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|Current Concerns and Associated Research on Fires in Tunnels|
Issue 83: Current Concerns and Associated Research on Fires in Tunnels
By Haukur Ingason
Fires are major risks that can create devastating effects. Expansion in the transportation sector and the ensuing pressure on traffic systems forces construction of more complex and complicated underground systems. This, in turn, requires new fire safety engineering solutions that need support and validation before being put into use.
Recent developments in water spray techniques (sprinkler, water mist etc.) for protection of tunnels is of course a positive reaction to the need to minimize the risks and the consequences of fires in tunnels. In urban road tunnels, there is a new type of hazard, namely constantly increasing queues, which has forced searching for new solutions to existing safety problems.
Previously, systems were designed assuming a continuous traffic flow throughout the tunnels. A longitudinal ventilation system with jet fans in the ceiling could solve most of the problems associated with control of heat and smoke, assuming that the traffic on the downstream side of the accident site could simply drive out of the tunnel. Through prevention of smoke backlayering, tunnel users on the upstream side of the accident could easily escape from the tunnel through a smoke free area.
Semi-transverse ventilation systems, where the heat and smoke are extracted from the ceiling, or transverse ventilation systems where fresh air is evenly supplied and hot smoke extracted along the tunnel, are not as common today as they were some decades ago. Properly designed semi-transverse systems can solve the fire problem related to the queue situation, as they confine the region where the smoke may exist in the tunnel, and thereby increase the opportunity for evacuation on both sides of the accident site.
Using both a semi-transverse ventilation system and a water spray system is a very safe solution, but of course rather expensive, especially for long tunnels. Therefore, a longitudinal ventilation system in combination with a water spray system is a more frequent solution today.
The large scale testing of water spray systems and the research that has been performed in relation to such testing has improved our understanding of these issues and helped to debunk many of the myths associated with the use of water spray systems in tunnels. The main questions today concern technical trade-offs and maintenance costs together with reliability issues.
Traffic control issues and detection of fires in relation to the use of water spray systems still need to be resolved properly. The challenging issue for the fire safety engineering community is associated with technical trade-offs when installing these systems. Should the design fire for the ventilation system to be lowered from say 100 MW to 50 MW, if a water spray system is installed? This would reduce the investment costs of the ventilation system considerably. An example of such a decision is found in the new Stockholm Bypass project where millions of dollars have been saved in initial costs.
Further, should the required structural fire load be reduced if a water spray system is used? The tendency at present is to pose that it should, but many are of the opinion that one must guarantee the reliability of water spray systems before such technical trade-offs can be allowed. An additional trade-off which is discussed is a reduction in number of escape routes used in a tunnel when a water spray system is provided.
Each of these questions is very important, but neither of them has been explicitly explored in large scale research projects. Design fires for tunnels with water spray fires have not been dealt with in guidelines or regulations to date.1, 2 The effect of water spray on fires related to new energy sources (electrical, hydrogen etc.) has not been tested. There is clearly a need for research dealing with these issues in the near future.
In rail and metro tunnels, the situation is different. The requirements for materials used in rolling stock makes it easier to deal with fire safety issues, although the latest large-scale testing carried out in Sweden3 and Canada4 indicate very high heat release rates from burning trains.
Properly designed carriages can minimize the consequences of a fire, but the consequences can be large in mass transport systems. The interior lining material, luggage and ignition sources are very important parameters in determining the fire development. Also, the compartmentation and quality of outer compartmentation like windows or the steel body play important roles in the risk of a fully developed fire.
The trend towards open gangway trains, with no separations between three, six or more carriages, has created new challenges for the fire safety authorities. There are no large scale tests performed that address the impact of open gangways on the fire development. Future research should focus on new type of carriages with long open gangway and trains with other types of bodies than steel, such as glass fiber or other lightweight materials.
Fire safety in road tunnels and rail tunnels require completely different approaches as the material and the fire performance requirements of the vehicles, as well as the number of users, are very different. The fire dynamics, however, are very similar. The challenges in future fire safety design require better education of fire safety engineers in the fire dynamics in tunnels. Lund University in Sweden gives courses in this field, and this important issue will increasingly become an important part of the education of future fire safety engineers around the world.
Haukur Ingason is with SP Technical Research Institute of Sweden
1st Quarter 2011 – Fixed Fire Fighting Systems in Road Tunnels – Andreas Haggkvist, Lulea University of Technology
Spring 2008 – Changes to NFPA 502: Standard for Road Tunnels, Bridges and Other Limited- Access Highways, 2008 Edition – Jason R. Gamache
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