Sinkholes are always the tip of the iceberg of ground control problems when tunnelling. You should be able to cut settlement to zero with a pressure-balancing tunnel boring machine, but that takes money. Already houses are cracking, and they are complaining about vibrations. This is going to be a darn train tunnel!
As well, the tiniest bit of settlement makes things a lot worse when the earthquake comes along.
Update: My one comment of the year has me making an addition. I will make up a totally fictional situation.
My company Cheapnsleazy Inc. has just finished a tunnel that may rhyme with a certain blue pill. I go to Vancouver, and impress them all with my astonishing low price. I had changed the name because I made a billion on charging extras, and a few people might worry. I get a crappy old boring machine and paint it up. What me worry? I charge ahead at full speed, running a huge ground deficit, with all the portals wide open. The trees tilt and the buildings crack. Not me! Finally I run into big trouble when the sinkholes open up way in front of the machine. Now is the time for my special trick. I go to court -- "You didn't tell me that dirt was dirt!" The judges always go for the contractor. Another extra billion in the pocket.
Update2: I am also yelled at because I'm not giving any information. You will never get information! The contractor makes a point of never hiring engineers, so he can go full blast with his Stupidity Defence. And the government people are all under the control of the PR department.
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