The Viaduc de Millau by JOHN LICHFIELD Foster:

The bridge spans the 1.5 mile valley cut by the Tarn

G Horcajuelo/Wostok Press

It is the elegant structure and well-toned concrete and steel muscle, that makes the Viaduc de Millau one of the engineering wonders of the 21st century. It claims to be the tallest viaduct ever constructed and will carry traffic across the valley of the river Tarn in south central France.This link will be joined as a continuous motorway from Paris (and the Channel tunnel) to Languedoc and Barcelona. The viaduct at Millau, was designed by British architect Lord Foster and at its highest point, is 60ft higher than the Eiffel Tower.

It comprises 7 pillars and three and a half suspended lengths forming a slight curve for one and a half miles between two high plateaux. The tallest pylon over the road-deck reaches 1,125 feet (343 metres) above the valley.It is the apparent delicacy of the structure that is most impacting. The design uses light, strong, modern materials combined with sophisticated computer techniques, to reduce cost and time and to diminish its impact on the craggy landscape of the southern Massif Central.

This location at Millau has been best known for many years as a major bottleneck on the incompleted motorway route through central France to Montpellier, the Languedoc coast and Spain. Previously, the A75 Clermont Ferrand-Beziers motorway stopped on the granite clifftops to the north and took 20 miles on a twisting, steep road into and out of the valley adding over three hours to the drive, before resuming on the 2,500 feet high plateau to the south. With the viaduct, this part of the journey will be reduced to 10 minutes.

Local sensitivity on the landscape value of the gorge required an architectural appointment along with the Engineers. Lord Foster worked with the French engineer Michel Virlogeux, head of the French government s bridges and roads department and the designer of the Pont de Normandie across the mouth of the Seine. The British architect says that there has been "fantastic co-operation" from the beginning between his team and the French engineers and contractors. Engineering and aesthetic, or architectural, choices have intertwined from the beginning, he said.

The seven pillars which support the viaduct look tubular and monolithic from a distance but are in fact split into curving, sculpted, gently separating columns, which resemble the fibia and tibula in a forearm. Lord Foster said that the design was partly driven by aesthetics but took account from the beginning of what was possible, and even desirable, from an engineering point of view.

The viaduct, costing EUR400m (pounds 278m), has been built in record time (just over three years) for a project of this size. The French construction company, Eiffage, the direct descendant of the company started by Gustav Eiffel, the builder of the celebrated tower beside the Seine, has raised the money entirely from private financing. In return, the company has been given a 75 -year concession to run the viaduct as a toll-bridge (likely to charge around EUR5 per car, EUR6.60 in the peak summer months).

The deck of the bridge - on which the four-lane road rests - has been

constructed from special steel, not from concrete. This is one of the choices which has made the bridge relatively light-weight and so seemingly delicate to the eye.

Eiffage devised a method for preconstructing the 32-metre wide road- deck in 2,000 pieces at its factory in Alsace. They were welded together on the hills on either side of the valley and then shoved out over the abyss 60 centimetres at a time. Roughly one mile of decking was pushed out from one side; half a mile from the other.

Satellite positioning technology ensured that the two ends connected

correctly (at the end of May). While this was going on, enormous, temporary trestles were built from the valley bottom to prop up the roadway. Once the "deck" was completed, A-shaped steel pylons or masts were constructed up to another 90 metres above the road level to hold the cables which support the bridge. As the cables have been attached in recent weeks, the temporary trestles have been taken away, leaving the roadway suspended spectacularly for 350 metres between each pair of pillars.

Alas the last sections, linking the A75 to the A9 south coast motorway, at Montpellier and Beziers, have not even been started yet. So the summer traffic jams around Millau are likely to last for several years yet but to be transferred 60 miles to the south. That however is not Lord Foster s problem. Will he be in Millau on 17 December for the opening of his bridge? "Nothing would keep me away."