I believe they're hoping to mitigate 3 and 4 by going deeper than most things. But wouldn't that make 2, emergency escape routes, a lot more expensive?
How do you make a very long, very deep, narrow-diameter, elevator-accessed tunnel safe from fires and toxic gas releases? You can make it a vacuum but then what if you have to escape a vehicle? How does this thing not become a death trap in the event of a fire?
Fire is the number one cause of fatalities in tunnels (remember it's the smoke that kills). Tons of engineering and expense goes into trying to devise shelter and evacuation solutions.
How do you make a very long, very deep, narrow-diameter, elevator-accessed tunnel safe from fires and toxic gas releases? You can make it a vacuum but then what if you have to escape a vehicle? How does this thing not become a death trap in the event of a fire?
Fire is the number one cause of fatalities in tunnels (remember it's the smoke that kills). Tons of engineering and expense goes into trying to devise shelter and evacuation solutions.