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FLEXIBLE DEVICES FOR SMOKE CONTROL IN ROAD TUNNELS Bettelini M., Rigert S., Seifert N. Amberg Engineering Ltd., Regensdorf-Watt, Switzerland

ABSTRACT This paper summarizes the investigations carried out for systematically assessing the applicability of flexible devices for smoke control in road tunnels. After a thorough investigation of previous efforts, all possible applications of flexible devices in existing and new road tunnel were systematically evaluated. Permeable devices for controlling the longitudinal air velocity are being used in one tunnel in Austria. Three additional types of applications emerged from this screening process, which appear both technically feasible and practically interesting: closure of tunnel entrances by tight or permeable devices, to be applied to short, steep tunnel with natural ventilation, and smoke curtains in longitudinally ventilated tunnels with either bidirectional traffic or unidirectional traffic with high congestion frequency. The findings for smoke curtains, which allow for an excellent control of smoke propagation with low longitudinal air velocity, are particularly promising and are illustrated in some detail. Keywords: smoke control, road tunnel, fire compartment, smoke curtain, smoke barrier 1.

INTRODUCTION AND OBJECTIVES

Flexible devices for sub-dividing large rooms into fire compartments are commonly used in large buildings. They can be used for preventing fire and smoke propagation in case of fire emergency while preserving a great flexibility for building exploitation. A Swiss national research project was launched for investigating the use of similar devices for controlling smoke propagation in existing and new road tunnels. The issue of fire compartmentalization has been discussed in the past as an innovative approach for fire and smoke control in road tunnels. The specific objectives, the physical principles involved and the resulting physical realization depend on the tunnel configuration considered. In principle the physical scope is very wide and ranges from the pure control of longitudinal air velocity (as investigated in previous efforts in Austria, discussed in the following chapter of this paper) to fire-compartment building, with the objective of oxygen depletion (self consuming fire) and direct prevention of smoke propagation. Fire compartmentalization can moreover enhance fire extinction using water-mist systems (Bettelini & Seifert, 2009, [2]). These effects follow the fire extinction principles of the “fire triangle”. A number of different devices, including massive doors, water or air jets and many more could be used in a similar manner for achieving congruent objectives. The field of investigation had to be narrowed for preventing an excessive dispersion of energies. The scope of this research project was therefore restricted to flexible, solid devices. Its focus was moreover more on physical principles than on specific constructive characteristics. Based on the results, the development effort needed for providing suitable devices shall be left to the market. Conversely, with respect to tunnel configuration maximum generality was striven for, including single and double-tube tunnels, fluid traffic and congestion, natural or mechanical ventilation, cut-and-cover and excavated tunnels, existing and new tunnels. 6thInternational Conference ‘Tunnel Safety and Ventilation’ 2012, Graz

- 266 The key objectives of the research was the identification and assessment of the most promising configurations in term of prevention of loss of human life, reduction of material damages and minimization of tunnel unavailability in case of fire. 2.

PREVIOUS INVESTIGATIONS

Results from previous investigations were gathered by means of a comprehensive literature study and of questions sent to 11 leading international experts in the field of tunnel ventilation and safety. The results were very satisfactory and allowed for a full overview of previous investigations and experiences. The most important and best known application of flexible devices for reducing the longitudinal velocity of air in fire incidents is described in Öttl et al. (2002, [6]). The devices considered consist of a number of strips formed by a fire-resistant textile, which can very effectively reduce the longitudinal air velocity in the tunnel while allowing for the transit of vehicles and persons. The investigation, which also included full-scale tests in the tunnel Roppen (Henn & Sturm, 2010, [4]), resulted in the installation in the Austrian tunnel Roppen (double-tube with unidirectional traffic, 5’069 m, concentrated smoke extraction in case of fire) and in the commercialization of the devices (system FIREcurtains by Aigner Tunnel Technology). Further investigations were carried out for an inflatable tunnel plug in a real-scale tunnel fire test during the UPTUN project (Bergmeister, 2005, [1]). The goal was to isolate the fire in short tunnel sections. The tunnel barrier efficiently reduced smoke concentration outside the fire section. Complete sealing of the fire compartment was not possible. However, this kind of confinement of the fire leads to the formation of a highly flammable gas mixture in the compartment and, as Bergmeister (2005, [1]) recognized, “…this would increase the risk of an explosively burst of the plug with all consequences” (backdraught). The same concept, building of fire compartments with the aim to extinguish the fire, was experimentally investigated by Kohl. He observed that, after closing the barriers of the fire compartment, fire power increased for a short time, before being drastically reduced, due to lack of oxygen (Kohl et al., 2005, [5]). Although a fixed construction was used to seal the fire compartment, fire caused damage to the structure, which enabled some fresh air to penetrate into the compartment. 3.

NEEDS AND OPPORTUNITIES

3.1. Safety needs in road tunnels The main safety issues in road tunnels arise from fire scenarios. Tunnels of a certain length normally have a powerful ventilation system for extracting or controlling longitudinal smoke propagation. Shorter tunnels have mainly longitudinal ventilation or only natural ventilation. As showed by several incidents in the recent past (e.g. Mont Blanc, Gotthard and Viamala), incomplete control of smoke propagation can lead to dramatic problems for self-rescue of the tunnel users and for intervention. Particularly unfavorable conditions are generally observed in short tunnel with high longitudinal slope (Bettelini & Seifert, 2010, [3]). A systematic preliminary screening was conducted for all practical tunnel types, traffic conditions and ventilation systems. The main goal was the identification of the situations, for which the greatest needs for innovative solutions arise. Based on this, it was decided to focus on the following configurations: •

Short tunnels with natural ventilation and high longitudinal slope, where high longitudinal air velocities arise because of the “stack effect”.

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- 267 •

Short, steep tunnels with longitudinal ventilation (uni- & bi-directional traffic), where mastering the longitudinal air velocity is frequently extremely difficult (jet fans in the smoke, complex regulation of longitudinal air velocity based on measurements of uncertain accuracy because of the presence of smoke).

•

Tunnels with smoke extraction systems but insufficient control of longitudinal air velocity, where smoke extraction alone does not allow for a sufficient control of smoke propagation because of the excessive longitudinal air velocity.

Additional needs arising from insufficient ventilation power or wrong design in existing tunnels are unfortunately not uncommon but shall not be considered specifically, since they call in most cases for very specific solutions. For the application of different types of flexible devices in tunnels the key issues to be accounted for are: •

Aerodynamic characteristics (influencing fire development, preventing smoke destratification, influencing smoke propagation, etc.).

•

Self-rescue conditions for tunnel users (fire and smoke compartmentalization without endangering the tunnel users during self-rescue).

•

Conditions for intervention (favorable conditions for rescue and fire fighting, flexible intervention options and constraints, self-protection conditions for fire services).

3.2. Physical effects and safety implications The following physical principles could be exploited by means of different devices: •

Reduction of longitudinal air velocity. This is important for all tunnels where vehicles are trapped on both sides of the fire (bidirectional or unidirectional congested traffic) and where high longitudinal airflows could be expected (e.g. stack effect in case of large longitudinal slope or large meteorological pressure differences on Alpine tunnels), independently on the ventilation system.

•

Partial smoke blockage by means of smoke curtains in the upper part of the tunnel profile. Unlike common application of smoke curtains e.g. in train or metro stations, the focus is here on the reduction of the critical velocity.

•

Form a barrier for smoke confinement, on one or on both sides of the fire.

•

Form a closed compartment around the fires for fire extinction by oxygen depletion.

Most practical devices act on more than one way. Some degree of blockage of longitudinal air velocity is e.g. clearly provided by almost all practical systems. Similarly, some level of oxygen depletion is provided by several devices. It is nevertheless important distinguishing between main and side effects, for a proper identification of the characteristics of the best possible device for every potential application.

Partial closure / smoke curtain

Full closure / smoke barrier Permeable closure / permeable curtain

Figure 1: The basic devices considered.

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- 268 A number of disadvantages and potential risks had to be accounted for from the beginning. The most obvious is physical blockage, which hinders the transit of persons and vehicles. Smoke barriers typically improve the conditions on one side, while higher smoke concentrations or some degree of stratification loss could be expected on the other one. This is particularly important during the self-rescue phase but has important consequences also for intervention. As a general guidance for the project it was decided that no device is acceptable if self rescue is not possible for all users in the tunnel. Thus all devices and combinations, which are not in line with the principle of a fair chance of survival for all users during the self-rescue phase, were rejected. 3.3. Investigation methodology The investigation was carried out in two steps: •

Detailed investigation of the aerodynamic characteristics of all devices, for assessing their suitability and effectiveness in terms of control of smoke propagation and survivability conditions.

•

Discussion of all safety-relevant issues, including self-rescue and intervention.

The investigation was carried out by means of a combination of one-dimensional (1D) and three-dimensional (3D) analysis. 3.4. Preliminary investigations of potentially useful applications Useful applications must be effective, practically relevant and feasible with reasonable cost. Thus a systematic, preliminary screening of all possible combinations was carried out for different tunnel configuration, traffic conditions and ventilation systems. Attention was focused on issues, which can’t be entirely mastered by conventional means, first of all tunnel ventilation. This includes e.g. short, steep tunnels with either natural or longitudinal ventilation. As shown by Bettelini and Seifert (2010, [3]) smoke propagation in such tunnel can’t be effectively mastered by means of conventional ventilation systems. The preliminary investigations showed among others that the option of realizing air-tight fire compartments is not feasible, because of the practical difficulties (highly tight closures at very short distances would be required), limited effectiveness (if the compartments are not very small and well sealed) and of safety consideration (self-rescue issues and risk of backdraught during intervention). Based on the preliminary investigations it was concluded that the most relevant applications of flexible devices in road tunnels are the following ones: •

Permeable curtains or smoke barriers for naturally-ventilated tunnels with bidirectional or unidirectional traffic with high risk of congestion (application at the portals or in the tunnel) for reducing the longitudinal air velocity.

•

Smoke curtain for longitudinally ventilated tunnels with bidirectional or unidirectional traffic with high risk of congestion. The smoke curtains are used for reducing the critical velocity and allow for an excellent control of smoke propagation with small longitudinal air velocities.

•

Control of longitudinal velocity in long tunnel with smoke extraction. These devices have already been investigated in great detail in earlier studies and did not call for additional efforts within this project.

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- 269 In this paper we will focus on the second application, more innovative and interesting from the point of view of tunnel ventilation. A few representative results are presented in the following chapter. 4.

AERODYNAMICS AND SMOKE PROPAGATIONS

Representative “pilot” tunnels were selected for investigating aerodynamics and smoke propagation. Their key characteristics are: •

Portal permeable or total closure: length 500 m, “horseshoe” profile, cross section 56 m2, longitudinal slope 3%, natural ventilation.

•

Partial closure: length 650 m, “horseshoe” profile, cross section 56 m2, vanishing longitudinal slope, longitudinal ventilation with jet fans.

A large number of simulations have been carried out for many relevant configurations. The findings will be illustrated based on smoke curtains with longitudinal ventilation. For all other devices only the main findings and general conclusions will be presented in the final chapters of this paper. The considered scenario was as follows: •

30 MW fire in tunnel center, which is reached after 5 min

•

Detection of fire after 1 min

•

Activation of ventilation and curtains after 1.5 min, complete closure of curtains after 2 min

•

Vehicles in the tunnel: 14 cars and 1 lorry on every lane.

Smoke curtains are lowered from the ceiling to a minimum height of 2 m above floor, which represents an obstacle only for buses, lorries and HGV. Persons and cars can still pass the curtains without any difficulty. Smoke propagation in tunnels with bidirectional traffic and longitudinal ventilation is in most cases difficult to control. Key issues are the ventilation direction, the high longitudinal velocities necessary for suppressing backlayering, (roughly 3 m/s), which would disturb smoke stratifications and drastically worsen the conditions downstream of the smoke source, and the danger of direct smoke destratification by the jet fans. A common approach in such situations is to establish a longitudinal airflow of 1-1.5 m/s in the direction of the initial flow. This ventilation regime allows to keep smoke stratification and to slightly influence smoke propagation but is, in most cases, far from optimum.

Figure 2: Visualization of smoke layer behavior at the smoke curtain. The new investigations showed that the curtains, if installed upstream of a fire, represent a very effective barrier against smoke backlayering, since the critical velocity is locally significantly reduced by the smoke curtains. The effect of the curtain on the smoke layer is illustrated in Figure 2. Drawback of the installation is that downstream smoke stratification is disturbed.

6thInternational Conference ‘Tunnel Safety and Ventilation’ 2012, Graz

- 270 A combined system of smoke curtains and longitudinal ventilation can prevent backlayering with a longitudinal airflow of about 0.5 m/s. Smoke curtains show the tendency to smoke destratification, but have the ability to keep a wide range of the tunnel free of smoke.

Figure 3: Evolution of visibility conditions in a longitudinally ventilated tunnel (30 MW fire, longitudinal air velocity 1 m/s, no smoke curtains).

Figure 4: Evolution of visibility conditions for a combined ventilation concept consisting of longitudinal ventilation and smoke curtains (30 MW fire, longitudinal air velocity 0.5 m/s). Figure 3 presents the simulation results for the purely longitudinal ventilated tunnel. Ventilation was activated 1.5 min after fire ignition, enabling airflow from left to right with a velocity of 1 m/s. As expected, smoke stratification is quite stable and backlayering can not be prevented. Figure 4 illustrates visibility conditions for the same tunnel activating two smoke curtains and longitudinal ventilation with 0.5 m/s for smoke control. Upstream, smoke propagation is effectively limited by the smoke curtains. Downstream, smoke stratification is disturbed by the smoke curtains’ recirculation. This significantly worsens the condition between smoke curtain and fire location. 6thInternational Conference ‘Tunnel Safety and Ventilation’ 2012, Graz

- 271 Comparing the two ventilation concepts, purely longitudinal and longitudinal ventilation combined with smoke curtains, a clear advantage for the self rescue phase is not observable for the investigated conditions. Since under these conditions, according to simulation results, smoke stratification is quite stable, this situation does not represent a major problem for the self-rescue phase. However one should be aware that even small disturbances (traffic signs, moving vehicles, lay-bys etc.) could easily lead to a disastrous smoke destratification. Moreover, smoke curtains limit smoke spreading upstream of the incident location. This represents a clear advantage for rescue and intervention forces, which could get close to the fire under favourable conditions and clear view, without change of the ventilation regime. 5.

SAFETY ISSUES

The different types of devices lead to specific safety-related issues, which need to be addressed carefully for any application. Only a few key issues are addressed herein. 5.1. Devices control Clear specifications for the activation of these devices need to be defined. The general goal is a rapid activation in case of fire. Smoke curtains are fairly uncritical, since small vehicles and persons can pass them unhindered. In principle they can be activated very rapidly after fire detection and automatic activation could be envisaged. Permeable or impermeable full closure systems are far more critical because of the potentially dangerous interactions with moving vehicles. Traffic accidents and damages to the devices could easily results from a sudden, unexpected closure while the traffic is still running. In short tunnels this can be easily prevented by a reasonable waiting time before closure, of the order of 30 to 60 seconds. More generally, a visual verification (CCTV) will be needed before device activation. Additional supporting measures, such as specific signalization (e.g. additional traffic lights, VMP, barriers, possibly loudspeakers) could be necessary for preventing dangerous secondary incidents. Moreover the performance of fully closing devices may drastically deteriorate if vehicles are located under the device. In general it must be pointed out that the installation of flexible devices will require the installation of additional tunnel equipment, which will increase the overall costs. This is particularly true for short tunnels with natural ventilation, where in most cases only limited equipment is in place, which in many cases does not include e.g. fire detection or CCTV. 5.2. Implications for self-rescue As a basic principle, all tunnel users must be able to leave the tunnel with their vehicles or attaining safe heavens on foot. Any device which does not allow for a fair chance of rescue to all tunnels users is not acceptable. Escaping tunnel users need to go through such a device without great efforts. This requirement is entirely satisfied by smoke curtains and excellent solutions can be found for permeable closures, as demonstrated by Aigner’s FIREcurtains. In the case of full closing devices specific solutions are needed, which block smoke and air propagation but allow for person’s passage (e.g. clear marked doors or highly flexible parts of the structure, such as lamellae). Normal “doors” without locks might be possible if smoke tightness is not necessary. Such devices might be fairly complex and expensive and acceptability by the tunnel users will certainly represent an important issue. 5.3. Implications for intervention Flexible devices being lowered down to create some sort of compartmentalization may cause concentrations of hot smoke and combustion gases behind the device. When approaching to

6thInternational Conference ‘Tunnel Safety and Ventilation’ 2012, Graz

- 272 the smoke barrier, fire fighters have to be very careful because the sudden availability of oxygen could cause a dangerous flashover. Smoke barriers, permeable curtains or the accumulated smoke behind them can close the view towards the fire site. On the other hand such devices can also represent an important element of protection for fire fighters (thermal & smoke protection). The intervention conditions at the upstream side of the fire site are generally much better with such a device in place. Protecting devices like smoke curtains can enable fire fighters to approach easier and to use special intervention tactics or means (e.g. use of fire fighting supporting machines like the LUF60 with radio-commanded capabilities to work in front of the fire team and cool down the environment). 6.

CONCLUSIONS AND OUTLOOK

A systematic investigation on the use of flexible devices for controlling smoke and fire propagation in road tunnels was carried out. The results showed that flexible devices have a significant potential for contribute solving some safety-relevant issues, which can’t be tackled using conventional tunnel equipments, such as ventilation. The following applications were identified as excellent candidates for additional investigations: •

Permeable curtains or smoke barriers for naturally-ventilated tunnels with bidirectional or unidirectional traffic with high risk of congestion (application at the portals or within the tunnel) for reducing the longitudinal air velocity.

•

Smoke curtain within longitudinally-ventilated tunnels with bidirectional or unidirectional traffic with high risk of congestion for reducing the critical velocity.

•

Permeable curtains in long tunnels with smoke extraction and insufficient control of longitudinal air velocity.

The investigations conducted to date were focused on aerodynamics, smoke propagation and user interaction. Costs and benefits will be investigated next. This research was made possible through the kind financial support provided by the Swiss FEDRO under research grant number VSS 2010/202_OBV. 7.

REFERENCES

[1]

Bergmeister K. (2005) UPTUN, Workpackage 6, Fire effects and tunnel performance: system response D62. Bettelini M., Seifert N. (2009) Automatic fire extinction in road tunnels – State-of-theart and practical applications; in Safe Tunneling for the City and Environment - ITAAITES World Tunnel Congress 2009, Budapest, Hungary, May 23-28, 2009 Bettelini M., Seifert N. (2010) On the safety of short road tunnels. Paper presented at the 5th Symposium ‘Tunnel Safety and Ventilation’, 3-4 May 2010, Graz. Henn M., Sturm P. (2010) Test der textilen Vorhänge im Tunnel Roppen. Bericht Nr. FVT-51/09/Stu V&U 09/38/6400 vom 07.12.2010 (in German). Kohl K., Kutz M., Wieneck F. (2005) Brandschutzforschung der Bundesländer, Die Wirkung von mobilen Abschottungs- und Belüftungsmassnahmen bei der Rettung und Brandbekämpfung bei Tunnelbränden – Teil 2, Heyrothsberge, Januar 2005 (in German). Öttl D., Sturm P., Almbauer R., Öttl W., Turner A., Seitlinger G. (2002) A new system to reduce the velocity of the air flow in the case of fire, Proceedings of the Int. Conf. On Tunnel Safety and Ventilation, Graz, 8.-10.4.2002, 279-286.

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6thInternational Conference ‘Tunnel Safety and Ventilation’ 2012, Graz