"asdf" wrote

I'm not familiar with the economics involved, but I suppose tunnelling
would be out of the question?

Not a motorway, but a very busy D3 trunk road - a railway has just been
tunnelled under the A2 at Southfleet (CTRL Phase 2). Not that long ago, the
Thames Flood Relief Channel was constructed under the M4 and the GWML at
Taplow.

Peter