Concerns about tunnel fire safety existed long before terrorism exposed the vulnerability of roadway, railroad and subway tunnels. Increased numbers of automobiles worldwide have resulted in a corresponding increase in fatal tunnel fires. Several efforts under way illustrate the urgency of finding ways to improve tunnel fire protection.

Canada's National Research Council and the Institute for Research in Construction is involved in a two-year international research project with the Fire Protection Research Foundation to investigate available fire detection technologies suitable for protecting tunnels. The project will provide information for use in the development of performance criteria, guidelines and specifications for tunnel fire detection systems.

These researchers are conducting fire tests in their tunnel facility to evaluate the performance of five different types of existing fire detection technologies, including both traditional and newer options, such as long-distance fiber-optic temperature sensing and closed-circuit television.

A number of recent papers in Fire Technology detail efforts to understand tunnel fire behavior using sophisticated models. One paper, “Numerical Simulation of the Howard Street Tunnel Fire,” presents numerical simulations of a severe fire in the relatively narrow Howard Street Tunnel in Baltimore following the July 2001 freight train derailment. A fire model was used to predict what the peak temperatures might have been in the tunnel during the fire.

“The temperatures predicted were those typical of fully engulfing hydrocarbon pool fires,” said Kevin McGrattan, a researcher at NIST's Building and Fire Research Laboratory. “The limited oxygen supply in the tunnel probably served to lessen the temperatures from roughly 1,100°C to about 1,000°C, but it's hard to say because this difference … is roughly comparable to the uncertainty of the calculation.”

The work was sponsored by the U.S. Nuclear Regulatory Commission as part of an assessment of the hazards to waste casks being transported by rail.

Another paper, “Theoretical Model of Major Fire Spread in a Tunnel,” examines how fire spreads from source to secondary vehicles. Knowing these conditions should help engineers design tunnels less conducive to fire spread, and also help tunnel operators and firefighters adequately plan emergency response.

Tunnel fire can spread from source to secondary vehicles five ways: by spontaneous ignition without flame impingement, by flame impingement, by burning brands, by transfer of fuel or by explosion. The paper deals with spontaneous ignition only.

“The paper attempts to discover what critical heat release rate an initial fire needs to spread from initial source to trucks downstream in a single-lane tunnel with a forced longitudinal ventilation,” said author Alan Beard, an engineer at Heriot Watt University in the United Kingdom.

Beard's model predicts the critical heat-release rate for spread from an initial fire to a secondary vehicle to be between 30MW and 40MW, at a ventilation velocity of 2m/s and a separation of about 6.5 meters.

“This … release rate is not very high,” Beard said. Truck fire release rates can be over 200MW in tunnel fires, as shown by Norwegian experiments at the Runehamar Tunnel.

The basic message, then, is that tunnel fires must be extinguished as soon as possible to try to stop the fire reaching a critical heat-release rate.