This is a UK construction project doing something that sounds totally ridiculous when you describe it.
They need to tunnel under the motorway so that new train lines can be built, and they need the motorway to then pass overhead on a bridge. But they cannot close the motorway for months while they do it.
So they are building the bridge and tunnel as a combination giant concrete box next to the motorway. They will then close the motorway for three weeks, during which they will demolish the carriageway and tunnel through. They will then push the box into position and reinstate the road deck.
I would love to have been the person who suggested this as a solution. Just to see the look on the faces of everyone around me.