It's also really hard to make the tunnel remain a tunnel over its expected 150 year lifespan - given that it basically runs through a fault line. They had to study and test local geology for about 15 years, build certain sections to expect some movement over time, as well as kit everything out with a lot of sensors.
Overall an amazing achievement, and unsurprising it took this long to figure out!
After seeing some of the safety features in a short video I linked in another comment, I get the impression that this is either going to last much longer than 150 years or something so catastrophic will happen that nothing that could have been built would've persisted.
Overall an amazing achievement, and unsurprising it took this long to figure out!