How much extra force does a vacuum tunnel have to withstand to avoid collapse, compared to a regular tunnel?
Dirt weighs ~1.5 tons per cubic meter. So one meter of soil depth is 15,000 N/m^2^ (1500kg * 9.8 m/s^2^) so 15 kpa/m (which is 2 PSI per yard of depth).
Vacuum can't provide more force than atmospheric pressure. Atmospheric pressure is 100kPa or 15 PSI. Adding vacuum load adds the equivalent crushing force as digging a tunnel 6.5-7m deeper (7.5 yards). I don't know why this is totally insurmountable. Take a thick walled concrete tube, coat the outside with some sort rubber, bury the tube and pump out the air.
Yes you need to seal between sections, and keep sections aligned. I'm not a civil engineer or whatever but it doesn't seem crazy hard. It just seems like... not important. It seems a lot more expensive, and less beneficial than just making a good surface transit network. Who cares if it would let me get from Vancouver to Seattle in... half an hour? If it's already taking me an hour to get to the station and find parking, and get through ticketing/security/ customs.
It's only 2 hours from Surrey to Everett in a car (ignoring customs). Maybe we should just like have a bus?