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05 November 2008

Tunnel Trouble

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Whether to go for a deluge system or rather opt for a water mist model spraying small droplets, several European countries are rethinking fire suppression mechanisms for their tunnels

Whether to go for a deluge system like the one which worked wonders for the emergency services during the 2007 Burnley Tunnel fire in Melbourne, Australia, or rather opt for a water mist model spraying small droplets of about 100 micron with a low terminal velocity and a high surface to volume ratio, several European countries are rethinking fire suppression mechanisms for their tunnels.

Lessons from the Burnley Tunnel fire, in which a major disaster was successfully averted by the brand new safety system working like clockwork, are being taken on board in Europe, says Anders Lonnermark, a senior scientist in the fire technology department with the SP Technical Research Institute of Sweden, which also organises the International Symposium on Tunnel Safety and Security every two years.

The pile-up of trucks and cars inside the 3.4 km long Burnley Tunnel that killed three people on March 23, 2007, burst into a wall of fire which reached temperatures of over 1,000 degrees. But any further casualties were avoided because the flames automatically activated the tunnel safety systems, including a huge deluge of water which, according to the Melbourne police assistant commissioner, was “a godsend” to the fire brigade that rushed to the scene.

Hundreds of motorists were immediately advised to leave their cars with their keys in the ignition and evacuate the tunnel. Most took the emergency exits, which lead to separate pedestrian tunnels, though some took the riskiest route by walking back to the entrance of the tunnel. But the smoke extraction system functioned as planned and pumped the toxic fumes out.

As a consequence, the Burnley Tunnel, which was opened in late December 2000, is now widely regarded as a modern safety model. “The deluge system worked very well there”, Dr Lonnermark says. “In Australia, most new tunnels have one”. This is all the more important, since over the past few years, he adds, “we have shown here that also ordinary cargos can cause very large fires. Prevention could go many ways: you could have extinguishing systems on the vehicles, or a more secure way to shield your cargo, or limit the risk of fire spreading from one vehicle to another”.

Recent European research seems to have been focussing on better fire containment, risk management models and scenario analysis. Since Europe laid down the rules in 2004 in order to avert major disasters like the ones of the past few years, big differences in the national guidelines followed by member States still seem to remain, though.

Fire containment is perhaps the first big issue. Sprinkler systems should be designed to prevent a fire from reaching its peak, but they can be affected by high or low ventilation. Early light or sound beacons that can be immediately seen or heard, plus the availability of plenty of viable escape routes along the tunnel, are also crucial.

Sprinkler systems, Dr Lonnermark says, could certainly be feasible in Europe, but “there needs to be a little more testing to do. This depends on how large a fire you should be able to extinguish, but there is also a discussion about how to avoid pushing the smoke from the ceiling to the ground while extinguishing the fire, which would make escape more difficult. But if you have a well designed system with good detection, you should be able to start these systems before the fire gets too large”. In order to prevent what is called as “concrete spalling”, such as pieces dangerously falling off and the supporting structure losing some stability, special types of concrete or insulation have already been used in some tunnels.

The discussion about sprinklers, Dr Lonnermark says, has “delayed the development of these systems a little over the years, but now the whole thing has started. Perhaps sprinklers will not be installed in all tunnels; the type of system could depend on the traffic situation and the fire risk. But there are already some quite large water mist system producers in Europe”. The ISTSS Symposia (the third took place last spring in Stockholm, the next one is scheduled for 2010 in Frankfurt) are a showcase for the latest research, with fire and explosion simulations. They also look at security implications, to counter horrifying scenarios such as terrorists trying to blow a tunnel up.

In Europe there is now a better knowledge of evacuation procedures, Dr Lonnermark says. But motorist behaviour can still be an issue. According to the Centre for Tunnel Safety in the Netherlands, not enough attention has been paid so far to the fact that 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, and they need to be made better aware of this.

Intelligent Trasport Systems (ITS), warning motorists of any impending danger and giving them valuable early directions, could be the key here. “I know people have tried them in some hotspot tunnels”, Dr Lonnermark says. But the question remains, when they might effectively become part of a more sophisticated, common risk management strategy for good. Perhaps a whole array of efficient and widespread vehicle warning systems, as highlighted by the European Commission’s 2010 focus on the “Intelligent Car Initiative”, could help to integrate any local danger warning system worked by tunnel operators in the event of a crash or a fire.

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