Watch Secrets of the Ancients online: Episode 2 Caesars Bridge

In 55 BC Julius Caesar marched through Europe, crushing the rebellious tribes in the North. In mid-summer he arrived at the mighty Rhine. He was determined to get his legions across to teach the tribes on the other side a lesson, but considered it beneath the dignity of the Roman army to cross the river by boat. Consequently, he ordered his men to build a wooden bridge over a river which was at least 400 metres wide, up to 8 metres deep and flowing at 2 metres a second. In his account of 'The Gallic Wars' he claims that from the felling of the first tree to the completed bridge, this massive structure was built in only ten days. This programme attempts to discover if Caesar was boasting. Caesar left a surprisingly detailed account of how the bridge was built. Based on a series of trestles spaced across the river, it was unusual in a number of ways. For example, the pairs of timber piles that formed the trestles were driven in to the river bed at a slanting angle to provide more stability.This is much harder to achieve than the more usual vertical piling. Caesar tells us that 'the stability of the structure was so great and its character such that, the greater the force and thrust of the water, the tighter were the balks held in lock.' But how did Caesar's engineers pile 8 metre long oak timbers into the pebbly bed of the Rhine at an angle? The first challenge is to design and build a Roman pile-driver, with no evidence to go on. Engineer Chris Wise takes on the challenge and attempts to build a timber bridge across the North Tyne, in Hadrian's Wall country, another outpost of the Roman Empire, using the same techniques as the Romans had at their disposal. Chris has had a major role in building the Millennium bridge over the River Thames which will take a year to complete, and here again his main challenge is the time limit. Just like Caesar, he has only a matter of days to do the job with a small dedicated team - hardly the legions that Caesar had at his disposal. He soon runs into trouble with his machine for driving piles into the river bed. It has to be built on a raft and the whole thing takes far longer than expected. Time is running short and he and his helpers from the local squadron of TA Royal Engineers from Tyneside struggle desperately to finish the bridge in time. (105 Field Squadron, 72 Engineer Regiment)

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