Canadian Consulting Engineer

tunnels: the fire within

Travelling into the bowels of the earth in an underground tunnel takes one just a little closer to that fabled and feared place of fire and brimstone. And for the scores of people involved in four maj...

May 1, 2001  By Glenn McGillivray

Travelling into the bowels of the earth in an underground tunnel takes one just a little closer to that fabled and feared place of fire and brimstone. And for the scores of people involved in four major tunnel fires in recent years, the experience was a lot closer to hell than they would have liked.

The earliest of the four fires took place in the Channel Tunnel, or “Chunnel” linking Britain and France. Firefighters from both countries battled a blaze that started late November 18, 1996 aboard a truck carrying polystyrene. Though the Chunnel provides a rail-only link, drivers can load their vehicles into specially designed carriers (“rakes”) and enjoy the trip in the comfort of an amenity coach. At least 34 people were injured, two seriously, in the fire, which also caused Cdn. $85 million in damage. The blaze was the first major incident since the tunnel opened in 1994.

On March 24, 1999 a fire in the Mont Blanc tunnel in the Alps between France and Italy was sparked on a Belgian truck hauling flour and margarine. The blaze, which raged for two days before it was put out, took 42 lives. By some estimates the flames produced temperatures of up to 1,000 degrees C, hotter than the air that is directed into blast furnaces to make steel (usually about 900 degrees C).

Also in 1999, a markedly disastrous year for such events, a blaze began in Austria’s Tauern Tunnel that links the western portion of that country with Italy. According to firefighters, the May 29 blaze started when a truck loaded with paint plowed into the back of a car hundreds of yards inside the 6.4 kilometre-long tunnel. The car exploded immediately, setting off a chain reaction of explosions and fire. At least 12 people were killed, and 50 injured. Temperatures in the tunnel reportedly soared to more than 1,800 degrees C.

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And most recently, 155 people were killed on November 11, 2000 when fire tore through a funicular train climbing a slope in a tunnel in Kaprun, Austria. The train was packed with people when it became trapped in the 1.6 kilometre-long tunnel on the Kitzsteinhorn mountain in the Austrian Alps.

As with any loss event — natural or man-made — a number of conditions linked by a precise chain of events leads to a disastrous end. In themselves the elements may be harmless. Take one out of the chain, and the event doesn’t occur — or at least not to the same degree. But combine them in a certain way, and there’s havoc. With the four incidents mentioned above, the mere existence of fire was not enough to trigger such carnage.

Factored into the four events were such exacerbating elements as poor emergency procedures, malfunctioning equipment, miscommunication between command and control centres, driver error and other forms of human error. Because three of the four incidents were triggered by trucks, there were also broader issues surrounding maintenance and inspection of commercial vehicles and driver training.

Aside from these ancillary matters, however, two engineering design factors played very important roles in the outcome of the incidents: tunnel configuration and ventilation system design and use.

Tunnel configuration

The issue here is straightforward. If an incident affects one tube of a two-tube tunnel, presumably people and traffic in the non-incident tube are far less likely to be impacted. The presence or absence of parallel tube passages plays a key role in the generation of casualty figures and death tolls. The Mont Blanc, Tauern and Kaprun portals are all single-bore, while the Chunnel is, in effect, a three-tube tunnel with one “running” portal leading to France, the other to the United Kingdom, and the third used for service. More than 200 people died in the three single-bore incidents, while no-one died in the Chunnel fire.

Tube configuration is also important if there are few such links in a geographic area (which is often the case in Europe) and the movement of people and goods relies greatly on the route. With facilities such as the Chunnel, if there is an incident or when routine maintenance must be conducted, traffic can be redirected from one tube to the other (the Chunnel has two undersea crossovers and two land crossovers). Also with two-tube tunnels, the non-incident tube can be used to shuttle emergency equipment and staff closer to the scene. This ability proved to be very important in the Chunnel incident because the link is almost 50 kilometres long and the incident train was 10 kilometres into the tunnel before the rail control centre received its first confirmed fire alarm. Unfortunately, by their very nature even basic tunnels are extremely costly to construct. Building two separate tubes increases the price tag substantially.

Hand in hand with the issue of tube configuration is the question of service-evacuation tunnels. In Austria, some people who survived the Kaprun blaze went down the tunnel. But others who managed to flee the train carriage went up the tunnel in the hope of escaping the flames and heat, which some estimate rapidly approached 1,000 degrees C. They tried to climb hundreds of steps in darkness but were overwhelmed by the toxic smoke driven toward them by a rush of air. About 270 metres from the train lay a ventilation shaft that might have afforded a means of escape — but no-one ascending the tunnel reached it.

In the Chunnel, each running bore has a walkway on the service tunnel side for the evacuation of passengers and crew in an emergency. The running tunnels are connected to the service tunnel at roughly 375-metre intervals by cross-passages. These have fire resistant air-lock doors on each side. Once confirmation of the fire on board train 7539 reached command and control, a tourist shuttle that was in the non-incident tunnel at the time was ordered to stop by one of these fire doors. This action proved to be crucial in the successful evacuation of 26 passengers and the engineer of 7539.

Though not usually as pricey as running tunnels, service tunnels are still costly additions to a tunnel-boring project.

Ventilation systems

According to researchers at Heriot-Watt University in Edinburgh, fires in road or rail tunnels could increase in size as a result of current recommendations for ventilating tunnels to mitigate the effects of smoke.

According to the research engineers, a construction feature used in many tunnels throughout the world can actually be quite dangerous — the use of forced longitudinal ventilation. When a fire starts in a tunnel equipped with longitudinal ventilation, the system is switched on in order to blow smoke down the tunnel away from the blaze, allowing the passengers from the vehicles to escape in the opposite direction and giving a route for the fire crews to use in tackling the fire. But according to Heriot-Watt’s Dr. Alan Beard: “This, of course, could fan the flames, spreading the fire to more vehicles or coaches further down the tunnel. This may be what happened in the Mont Blanc fire.”1 Beard hopes that his findings, taken in conjunction with other research, will help to indicate rates of airflow that could be used to provide the ventilation needed while preventing the fire from spreading significantly.

According to a report released July 8, 1999 by French and Italian investigators as cited by Reuters, a ventilation duct switched to the wrong position fed the deadly fire in the Mont Blanc tunnel. An Italian tunnel worker saw cars suddenly turning back and passengers getting out of cars and decided to switch equipment to send oxygen into the tunnel instead of filtering out smoke. Duct equipment operated by the French was reversed to “extraction mode” minutes after the fire broke out. But as for the duct system operated by the Italians, their officials apparently waited for 21 minutes before trying to switch the flow of air. Even worse, they initially put the air flow at the maximum level, as investigator Michel Marec told the Associated Press. When they tried to switch the duct mechanism to the correct position through an automatic system, it apparently didn’t work. An attempt to switch the equipment manually also
failed. An interim report by French experts released earlier in 1999 also pointed out the ventilation error, and concluded that there was a lack of co-ordination between the French and Italian companies running the tunnel.

The Chunnel has a sophisticated ventilation system to extract smoke from the running tunnels and to provide good quality air in the service tunnel should it be needed for evacuation. However, during the 1996 fire the system was confounded by three major problems — two were caused by equipment failures and the third was probably a procedural issue or a human error. First, due to a lack of reliability in the opening/closing mechanism, the heavy steel doors used to close off the tunnel crossovers were left in the open position. Second, while piston relief dampers used for ventilation were ordered closed once the fire became known, one remained open. These two problems caused a great deal of acrid smoke to migrate from the incident tunnel to the non-incident tunnel. Regarding the third issue, variable pitch fans used in the system were left at zero pitch, rendering them useless for several minutes. Once they were stepped up, they began to remove smoke at a fairly rapid pace.

Canada’s transit fires

Canada has several rail and highway links that cut through the Rocky Mountains, as well as rail portals that link Canada and the United States via the St. Clair River at Sarnia and the St. Lawrence Seaway. There are also light rapid transit tunnels, such as Montreal’s five kilometre-long Mount Royal tunnel. Of course, there are also subway tunnels in Montreal and Toronto, and it is with the latter that probably the most fire experience has been gained.

There have been three incidents in Toronto Transit Commission subway tunnels in recent years. On August 11, 1995, three riders were killed and hundreds were injured when one subway plowed into another that was stopped between stations. Almost two years later to the very day — August 6, 1997 — at least 33 people were taken to hospital after dense smoke filled the Donlands subway station. The fire started in a pile of rubber pads used to cushion railway tracks. And most recently, a converted subway car used to haul trash burst into flames at Old Mill Station, early morning December 8, 2000.

While not belittling their importance, these urban subway accidents only mildly resemble the four European tunnel incidents cited above. Urban subway accidents can be nasty; there is no doubt. However, the distance between stations is seldom great. There are often escape routes located between stations. Intense 1,000 or 1,800 degree C fire is rare, and dangerous cargo rarer.

Subterranean links, on the other hand, often span great distances, either snaking through large mountains or below bodies of water. If they have escape routes, they are often few and far between. And it can take rescue crews a fair amount of time to reach the scene. What’s more, such tunnels often contain burning vehicles, which themselves contain fuel and give off toxic smoke from burning rubber and plastic. The vehicles may also be hauling dangerous substances: styrene plastic as in the Chunnel incident; paint in the Tauern fire. The ensuing heat often causes great damage and makes it very difficult for firefighters to get in close to the source. As critical transportation links, they are down for many weeks or months following the incident. The U.K.-bound Chunnel wasn’t reopened to passenger traffic until almost a month after the blaze.

In recent years, a great deal has been learned about tunnel fires. Unfortunately, it has taken several incidents with great loss of life and millions in damages to make the lessons meaningful.

Authorities responsible for the safe movement of people and goods in Canada must ensure that the lessons learned by others in the world — usually the hard way — are made meaningful here and that everyone who enters a Canadian tunnel gets to see the light at the other end.

Glenn McGillivray is assistant vice president and head of corporate communications at Swiss Reinsurance Company Canada, Toronto

1 Research Highlight, “Control of Fires in Tunnels,” www.epsrc.ac.uk, Engineering and Physical Sciences Research Council, U.K. Also see “Tunnel fires could be fanned by ventilation,” www.GlobalTechnoScan.com, October 25, Vol. 1. Issue 39.

Sources

“Channel Tunnel Fire November 1996,” by J. Lindley, IChemE Loss Prevention Bulletin, Issue 136, August 1997. The Institution of Chemical Engineers, England.

“The Hole Storey,” Fire Prevention, March 1999, pp. 20-21.

“Austria blaze fans tunnel safety fears,” BBC, November 11, 2000.

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