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The reason they are replacing Halley V is that there is an increasing risk of the ice shelf breaking off after 2010, taking the site of the current base with it. Not only will Halley VI be built further inland but it is being designed to be movable so that it can relocate if it is in danger of breaking off.
If it wasn't for the risk of carving, the current station certainly could survive in its present form for a number of years. The only serious problem with Halley V is that the sub-surface tunnels that contain the fuel stores and services between the buildings are getting very deep and increasingly crushed by ice. If they abandoned the old tunnel system and started on the surface again I don't see why the base couldn't continue for another 10 years.
As far as I was aware, there isn't much of a choice. Halley is fairly unique amongst current Antractic bases in that dropping off the end of the ice-shelf into the sea is an inevitability in the not so far future - Halley 3 has already started to fall into the sea if it's not completely done so by now.
Other ice shelves have broken back by unprecedented amounts in recent years and so it has to be a factor in forward planning that break-up of the ice shelf could happen quicker than current rates would imply.
Also, stations get old and wear out, requirements for new facilities increase as time goes by, so eventually the most economic long term plan becomes to buy a brand new station.
Having said that, the new new design doesn't look the best to me (on my superficial inspection), why are all the buildings arranged in a line? Not very community-minded. I like the idea of a huge window with a massive vista though - I can imagine standing and looking out at the sunrise/set from the comfort of base, or maybe even an aurora.
Paul: Originally this design had small modules all connected to a central area (see here). The problem with that layout was snow management - anything to the west of a structure would quickly get buried by blowing snow. The long thin layout (which will run north-south) will certainly reduce the annual maintanence (one of the key requirements of the new design).
koldK: The designers are Faber Maunsell with Hugh Broughton Architects. Faber Maunsell are a pretty big international company who have had experience of constructing buildings at the South Pole station. A member of each of the three finalists visited Halley V last summer and spent a couple of weeks talking to everyone on base about what would and wouldn't work.