The airport that rose from the ooze: Osaka wanted an offshore airport, but had no island. No problem: they had one built, three miles long, out of crushed rock on soft clay. Jonathan Glancey marvels at an extraordinary marriage of design and engineering

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To write about an airport before using it as a regular passenger might seem almost as silly as flying without wings. But here goes. Kansai International Airport opened at the beginning of this week. Seen from the air, it sits like some immensely long, infinitely delicate steel insect on a man-made island three miles out to sea in the Bay of Osaka and 25 miles from the Japanese city it serves, reached by a bridge carrying road, railway and power.

Together, island and terminal form one of the most extraordinary works of engineering and architecture of all time. Yet, unlike the pyramids, medieval cathedrals or the Great Wall of China, decades in the making, the Kansai project has taken just six years. The cost and ambition of the project are on much the same scale as the Channel tunnel.

Why build an airport at sea? Because commercial aircraft are not allowed to take off or land around the clock over Japanese cities. Long-haul operators, however, are keen to do just that. Their aircraft are immensely expensive; a brand new Boeing 747-400 costs pounds 100m. A mere half-dozen of these magnificent machines would cost as much as the Kansai terminal.

The one sure way to keep these jumbo investments paying for themselves was for the airport corporation to go offshore - literally. Building an island three miles by one was not an easy thing to do. It is made of crushed rock, piled 65ft high, resting uneasily on natural clay so soft that you could not walk on it without sinking. This layer of sunken ooze lies, in turn, on 200ft of harder clay. Not surprisingly, the island began to sink as soon as construction began, causing delay, scandal, resignations and media-fodder for months on end.

When your 747 finally touches down at Kansai, rest assured that the island is safe, although it will continue to bob up and down for the rest of its life - the Leaning Tower of Pisa of our age. It will also be battered by storms, hurricanes, tidal waves and earthquakes.

The task of designing the mile-long terminal building - the world's biggest - has been in the hands of Renzo Piano's Building Workshop (of Genoa and Paris), Ove Arup & Partners (the brilliant multinational team of engineers based in London) and Nikken Sekkei, the giant firm of Japanese architects and engineers.

The design contract was won by Piano through a competition in 1988. His closest rival was Sir Norman Foster, whose Stansted airport terminal (1992) is one of the world's finest - but, in scale, a Dakota to Piano's Jumbo.

Piano was joint architect, with Richard Rogers, of the Pompidou Centre in Paris (1971-77). Since then, his architecture has matured from Meccano-bombast into a more sensual and environmentally responsive technologia; he is the designer of soft machines, buildings that go a long way towards reconciling the natural and the man-made.

The Kansai project was run by Piano's lieutenant, Noriaki Okabe, who worked on the Pompidou Centre from its earliest days. So too did Kansai's principal engineer, the late Peter Rice (of Ove Arup), and other members of the highly integrated design team. This is a building where the work of architect and engineer is all but indistinguishable.

Aircraft, 41 at a time, nuzzle up to the gleaming aerofoil-shaped 'wing' of the terminal. Passengers - international and domestic, arriving and departing - are taken by transit shuttle along the length of the wing to the core of the building, known as the 'Canyon'. This is a multi-storeyed, 300ft-high hall of trees, internal bridges and daylight supported by bone-like steel columns. It houses immigration, customs and baggage retrieval, and leads out to fresh air and the bridge across the bay.

The trees inside the Canyon are meant to continue out of the terminal in a seamless progression, marching its whole length. But as trees attract birds, mortal enemy of jet engines, they may be left unplanted. At one point the airport authorities seriously considered radio-controlled robot hawks to do battle with any bird taking a shine to the island: a savage diversion for passengers.

Travellers can walk the length of the beautiful steel-and-glass wing if they choose. In doing so they will grasp the staggering scale of this mighty yet gentle giant of a building. They may be reassured to know that it is supported by hundreds of steel columns sunk into a raft of concrete. Each column can be jacked up or down as the building flexes in response to shifts in the structure of the artificial island.

Such shifts are monitored by a computer that, like Hal in Stanley Kubrick's 2001: A Space Odyssey, keeps tabs on every aspect of the building's services and structure and pumps cool air through the wing for much of the day.

Piano's major contribution, aside from shaping a supremely elegant building, was to design a layout that requires the fewest possible signs. Departing aircraft, for example, are visible from the moment passengers arrive by road or rail. Passengers ascend by lift, stair or escalator to the top of the building: from here they pass under the spectacular steel waves of the roof with their aircraft in sight all the way.

Shops and cafes have been separated from the main flows of passengers. This streamlined arrangement would be incomprehensible in Britain, where airports have become edge-of-town superstores with terminals attached, where passengers are 'customers' and the path to an aircraft is a fight against the smell of hamburgers and the siren call of luxury goods.

Kansai is a hugely significant building. It shows how the most sophisticated use yet of computer-aided design and unrepentant advanced technology does not necessarily lead to a building that makes an overt display of Boy's Own technology; it is infinitely more subtle. It shows how architects and engineers can work together without anyone being able to tell where one began work and the other stopped. It is the fruit of a truly international collaboration. Much of the steelwork was supplied from Britain (Watson Steel of Bolton). This has been like shipping coals to Newcastle, since Japan produces more steel than the whole of Europe, but the steelwork from Bolton, forged during the depths of the recession, was cheaper and superior to that available in Japan.

Whether we want to go to such extraordinary lengths to encourage international travel is not a question that the designers of Kansai International Airport had to face. Given the apparently inexorable growth of exotic travel, this is the first major airport to match the dream of 24-hour-a-day, round-the-world flight by pounds 100m jet.

You can see more of Kansai airport in the Channel 4 programme '21st Century Airport', produced by Chris Hale, 8pm, Sunday 11 and 18 September.

(Photographs omitted)