You're missing the biggest one - evacuation. The reason many tunnels are oversized is to allow walkways for people to evacuate using in an emergency. This drives the size up a fair bit.
It's also 12 stories deep in the ground.[1] I was also looking for the other dimensions for the tunnel. It's just crazy to think about digging holes like that. Just like the Channel Tunnel along the English Channel. Insanity is just the name of the game when it comes to Tunneling.
Except that in tunnel boring the circumference is not that relevant, it's the surface area of the cross section of your tunnel. In the first instance that's 113.1 square feet, in the second it's 452.4. Every time the radius doubles, the surface area quadruples.
The perfect tunnel transports people horizontally in a very narrow tube.
The length of the tunnel doesn't have much to do with earthquake survivability, beyond the obvious linear scale factor. What the tunnel is tunneling through has a lot to do with it.
Seattle's geology is somewhat different from any of the usual cited examples, as we've learned by watching everyone from journalists to politicians to engineers stand around with dumbfounded looks while the Bertha saga unfolds.
The tunnel boring in Seattle began right at the edge of downtown, went under a highway and a monumental retaining wall (They covered that wall with surveying targets and monitored it the entire time), then pretty deep under a residential neighbhorhood and a canal.
I think the only time it's moving close to high-rises again is over by the University.
Edit: I believe they went so deep because of the volume of glacial deposits in this area. Otherwise they would have been going through gravel. That made building the stations a bit of a pain (the Beacon Hill station still has chronic problems with water. It smelled like a moldy towel for a long time).
Bertha dug a much wider, deeper tunnel through a major downtown area next to the ocean, underneath an active highway.
The Boring Company bought an off the shelf machine used for sewer lines and dug through a desert at less than a fourth of the maximum depth Bertha did.
It is not, per general EU regulations no 'single bore' tunnel can be longer than 1km (0.62miles0. Tunnels that are longer than that, either have to be double tunnels, with escape doors between them, or one large tunnel and an emergency smaller one.
This looks the size of a emergency tunnel. Still, the cost of building it seems pretty low. Boring Co could build two side by side, (one for each direction) and have emergency hatches between them.
reply