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Segment survey

The giant segments for the tunnel are made in a high technology factory at a nearby site in the harbour. In charge is Heinrich Simmering, who also ran the precast factory for the Storeblt tunnel segments in Denmark.

'But these are much bigger,' he says. Extraordinarily that means they must be much more accurately formed than usual with face tolerances of no more than 0.6mm on a 2m width. 'Their size means any point contact has its effect magnified by the greater length,' he explains.

To achieve accuracy, segments from CBE in France are made in special steel forms, the four sides of which lock together with a male/female joint which includes an engraved reference line which must line up. A centre bar which passes over the top has a pointed end to fit into a reference slot as a further check.

A three dimensional survey of units to check quality uses a spatial reference system developed by Kern Space Systems of Switzerland. Four motorised post-mounted theodolites take corner readings of a finished unit in a test chamber, to establish a reference grid. They then automatically survey 180 surface points, building up an image of the unit which can be displayed in real time on a monitor.

'If the unit is accurate we clear the preceding 19 units,' explains Simmering. 'If not we go back to number 10 and clear half of them and then work to number 15 or number five depending. All units up to a flawed one are scrapped.'

Concrete is a high strength 50N/mm2 mix designed for low heat of hydration. With units this size heat build up is a danger 'to the point where one could explode', says Simmering. Even so the four lines of nine forms, plus one keystone form, are heated for curing with temperatures monitored.

Units can be broken out after about 15 hours, but have to stay under cover for three days and remain individually wrapped in plastic in the storage yard outside. The yard produces 55 units daily and will make 11,560 in total.

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