NOT MANY cities with a population of just 200,000 can boast an underground metro system. In fact, Rennes believes it will make the Guinness Book of Records as the smallest city in the world with a full metro once its new line is opened in 2001.
The Rennes line will be a light rail system on a route just under 10km long, with about 3.7km in bored tunnel, a similar length in cut-and-cover tunnel, and some of the rest on viaduct.
When studies began in the 1980s the city considered building a tram, says Jean-Pierre Potin, a co-ordinator for the Systra group which has done most of the engineering design for the project - 'and, of course, it would be a lot cheaper'.
But Rennes is an old city and its centre has historic buildings, some dating back to the 17th century. It therefore decided to go ahead with an underground, which comes in with a FFr2,942M (pounds310M) price tag - pounds150M for the civil works and pounds110M for the VAL automatic light rail. VAL is a two-car driverless system developed by French manufacturer Matra together with the Systra group, a consultant born from the merged engineering bureaux of the Paris metro authority (RATP) and French Railways (SNCF). The remainder pays for parking areas and temporary works.
The biggest contract has gone to a consortium of French firms for the main 3,765m long underground tunnel beneath the city centre, the river and the mainline railway station. The group comprising Campenon-Bernard, SGE, Razel and Pichenot, began excavations last year for the seven underground stations in this section.
The 7.73m diameter TBM, made under licence from Japan's Kawasaki (which completed work last year on the second Lille line) has been reconditioned with increased power for the drive through the metamorphic schist which underlies most of Rennes. It is currently about 700m in and 'going quite well', says Potin.
Care will be needed in the centre of the city where the line passes under some of the very oldest buildings. Cover is typically 12m, and a maximum 10mm settlement is allowed. Condition surveys have been undertaken, and the at-risk buildings instrumented.
Another 3,770m of line is to be built by cut-and-cover, mainly within 'Berlin walls' - driven steel piles with timber lagging. This method suffices for most stations except one near the river, where Soletanche Bachy has formed a perimeter of diaphragm walls between 18m and 20m deep.
The most complex station is at the other northern end of the line. The JF Kennedy station is being built underground in a shopping, residential and parking complex.