Engineering question reguarding sunken tunnels

[Dennis Toy]

I was riding on the metro and the train i was on ( The green line) runs under the anacostia river in a sunken tube. I pondered this somewhat morbid question.

Everyone in here knows that in some projects, tunnels are built on land and sunk into a trench that was dug in the river and covered with rocks and dirt from the river to weigh it down and used for roads and rail. I was wondering what if for some freak reason like a hurricane or weak construction all the river dirt or rocks were to float off. Would the tunnel stay inside the trench or float away and allow the water flood the other tunnels?

[CaptainChewbacca]

I was riding on the metro and the train i was on ( The green line) runs under the anacostia river in a sunken tube. I pondered this somewhat morbid question.

Everyone in here knows that in some projects, tunnels are built on land and sunk into a trench that was dug in the river and covered with rocks and dirt from the river to weigh it down and used for roads and rail. I was wondering what if for some freak reason like a hurricane or weak construction all the river dirt or rocks were to float off. Would the tunnel stay inside the trench or float away and allow the water flood the other tunnels?

Generally a tunnel section is bolted down to the bedrock, and the weight of the roadbed materials would be enough to weigh it down in any event.