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Road design

 

roman road

Via Domitia at Arbessum, near Montpellier, France. Photograph by Benoît Strépenne, 2006 (Wilkipedia)

The Romans built roads to last. In fact they were like massive walls, up to one metre high, sunk into a trench with the top providing the road surface. Following the digging of drainage ditches, which also provided some of the fill, the road was built up in four layers: slabs embedded in mortar formed the foundation; masonry made up the second; the third, called the nucleus, consisted of agglomerates; finally, the rolling surface could be simply broken stones, paving stones, or bricks depending on traffic.

With the collapse of the Roman Empire this or in fact any technology was lost for more than a thousand years. The absence of durable roads only really began to be felt with the increase in numbers and size of wheeled traffic during the seventeenth century following a rapid expansion of trade. At first vehicles were seen as a damned nuisance by those unfortunates made responsible for roads but without funds or knowledge do anything about them. Having tried unsuccessfully to minimise wear and tear by use of controls on weight, axle width, number of horses used, and width of wheels, all of which were easily evaded, serious attempts began during the middle of the 18th century to design roads that would accommodate the traffic.

People disagreed about how roads should be constructed. Some proposed that they should be concave, or even placed in a trench and periodically flushed out. Many travellers remarked that this had happened anyway, since many roads had sunk so much due to erosion and misguided maintenance that they could no longer see the surrounding countryside. The more enlightened suggested they be sloped across their width.

However, certain basic principles began to be defined, notably the most important, that of sound drainage. Around 1750 Trésaguet in France and Metcalfe in the UK proposed a method of construction relying on a firm and  well-drained foundation of large rocks topped by progressively smaller ones, forming a  convex surface to allow water to run off it.

This proved extremely strong but also highly expensive to build and maintain. Around the end of the eighteenth century Telford in the UK proposed a similarly robust concept, perhaps too much so for traffic needs (he had been looking forward eagerly to steam-powered vehicles but was nearly a century too soon), with the result that it was rather expensive due mainly to its thickness and very solid foundation, intended to compensate for unstable roadbeds.

telford

It was left to Macadam at the beginning of the 19th century to develop the far more economical approach which is still used today in adapted forms, ususally with a coating of bitumen to seal it.

macadam

This calls for very good subsoil drainage to keep the roadbed firm, resulting in a road much thinner and cheaper to build. In fact he insisted that thickness should be dictated only by the need to protect the roadbed and keep it dry rather then to provide the high load bearing capacity underpinning the approaches of his predecessors.

The Stone Road, Homer Watson, 1881, Canada