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Superstructure design

'Our conclusion is that if the piles supported some sort of platform, they were probably connected by horizontal members,' suggests Ove Arup maritime business leader Greg Haigh. 'However this raises questions as to why there are so many piles.'

In true Arup form Haigh and Arup associate Clon Ulrick have suggested not only what the bridge may have looked like but developed a whole new technique of design and build construction - the Bronze Age Observational Method.

'The structure started life as a few piles,' explains Haigh. 'but as they rotted or needed more lateral support, additional piles were driven through the deck or alongside. As a result the form of the structure evolved in response to the ground conditions encountered and the strength that had been achieved in the elements installed previously.'

The Arup team speculates that the construction sequence was:

Cut down oak trees. Select straight timbers up to 390mm diameter and up to 3m long for piles.

Lay willow mats on marshy intertidal foreshore as a construction platform. Hold mat in place by placing stones and gravel on top.

Float oak down river and place on the foreshore.

Strip bark off lower end of piles, leave stub branches at upper end.

Pitch piles to roughly vertical. Possibly weigh down one end and use the tide to lift the other.

Connect across the top of piles using temporary wooden beams or rope.

Pile will sink into peat a few feet under self-weight as the as the tide goes out.

Pull piles towards each other to improve lateral stiffness.

Float in heavy logs at high tide and place across stub branches.

On the falling tide, the weight of the logs pushes piles further into the ground.

Raise logs to higher notch, using the tide, as piles sink further into the ground.

Hew one side of smaller timbers to form planks to span cross beams.

If pile sinks excessively or rots, place larger pile outside the existing one and repeat process.

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