Moffat Town :- John Loudon McAdam, Engineer
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They Were Here :
John Loudon McAdam, Road Engineer (1756-1836) :
Portrait from 1800, gravestone erected 1836.
John McAdam was born in Ayr, but lived for the last part of his life in Moffat, and is buried there. His name is preserved in 'Tarmac', the name given to a version of his most successful discovery about road metalling. He became the tenant of Dumcrieff Farm in Moffat Water in 1790; a staunch Royalist, though settled for a time in America, he came home during the American Revolution.
During the period from the Roman Empire onwards, the most successful roads had been built with a bottom layer of heavy stone, then succeeding layers of finer aggregate with a top layer of gravel. The best roads (for example, in towns, on bridges and on important routes) might have slab or cobble paving. By the time of John McAdam, roads repair was in the hands of parish 'road gangs', men more interested doing the least work at the most profit, than in laying anything lasting. One of the few statutes in Scotland stated that stones used in road repairs should be no bigger than could go into a man's mouth. This lead to each gang having at least one man with the largest mouth in the neighbourhood, and to wildly unsuitable stone being tipped into deep ruts. The snag was that the stone was frequently water-rounded field or river stone, which refused to lock together under load. Unless regularly raked, even the round river gravel tended to be thrown by cartwheels to the middle or sides of the roads.
McAdam discovered that the best stone or gravel for road surfacing had to be broken or crushed, then graded to a constant size of chippings. When a roller or a cartwheel passed over the chippings, they locked together and did not slip. He evolved a system by which the roads were covered with from 4 (100 mm) to 10 inches (257 mm) of graded broken chippings, laid upon the natural soil. The chips were allowed to be no larger than 1½ inch (38 mm.) and no heavier than 6 ounces (170 gm or 0.17 Kg) and had to be spread in layers that could be rolled flat so that they bound together. The stone was laid so that the center was higher than the edges (cambered) to assist rainwater in draining off. Drains either side of the road carried away the water, providing a hard and dry road free from ruts.
This in itself was a remarkable improvement, though calling for a labour hated by the road gangs. Breaking or 'knapping' stone was considered one of the most unpleasant tasks of all, and was famously used as a prison punishment at Dartmoor. However, the hammers needed were small one-pound weight ones, together with a rake, a shovel and a pick, so the work was tedious as much as arduous. Established roads were sometimes as much as three feet deep in unsuitable stone, so 'Macadamisation' called for little more than clearing the stone, ditching upon either side for drainage, then laying the broken stone upon the road mound produced by excavation. This nevertheless called for good surveyors and superintendents to ensure that the work was not skimped; a brass ring-gauge two inches in diameter was used to test the sizes of stone, those unable to pass through being broken further. Rather to the surprise of his contemporaries such as Telford, McAdam laid his gravel roads even across marshy land without a foundation. He had discovered that the gravel spread the load and provided resilience; on study of a road near Bristol, half on marsh and half on bedrock, it was found that the marsh had needed only five inches of additional stone, as against the seven inches of additional stone on the hard rock bedrock.
Despite initial doubts, the macadamisation of London was begun in 1824, granite cobbles being broken up to provide the necessary gravel, with excellent results. American visitors to London later reported that the later upkeep of the roads had not been as good, but the work so started carried on and eventually gave London some of the best streets in Europe.
Although McAdam was remarkably successful in constructing roads that lasted, the conventional wisdom of Telford was small stones on large. Telford built mail roads up to Holyhead and to Stranraer, as is told in the page on him. The expense of his cobbled roads was against Telford, as was the point of McAdam that frost-heave (freeze/thaw stone movement) inevitably brings large stones to the surface through the smaller stone, as does traffic vibration.
The success of Macadamisation was rapid and influenced roadmaking throughout the world; for the first time, rapid road transit was possible for large numbers of vehicles and goods. The stage coaches and mail coaches briefly ruled the roads, but by the 1830s they were being replaced for mass-transit by the railways. Those were less flexible than roads, but offered faster movement of heavy goods and large numbers from point to point. Rock crushers were introduced by the 1860s, for the same binding capabilities of broken stone (aggregate) were needed for railway ballast. Railway ballast was broken to a much larger size, but the principle was otherwise the same.
The development of cars, lorries and buses, was foreshadowed as early as 1827 by the invention by Gurney of a steam-carriage that could run on the Macadamised roads, but this was to be frustrated by ostlers and railway-owners, - an unlikely combination of protectionists. McAdam was for steam-carriages and steam-coaches, mainly because horses' hooves damaged the road surface and wheels compacted it. So it was that modern transport powered by steam was set aside until the competition from the more-polluting petrol and diesel engines made it possible for the railway to be doomed. The motorway beside Moffat is a descendant of both McAdam and Telford, the closure of Beattock Railway Station matched by the opening of the interchange to Moffat and the Blue Boar Service Station.
Modern road surfaces are still largely dependent on McAdam's discovery. Coal tar was first used to bind the stones together, hot-laid tarred aggregate or tar-sprayed chips providing an excellent road-metalling for the surface. Oil-based asphalt was later used from Trinidad and from refineries as a road surfacing, laid on reinforced concrete, but still owes a lot to McAdam as it is mixed with granite or limestone chippings.
McAdam never really achieved the respect that was his due. He was paid the sum of £ 5,000 for works done for Turnpike Trusts around Bristol, but a proposal for £ 5,000 from Parliament as a grant for his expenses was first refused, then cut to £ 2,000, mainly due to professional jealousy. Corruption in roadworks was appalling; by his own efficiency, McAdam exposed the abuse of road tolls by less scrupulous Turnpike Trusts, many of which were run at a deliberate loss despite high toll receipts. Travellers of all kinds respected McAdam, but those whose scams he had revealed remained his bitter enemies. His reputation has nevertheless survived, as the Scotsman who paved the way for development.
Note : The unusual load-bearing capacity of McAdam's system may rely upon a phenomenon called 'earth arching', where the load in the structure spreads sideways across the underlying surface, varying according to the depth of gravel. Thus, a four-inch tyre might have its load spread by the depth of broken stone across twelve inches width (four inch depth) to twenty four inches width of (ten inch depth) soil surface, which could carry the load if reasonably well drained. A Macadamised road could thus carry vehicles across a bog in which a man or horse would sink. The system would only fail if torn up by cleated or deep-treaded wheels accelerating too fast across the surface of the stone.
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All text and images © 1999 Richard Edkins of Dalbeattie Internet.
Moffat Town Website started 9th June 1999.
Last updated 14rd December 1999.