jets is to be small. spreading and full operation of the exhaust system with large dampers, the smoke may have spread 1 km or more from Fresh air jets entering from the ceiling are stopped imme- the fire site depending on fire detection and ventilation system diately after the fire alarm sounds in the ventilation section.

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89 FIGURE 25 Tunnel with a single-point extraction system (55). For longer tunnels, it is suggested that the fresh air outlets be Also, it is important that no jet fan is turned on in or near a positioned near the road surface. place where there is smoke, as this would immediately destroy the smoke stratification. Fans in the exhaust air duct are exposed to a mixture of very hot air from the immediate area surrounding the fire plus The usual way to control the longitudinal velocity is to cooler air farther from the fire. This mixture of hot and cooler provide several independent ventilation sections. When a air then travels in the duct and gets more cooled down. tunnel has several ventilation sections, a certain longitudinal velocity in the fire section can be maintained by a suitable Tests in the Zwenberg Tunnel in Austria or in the Memo- operation of the individual air ducts. By reversing the fan rial Tunnel in the United States gave air temperatures at the operation in the exhaust air duct, this duct can be used to sup- fan below 250C (482F), even when the fire was very near ply air and vice versa. the fan station. Memorial Tunnel test results are presented in Table 21 in chapter nine. A fire resistance of the fans to Whatever the means of controlling the longitudinal air 250C (482F) could be considered sufficient for most of the velocity are, their operation has to be preprogrammed accord- fire events, but needs to be checked by design. ing to the location of the fire in the tunnel to ensure the opening of the required dampers and activation of required When fans are located close to or in the exhaust air open- fans. ings of the single-point extraction system, the exhaust fan temperatures must be evaluated in the design. Tunnel ventilation fans that are to be used in a fire emer- gency shall be capable of achieving full rotational speed Control of Longitudinal Velocity from a standstill within 60 s. Reversible fans shall be capa- for the Single-point Extraction System ble of completing full rotational reversal within 90 s (NFPA 502). The emergency ventilation system shall be capable of To maintain smoke stratification, a low-speed longitudinal reaching full operational mode within a maximum of 180 s air velocity is required to push smoke to one side of the fire, of activation. Fans could be activated sequentially based on which can be achieved by jet fans or Saccardo (portal) noz- fire zones. zles. However, the process of activating the required number of jet fans within a few minutes after fire ignition is compli- cated owing to the turbulent nature of tunnel airflow, large Emergency Exits Pressurization cross-section area, and changing winds and other natural fac- tors. This requires air velocity measurements as average over NFPA 502 calls for a tenable environment provided in the cross section (1): means of egress during the evacuation phase. Emergency "exits" shall be pressurized in accordance with NFPA 92A. Required accuracy 0.3 m/s, Appendix F1 (web-only) provides a comparison analysis of Short response time and time resolution, and the pressurization requirements of emergency exits in the Proper positioning of sensors. national and international standards.