John McAdam

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1820: John Loudon McAdam was born in Scotland in 1756 and became a civil engineer. He was the inventor of "macadamisation" which was an effective and economical method of constructing roads.

In 1783 he became a trustee of the Ayrshire Turnpike and became increasingly interested in road construction. He came to the conclusion that roads needed to be raised above the surrounding ground, constructed from systematic layers of rocks and gravel and cambered so that rain runs off..

To our modern eyes, this seems fairly obvious but at the time it was the greatest advance in road construction since Roman times.

For centuries, the Roman method of making roads had been forgotten. Surveyors simply repaired roads with earth and cartloads of any stones which came to hand. Once this was done done their “repairs” were completed. The inevitable result was that the roads quickly deteriorated until they were as bad as they were before. 

The simple fact is that roads cannot be made with loose earth and stones. The only way to create a road is to to remove the earth to create a trench and then fill it with coarse-broken granite or rock. topped with a layer of small stones on top.

John McAdam realised that broken stones have angular edges which pack together, whereas random stone is mainly rounded and never packs together into a solid mass.

Generations of road builders and repairers had not spotted this simple fact. They assumed that the traffic would pack the stones down but in fact they remain as loose as a shingle beach and the surface is quickly destroyed by the wheels and hooves of passing traffic.

In 1802 John McAdam moved to Bristol where he became general surveyor for the Bristol Corporation and then surveyor to the Bristol Turnpike Trust. It was here that he decided to put his ideas into practice and remake all the roads under his care with crushed stone bound with gravel on a firm base of large stones. A camber, making the road slightly convex, ensured rainwater rapidly drained off the road rather than penetrate and damage the road's foundations.

The top layer in particular is interesting and became known as “macadam” It consists of smaller pieces of broken granite bound with “hoggin” which is red gravel mixed with clay. Hoggin has a binding quality. 

His method was so successful that it spread very quickly across the world. It became known as "macadamisation".  The first macadamised road in North America was the National Road, completed in the 1830s.

John McAdam died in 1836 but his method continued to spread until most of the main roads in Europe were macadamised by the end of the nineteenth century.

Even today, his name lives on in our own roads which are covered in “tarmac”, which is short for “tarmacadam” - the addition of bitumen tar to John McAdam’s top layer.

Incidentally, great scheme for macadamising out the whole of the Great North Road between London and Edinburgh was in progress when the successful trial of the competing locomotives at Rainhill, near Liverpool, cast a warning shadow over the arrangements, and finally led to the project being entirely abandoned. 

Had the work been done, it is quite possible that the railways to the north would have taken another direction.

Thomas Telford was the engineer chosen by the Government to execute the work, and the portion between Morpeth and Edinburgh was actually constructed.  The road between London, York,and Morpeth had not only been surveyed, but work was on the eve of being started, when the Rainhill trials in 1829 stopped them short, and caused the utter waste of the public money spent in the surveying.

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