Cods. As was pointed out previously, the Dutch have been doing it for centuries. Amsterdam is built on wooden piles, which appear to be still doing their job even after a few hundred years.
"A few hundred" years ago. silverstrata: the Dutch have had to re-claim land. A few hundred years ago the the "wooden piles" would have cost a fraction of what they today.
For a start labour would have been cheap, with no trade unions shackling the powers that be those days.
The UK does not need to built an airport on a river, we are British with British needs, not Dutch.
Private money might be attracted to a modest by comparison third runway at Heathrow..No chance of attracting capital on a Boris fantasy.
In addition, this is throwing good money after bad, so it does nothing for the economy. Had that £350 billion been thrown at infrastructure and R&D projects, there would be a million people involved in building things that will be useful (and wealth generating) for the next hundred years.
You are living in the past...Cameron's government did not give £350 billion to the bankers which I agree was wrong, but done by a socialist government.
If you think the Cameron/Osborne double act are going to waste public money on a Boris fantasy airport - Well it ain't gonna happen.