Plan the bridge so any ship big enough to hurt it grounds before it gets that close. Don't put pilings in the channel. It's just money. But it's a lot of money so sometimes it's better to just have shipping not suck.
Alternatively, the Chunnel will almost certainly never get hit with a ship.
Yet another idea: if a ship's motors fail, have a ship ready that can push it in the right direction, in time. Probably need 2x the amount of horsepowers to make up for lost time, but it's not impossible.
Yes, that's called a tug and in plenty of harbors a vessel of this size would not be permitted to do close quarters maneuvers without the mandatory assistance of one, or in this case more likely two, tug boats of a certain minimum size relative to the size of the vessel.
Plan the bridge so any ship big enough to hurt it grounds before it gets that close. Don't put pilings in the channel. It's just money. But it's a lot of money so sometimes it's better to just have shipping not suck.
Alternatively, the Chunnel will almost certainly never get hit with a ship.