Deck launching is taking place in 171m, half span stages, leaping across the valley between permanent and temporary piers.
The operation is carried out using a computer co-ordinated jacking system developed specially for the Millau job by American specialist Enerpac.
It was chosen because it limits bending forces imposed by jacking on the viaduct's slender concrete piers and temporary steel supports.
Jacks on each pier head lift and carry the deck forward longitudinally in a series of 600mm strokes. The system comprises two types of jack, one for lifting the deck and the other for advancing it. Lifting and pushing operations are strictly co-ordinated by computer to ensure forces are applied uniformly to the deck.
At the beginning of each launch, the 250t capacity lifting jacks push a horizontal wedge shaped piece of steel between the pylon head and the underside of the deck, to raise it off its bearings (see diagram). Then, two pairs of 60t capacity jacks push the deck forwards. The wedge is then pulled out, lowering the deck back onto its bearings so the whole operation can start again.
Each 600mm increment takes four minutes to complete. It takes around 40 hours to carry out a single 171m push.
As the end of the deck approaches a pier, a third set of jacks mounted on the pier head is used to correct sag and lateral deflection, guiding the steelwork into position.
These jacks have picked up the deck sections' temporary steel noses and used them to lift the following steelwork to the correct height to clear the piers.