The very image of a fire in a tunnel -- people and vehicles helplessly trapped below ground while flames consume limited oxygen -- is a horrifying one. But it is the smoke and heat, not the fire itself, that are truly deadly. Fortunately, smoke and heat can be controlled by proper ventilation. In the past, designers have had to rely on theoretical analysis in designing highway tunnel ventilation systems. Now, a new test program will give designers first-hand, real-life data and information on the behavior of heat and smoke in a tunnel fire.

For years, the international scientific community and the Federal Highway Administration (FHWA) have been looking for a way to obtain data on the smoke and heat flow resulting from a full-scale accident inside a highway tunnel. The Central Artery/Tunnel (CA/T) Project in Boston, Mass., provides such an opportunity. This project, currently the nation's largest public works effort, involves the construction of an underground expressway to replace the existing, fiercely overcrowded, elevated Central Artery, which was built in the 1950s. (The Central Artery was originally designed for an average daily traffic rate of 75,000 vehicles per day; today it carries about 150,000 vehicles per day.)

More than half of the project's 12.07 kilometers of new highway will be built underground. It is therefore imperative that the behavior of heat and smoke in a tunnel be fully and properly analyzed. Thus, the FHWA, in cooperation with the Massachusetts Highway Department and the West Virginia Department of Transportation, is participating in a test program in an old, abandoned highway tunnel -- the Memorial Tunnel.

The Memorial Tunnel is a two-lane, 0.85-km-long structure near Standard, W.Va. The tunnel was abandoned in the late 1980s when the highway was expanded and realigned. Today, however, it is more like a highly sophisticated laboratory than an outmoded highway tunnel. It has two ventilation buildings -- one at each end -- each with three fully reversible vane axial fans. Temperature sensors, video cameras, and velocity probes have been installed inside the tunnel.

Approximately 130 tests will be conducted in the Memorial Tunnel "laboratory" to determine the effectiveness of different ventilation systems and strategies in tunnel fires. Fires ranging in size from one that might be caused by a single-car accident to one caused by a fuel tanker spill will be simulated by burning No. 2 fuel oil in shallow pans on the floor of the tunnel. Steel silhouettes of passenger cars and trucks have been constructed and placed inside the tunnel to simulate the effect of a congested tunnel on the flow of heat and smoke.

Visibility, temperature, airflow, and carbon monoxide levels will be recorded during the tests. These data will be captured using various computer terminals in the tunnel's control room. Additionally, each test will be recorded by six video cameras: two of these are inside the tunnel, two are at the portals, and two are about 100 meters away from the portals; these last will monitor the impact of smoke, if any, on the adjacent highway. The cameras are state of the art, capable of zooming, panning, and rotating.

The current test program will provide guidelines on how ventilation systems can be used more efficiently -- that is, how they can extract smoke and heat more economically and, simultaneously, provide a higher level of safety during a fire. Ventilation systems to be tested include the following.

• Transverse. A transverse ventilation system is characterized by the uniform introduction of fresh air (typically at roadway level), and uniform extraction of vitiated air (typically at ceiling level), throughout the full length of the tunnel.
• Semi-transverse. A semi-transverse ventilation system either provides fresh air or extracts vitiated air uniformly along the length of the tunnel.
• Oversized single extraction ports. This experiment will test the effectiveness of an oversized extraction port located over the fire area.
• Jet fans. Once all of the previously mentioned ventilation systems have been tested, the tunnel's hanging ceiling will be removed, and jet fans installed and tested. Jet fans are currently used in many European tunnels; their application in the United States is not now generally supported by FHWA.
• Natural. Finally, the behavior of smoke and heat will be tested under natural (airflow) conditions.

Several proposed tunnels that are now being designed are awaiting the results of the Memorial Tunnel Fire Ventilation Test Program to see if the mechanical equipment used there could be downsized to provide a more efficient ventilation system. Data on ventilation from this program should be very beneficial - not only to the operation of the CA/T Project but also to the design and operation of ventilation systems throughout the world.

Determining Safety In A Tunnel Fire

Tunnel safety in a fire is based primarily on: (1) maintenance of a certain level of fresh air and (2) not exceeding a certain concentration of carbon monoxide. In the United States, common practice for determining the quantity of fresh air required during a fire is based on 9.29 cubic meters per lane/per linear meter of tunnel. For one component of the CA/T Project alone -- the Third Harbor Tunnel, which will directly link Logan International Airport and south Boston -- this will represent a flow of 60,800 cubic meters per minute for each tube. Regarding carbon monoxide levels, the U.S. Environmental Protection Agency and FHWA have jointly issued a guideline about the maximum concentration acceptable inside tunnels. This policy states that for an exposure time of up to 15 minutes, the maximum concentration should not be greater than 120 parts per million for peak hour traffic, 65 parts per million for 30 minutes, 45 parts per million for 45 minutes, and 35 parts per million for 60 minutes.

Jesús M. Rohena is a structural engineer in the Review and Design Branch of FHWA's Bridge Division. Formerly, he was a highway engineer for the Eastern Federal Lands Highway Division.