In a number of catastrophic tunnel fires, fire often spreads from vehicle to vehicle over surprisingly large distances, occasionally hundreds of meters. A new paper attempts to understand the conditions under which fire spread between vehicles will occur in a tunnel fire.

“Almost all tunnel fire research to date considered only a single burning object in a tunnel, such as an auto or heavy good vehicle, but the major tunnel fires in recent years have shown that real incidents frequently involve multiple vehicles,” said Richard O. Carvel, Centre for Fire Safety Engineering, University of Edinburgh. Carvel is co-author with Alan Beard of The Handbook of Tunnel Fire Safety.

In the 1999 Mont Blanc tunnel fire, for instance, the distances between stopped vehicles entering from the French side varied up to 45 meters, yet the fire was able to spread across these gaps. Even more remarkably, the fire also managed to spread over a distance of 290 meters toward Italy against a prevailing wind to involve a queue of eight heavy good vehicles, or HGVs, stopped there. The fire also ignited a fire apparatus 450 meters from the fire.

“Analysis of the Mont Blanc incident suggests that convective flow leading to spontaneous ignition might explain the fire spread to the fire engine, while transfer of burning fuels could be an explanation for the spread to the HFVs,” Carvel said.

Using Bayesian probability analysis, Carvel's tunnel fire study concludes:

• Small vehicle fires (car fires, which are highly unlikely to exceed 8 megawatts) are not likely to produce flames that will impinge on an object more than five meters away.
• HGV fires larger than 64 megawatts will almost certainly produce flames that will impinge on another HGV up to 20 meters away, in most ventilation velocities.
• In a single-lane tunnel, fires between 32 and 64 megawatts will almost certainly produce flames that will impinge on another HGV up to 10 meters away.