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On the bridge at Avignon

Further down the Rhne valley at Avignon, twin TGV viaducts will stride across the river on chunky columns with more than a hint of Roman stone pillars or pedestals for giant statues. These classical echoes are intentional: French Railway's SNCF architects were well aware of Avignon's importance in Roman times, and the fact that it was once home for a medieval Papacy at a castle just down river.

In contrast to the historic arched bridge of children's rhymes, which now reaches only halfway across the river, the first viaduct for the TGV is complete. Said to be the first ever TGV bridge to use precast concrete segmental construction, the FFr565M (£58.3M) project features spans of up to 100m. Joint venture contractor GTM/Bouygues placed the last of the deck segments, which weighed up to 130t, in late June.

The launching gantry, one of two built by Comtec for GTM's Second Severn Crossing contract in the UK, has been moved to the start route for the second line, which is due to be completed by March next year. Rail works have to be finished by mid-2000, ready for the opening of the whole line in early 2001.

The two viaducts will sweep the line up from Marseille and across the western plain towards Montpelier, and eventually Spain. Another line splits off north to Valence, Lyon and Paris. A third line between Montpelier and Lyon makes a third side to a major triangle junction.

External post-tensioning by 37 strand tendons is another first for the TGV. 'Other complications include the need to install a movement joint in each viaduct,' says JV design co-ordinator Olivier Martin. The joints are located near one of the higher piers, seated between two specially shaped and reinforced segments, and allow the structure to absorb dynamic forces from trains decelerating from 300km/h. The pier needed special reinforcement and a rigid connection between the deck and pier. Bearings came from Maurer Germany.

Variable segment shapes are also demanded by the viaduct's lower flyover or 'Saut de Mouton' and up to 18m high embankments where one line curves over the other. Wider segments were needed for the completed junction section, and narrower ones for the single track elements of the link. These are lifted in by a Liebherr 1400, carrying up to 80t units at 32m.

The vast majority of the total of 838 segments have already been produced in an on-site casting yard, where five specially made forms are serviced by Potain towers cranes and a on-site batching plant. Forms came from Metalform and Serimetal of Lyon. A total of 120,000m3 of high strength concrete will be needed, with 38,000m3 a special white mix designed to match the shade

of the dramatic river cliffs.

Other current work includes removing temporary works and piling for cofferdams in the river, where many of 343 bored piles, between 1m and 2.2m diameter were placed.

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