Not sure I fully understand that they will "deck"over the M25.
Does that mean the whole runway will be built on stilts? Will it be sloped up to cross the M25? Or do they do similar to the Queen Elizabeth aircraft carriers, which would make for an interesting (?) experience.
Whatever they do will cause problems with the M25, and how much will that cost?
The other problem raised in today's press, lack of construction labour. Redeploy the Orange Army (where do the signals go)?
If you want an extreme example of a runway built on stilts, head for Funchal and Aeroporto Internacional da Madeira Cristiano Ronaldo.
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(Creative commons copyright so far as I have been able to tell)
It isn't the first, just very visible. It will, as you say, cause problems with the M25 during building, although that is by no means unusual, and will be the subject of much discussion when planning the build. I say "when" rather than "if", because it will be the only certain end to this long-running saga. No other outcome will be anything other than a postponement unless a practical alternative to the large aircraft can be found for international travel within the next 5 years. It is a project than will be massive and controversial, both go without saying. I struggle to think how it could be brought to a state of shovel readiness before the next election, but if it isn't, it will be a major topic and would need all the dark arts of the spin doctors to sell as a manifesto commitment. And yet, it isn't impossible. It may be the hottest topic ever in the area around it and along the last part of the flight path, attracting not just NIMBYs but NOMBYs - Not Over My Back Yard - and the fiercest of opposition from ecologists of all waters. There will be, like for
HS2▸ or the nuclear power stations, a lot less interest from constituencies out of sight, though, some of whom may think it high time that London place took one for the team.