The notion of a Highway 99 tunnel raises its own set of fears among the public. What about fire, earthquake, tsunami or crashes? It turns out the Seattle project includes engineering solutions that the tunneling industry devised in response to catastrophes in other parts of the world.

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The proposed Highway 99 tunnel is supposed to replace a threat to public safety — that the old Alaskan Way Viaduct might topple in an earthquake.

But the notion of going underground raises its own set of fears among the public.

A quake might crack the tube, some argue. Commuters could drown in a tsunami. A crash might trap cars and block ambulances. Toxic fumes might outrun evacuees.

While some doomsday comments are sincere, others are attempts to toss sand into the machinations of pro-tunnel governments as the controversial project, set to open by the start of 2016, moves forward.

It turns out the Seattle project includes engineering solutions that the tunneling industry devised in response to catastrophes in other parts of the world.

“We’re looking at a tunnel that’s providing state-of-the-art technologies,” said project manager Linea Laird.

But tunnels, by their nature, have hazards. Which matter most, and what do they mean for motorists?

Emergency escape doors will be built every 650 feet. On the other side would be concrete-sheltered corridor, designs show. People could then walk to daylight at the Sodo or South Lake Union portals of the 1.7-mile tunnel.

Though firefighters would be dispatched to the scene, sheltering in your car would be a bad move.

“If you’re in the tunnel and something happens, we assume you will self-rescue,” says Gary English, Seattle assistant fire marshal.

Fire is main threat

Fire is the primary threat to drivers in a tunnel, in the view of state engineers, tunnel contractors and emergency responders.

Worldwide, there have been 188 fires in highway, transit and freight-rail tunnels, according to Promat, a maker of fireproof materials.

Among the worst was a 1999 tragedy in which a truck hauling margarine ignited in the Mont Blanc Tunnel, between France and Italy. Smoke and heat killed 39 people during the 53-hour incident.

Seattle Tunnel Partners is proposing a ventilation system that would suck smoke out in numerous locations, instead of pumping fresh air into the tunnel, which might push smoke and fumes the wrong way.

A confined tunnel reflects and traps heat, so temperatures reach extremes. Moisture within the concrete vaporizes and causes the wall to burst like popcorn, a phenomenon called “explosive spalling.” As shards fly, the next layer of concrete is exposed, until a large segment falls to ruin.

A 2008 nonfatal fire in the Channel Tunnel (also known as the Chunnel) between England and France, destroyed 750 meters of concrete lining. Spalling occurred in the Mont Blanc fire, in two other Chunnel fires, and during construction of the Great Belt Tunnel in Denmark.

The worst-case fires follow a pattern called the “RWS Curve,” coined by Dutch researchers. An engulfed gasoline truck would generate 300 megawatts of energy, nearly one-third of a nuclear-plant output. Temperatures can spike to nearly 2,200 degrees in 20 minutes, and stay there at least three hours.

U.S. fire codes are results-based. Tunnel walls must be protected so they never exceed 716 degrees.

“Seattle Amendments”

The Highway 99 tunnel codes include another wrinkle: special “Seattle Amendments” that mandate sprinklers. The Mount Baker Tunnel, Interstate 5 beneath the Washington State Convention Center, Battery Street Tunnel and Downtown Seattle Transit Tunnel all feature deluge systems.

“Seattle has more tunnel fire sprinklers than any other city in the country,” said English, a national authority. Meanwhile, the Chunnel is being retrofitted with four stations where a burning train can stop and be drenched in mist.

“The way to deal with a fire is to put it out,” said Susan Everett, a Washington State Department of Transportation (WSDOT) engineering manager.

Also, gasoline tankers will be banned from the Highway 99 tunnel. Therefore, authorities required that the tunnel withstand a lesser 100-megawatt fire that a freight truck might generate, Laird said, the project manager.

The Seattle Tunnel Partners construction team, led by Dragados USA, says it will install not only sprinklers but coat the interior with a 1.2-inch-thick calcium shield that is designed to withstand a gasoline fire.

Manuel Pardo, Dragados’ project executive, said his team was using the best practices from its experience in Europe, including the M30 highway project in Madrid, Spain.

English emphasized that if a gasoline truck, or any suspicious vehicle, enters the Seattle tunnel, control-room operators could identify it and close the highway to traffic.

For its basic design, the concrete tunnel tube must be at least 2 feet thick and contain steel-reinforcing bar, Laird said. New York City has needed to reinforce its older tunnels with steel as a special precaution against terrorism.

Natural disasters

The tunnel is subject to state seismic requirements to withstand a so-called “2,500-year” quake without life-threatening damage.

A severe quake would be expected to cause the cylindrical shape to bend like an oval, about a half-inch, said state engineer Tim Moore.

Tunnels have the advantage of being braced by surrounding soil, said Steven Kramer, a geotechnical-engineering professor at the University of Washington.

In 2005, scientists considered what might happen if a temblor of similar force to the 6.8-magnitude Nisqually earthquake of 2001 were to occur in the Seattle Fault, directly under the city. They predicted 1,600 deaths, severe damage to 29,000 buildings, and $33 billion in losses.

“I think the tunnel would probably be the least of our concerns at that point. The tunnel would likely be in much better shape than the structures over it,” Kramer said.

The tunnel team dismisses any risk of tsunami.

A worst-case event would be triggered by a rift in the Puget Sound seafloor generating a wave that covers part of Sodo in water between knee and head height.

However, the only known tsunami from the Seattle Fault occurred about the year 930 and generated a 10-foot wave. To breach the current seawall, which is 9 feet above the average high tide, it would require a record tsunami at high tide. Such an event might occur once in 23,000 to 60,000 years, according to consulting firm Parsons Brinckerhoff.

However, the WSDOT analysis done by Parsons Brinckerhoff doesn’t factor in climate change, which scientists believe could raise sea level worldwide by an average of 1 meter by 2100.

Pardo replies: “Even if the sea level rises 1 meter, you won’t get water in the tunnel, because the tunnel is higher than that.”

How to escape

All the tunnel’s emergency doors will be on the west side — left of the northbound lanes and right of the southbound lanes along the double-deck highway.

Cameras, speakers and phones would enable people to be located by workers in the tunnel control room and at WSDOT’s regional traffic center in Shoreline. Evacuees would walk either up or down a staircase to reach the passageway out. Disabled people would wait in a shelter zone for firefighters.

In the 2008 Chunnel fire, everybody survived.

“The reason there were no fatalities was due to the fact they had an emergency-evacuation system that looks a lot like the Highway 99 tunnel,” English said.

Opponent Christopher Brown, a retired engineer who opposes the tunnel, has argued the tunnel is too narrow for wheelchair ramps to be used, and the disabled wouldn’t be able to escape via staircases; the Federal Highway Administration says it’s reviewing his critique.

As with light rail in Rainier Valley, where cars and trains share the road, public safety depends on awareness.

An experiment for a European tunnel-safety association found that untrained drivers took five minutes to make the right decision — walk to an escape door — when smoke was pumped into a tunnel. Authors suggest education campaigns, even in-car placards such as those aboard aircraft. The Highway 99 tunnel would include loudspeakers and electronic signs.

Local firefighters have experience reaching crashes on the narrow Highway 520 bridge and Spokane Street Viaduct, English said. They can drive opposite the normal direction of traffic. They could stop on the southbound deck, then descend to a northbound accident scene. Or firefighters might enter through the escape passageway, then hook hoses to hydrants inside. The tunnel, he said, might be easier to deal with than certain high-rise buildings.

“Whatever it takes, we’ll carry enough people and enough hose.”

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com