Fire in the Mountains
if ca/t designers and operators got some surprises when they began simulating traffic flow, they got even bigger shocks when they began to look into another problem: how to protect a tunnel and its occupants from the ravages of fire. It wasn’t just the threat of direct burns that they were worried about. In fact, “Smoke and heat are the real killers in a fire,” says Richard W. Drake, operations manager for the Central Artery/Tunnel Project.
For years, explains Drake, engineers around the world have been building automobile tunnels with ventilation equipment large enough to handle the smoke from the biggest fire conceivable. But they had no way of knowing how their conceptions of fire would match a real blaze. Though tunnel fires-such as the one that broke out in 1996 in the Channel Tunnel that connects the U.K. and France-capture international media attention, none has burned under the watchful eye of high-speed scientific instruments. So engineers have always based their fireproofing designs on theoretical models, not hard data.
Uncomfortable with this uncertainty, engineers have for decades over-built their projects, adding more ventilation equipment, insulation, and structural support than they thought necessary-just in case their models underestimated the heat and smoke a tunnel fire could produce.
What engineers needed was a test bed-an experimental system for tunnel fires. And in the early 1980s, the Federal Highway Administration (FHA) came up with one, re-routing a section of I74 in a way that left an empty 1.1-mile tunnel in the hills of West Virginia. There the FHA teamed up with Parsons Brinckerhoff, one of Big Dig’s primary contractors, to perform a series of full-scale burns that would finally put the theories about tunnel fires to the test.
The team spent $10 million renovating the abandoned tunnel with a state-of-the-art ventilation system and another $10 million installing sophisticated monitoring instruments. “We outfitted the tunnel on a grid system so you could collect data on temperature, air flow, and carbon monoxide throughout the tunnel,” says Drake, who supervised the project.
In the middle of the abandoned tunnel the engineers built large steel pans measuring more than 10 feet on a side. They filled the pans with 6 inches of water (to protect the steel from the heat) and then an inch of fuel oil. A remote-controlled propane burner ignited the fuel.
Ultimately, Drake supervised 101 burns. The smallest was 10 megawatts (MW), simulating a small car bursting into flames. The largest was 100 MW-approximately the power released when a small gasoline tanker has a head-on collision with a truck.
“If you want to see what Hell looks like, we’ll show you a picture of a 100-megawatt fire,” says Drake. “It is absolutely astounding to see tiles blown off the wall. The asphalt and tar expansion joints bubble.”
To the team’s amazement, the tunnel and the ventilation equipment held up far better through these holocausts than the models had predicted. “Nobody thought we would ever get this number of fires off. They thought the tunnel would collapse long before we were done with it,” says Drake.
The resilience of the tunnel in West Virginia pointed toward a staggering conclusion: Worldwide, billions of dollars had been wasted making tunnels more fire resistant than was ever needed.
Although the results came too late to allow for a complete Big Dig redesign, Drake has still been able to save tens of millions of dollars in concrete and excavation costs by shrinking some ventilation shafts and eliminating others. “We saved about $25 million on this project in insulation costs alone,” says Drake. “We are very confident that we can show you $45 million in savings overall.”
More important, the tests have taught engineers how to “tune” the CA/T’s ventilation system. In the event of a fire, says Drake, conventional wisdom had always held that fans supplying fresh air to tunnel regions adjacent to the flames should be set at roughly 50 percent of capacity. It was an attempt to strike a delicate balance: “You don’t want to feed fresh air” to the fire, explains Drake, but you don’t want people trapped in their cars to suffocate either.
Again, conventional wisdom was wrong. The West Virginia experiments showed that it is better to turn the nearby supply fans way down during a fire-to just 10 percent or 20 percent of capacity. At these reduced settings, the tests prove, the ventilation system will still provide enough fresh air for trapped motorists, and it won’t fan the flames as high. It’s a strategic adjustment that might seem minor, but with a quarter of a million vehicles expected to negotiate the tunnel each day by 2010, its impact could prove enormous.