Super highways

6th May 2005 at 01:00
A good road must combine a relatively smooth surface with a durable structure. That principle informed the achievements of the Roman army's engineers as well as modern construction crews. The twin enemies of traffic and rain have constantly sought to wreck the surface.

The Romans started by clearing vegetation and driving a level trench along the chosen line. They then placed a layer of stones at the bottom, covered these with a layer of broken stones, pebbles, cement and sand that formed a firm base. On this went cement mixed with broken tiles and then finally paving slabs that were cut to fit together snugly. Either side of the road was a drainage channel to draw water away.

The modern roadbuilder likewise starts by removing vegetation. Soil is then cut away and compacted with heavy machinery to form a base course. On the top of the structure goes a smooth waterproof wearing course of asphalt (black top) or concrete (white top), the purpose of which is to prevent moisture getting into the structure.

The road is given a convex lateral surface known as a camber, to allow water to drain away to the edges where it is carried off by gutters and drains.

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