Three major tunnel fires have claimed hundreds of lives over the last two years, raising questions about fire precautions. None of these tunnels was equipped with sprinkler systems which could have brought the blazes under control before the emergency services arrived.
This week we ask: Should tunnel operators install sprinklers as a safety measure?
There are hardly any sprinkler installations in tunnels so there is no practical knowledge with which to justify their usefulness in this situation. We therefore have to look at experimental studies and design arguments which have been used, notably in Australia, to support their use in this context.
In the Memorial Tunnel fire, experimental tests were carried out using foam-water sprinklers to extinguish pool fires. 20MW fires were extinguished in about 30 seconds and the effect of strong ventilation was negligible. Tests in Sweden showed the effect of sprinklers on a freight truck fire in the tunnel-like environment of a mock ro-ro ferry. It was concluded that only a small reduction in temperature was achieved by the water spray system, although the duration of the fire was relatively short at six minutes. Thus higher discharge rates, or deluge systems, are indicated.
The general advantage of water suppression systems is the protection they would give to the tunnel, enabling facilities to be returned to use in a shorter period - important when considering the closure times following recent fires. The use of water drencher systems in Australian tunnels is argued on this basis, but with important qualifications over use. They are manual systems operated after evacuation, so questions of smoke destratification are less significant. They are only operated by trained staff and the system is only one of a number of complementary tunnel facilities that are provided for fire defence.
The type of the sprinkler system is also important. Water spray systems will only be effective if the discharge rate is high;
drainage needs to be good to limit spread by flammable liquids. Foam additives seem to increase effectiveness, but water discharge that is adequate for liquid pool fires would not be adequate for a vehicle fire.
After recent fire disasters in tunnels there is a great desire to find a universal panacea. This can lead to a knee jerk response and 'stable door' legislation. However, the responsible reaction should be based on risk assessment. For example, after the fire in the underground station at King's Cross in 1987, one proposed solution was additional means of escape from the lower levels of such stations. Quantified fire risk assessment showed that combinations of other measures could reduce risk much more effectively.
Tunnels vary widely in design and purpose. Accordingly, the most effective combination of fire safety measures will be related to the specific tunnel and the way it is used.
The total package of fire precautions may include: prevention - control of ignition sources, low amount and slow reaction to fire of materials. Detection and alarm - automatic and manual detection. Voice alarms and visual warnings. Containment - fire resistant physical barriers.
Ventilation and smoke extraction. Suppression - hand extinguishers, hoses, sprinklers, fire brigade. Escape - travel distance, way finding, lighting, protected escape routes. Management - control of uses, staff training, emergency planning and continuing risk assessment.
Sprinkler systems are designed to detect and control a fire while it is small and have an excellent record of performance in buildings. However, they are not so effective against covered fires because activation will be delayed, the spray will not reach the source of the fire and more smoke will be brought down to low level.
There are, of course, instances where the provision of sprinklers is crucial. However, the real challenge is to quantify the fire risks in the many diverse existing tunnels, so where improved fire safety is needed, the most appropriate solution can be implemented.
The Highways Agency is planning to trial sprinklers on the floor of part of the Mersey tunnel. Floor mounted sprinklers are thought to be better able to tackle the seat of a fire than those positioned in the tunnel roof.
One hundred and fifty six people died in an Alpine rail tunnel fire near the Austrian ski resort at Kaprun in November.
More than 40 people died in the 11.6km Mont Blanc road tunnel in 1999 after a lorry carrying margarine burst into flames. The blaze, near the tunnel's mid point, took 50 hours to put out.
Five people died in a blaze in the Tauern tunnel in Austria last year after a vehicle fire got out of control.
Fire fighters at Mont Blanc, Tauern and Kaprun were unable to get close enough to the blazes to get them under control because of intense heat and poisonous fumes.