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How it was done before

BRIDGES

The masonry arch bridge that replaced the old timber structure in 1828 had five spans over the river, the centre one measuring 18.2m. On the Richmond shore there are two land spans. The approach viaduct on the Kingston shore has long since been filled in, but one approach span remains, used for a subway taking the riverside walk under the bridge.

In 1914 the bridge was widened for the first time. Visually the 1914 addition is very similar to the original structure, with a replica of the original Portland stone facade and balustraded parapet. Beneath the skin, however, there is one key difference.

To reduce dead-weight there are voids in the mass brickwork over the piers. On the 1828 structure these are five circular horizontal voids of reducing diameter, while the later addition has single semicircular voids above each pier. English Heritage has insisted these voids remain - despite the fact they are quite invisible from the outside.

The pre-Industrial Revolution solution to the problem of founding bridge piers in a busy river was a shallow timber raft, very little different to what would have been used in the previous millennium. In 1914, however, a much more technological approach was adopted.

Steel cofferdams in the form of 2.4m deep round-ended open-bottomed boxes were floated out and sunk into position. Ballast was provided by mass concrete poured between the double skin of the cofferdams' 'lids'. Compressed air then forced the water out of the cofferdam, workers entered through an airlock and began hand excavating the river bed.

Once the cofferdam had sunk to the desired level it was filled with concrete. Construction of the superstructure could then begin.

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