[david]

david asked the Naked Scientists:

How did Romans or other early builders, put the centre supports for a bridge in deep/wide rivers.

What do you think?

[graham.d]

The Romans used (and invented) many bridge building techniques but, the specific question, of how they built a central support (or supports) in a deep or wide river is an example at how advanced their engieering was. If the river bed suited, they would create an island (or islands) by dropping rocks to the river bed in the appropriate place and then construct a pillar on top but the really clever technique was to use a "cofferdam". Here they drove wooden piles into the river bed to form a pair of concentric circles. They filled the gap between them with clay to make the construction water tight and braced the whole construction to withstand the pressure of water after they bailed out the centre. They then used concrete to fill the centre to form a circular pier. This technique is still used widely today though, of course, steel piling is used rather than wood.

[lyner]

I think it's fantastic how they used concrete all those years ago (the Pantheon roof in Rome is all concrete and doesn't use buttresses). I believe that some of the Roman techniques for concreting were forgotten and needed to be reinvented in modern times.

[daveshorts]

They did of course have the advantage of having volcanoes nearby which made cement for them, which I think gave them the idea.

[lyner]

Their lightest concrete certainly used pumice.
Can you buy it in Travis Perkins, I wonder?

Breeze Blocks, etc,  I suppose.