Macadam

Macadam is a type of road construction pioneered by Scottish engineer John Loudon McAdam c. 1820, in which crushed stone is placed in shallow, convex layers and compacted thoroughly. A binding layer of stone dust (crushed stone from the original material) may form; it may also, after rolling, be covered with a cement or bituminous binder to keep dust and stones together. The method simplified what had been considered state-of-the-art at that point.

Pierre-Marie-Jérôme Trésaguet is sometimes considered the first person to bring post-Roman science to road building. A Frenchman from an engineering family, he worked paving roads in Paris from 1757 to 1764. As chief engineer of road construction of Limoges, he had opportunity to develop a better and cheaper method of road construction. In 1775, Tresaguet became engineer-general and presented his answer for road improvement in France, which soon became standard practice there.

Trésaguet had recommended a roadway consisting of three layers of stones laid on a crowned subgrade with side ditches for drainage. The first two layers consisted of angular hand-broken aggregate, maximum size 3 inches (7.6 cm), to a depth of about 8 inches (20 cm). The third layer was about 2 inches (5 cm) thick with a maximum aggregate size of 1 inch (2.5 cm). This top-level surface permitted a smoother shape and protected the larger stones in the road structure from iron wheels and horse hooves. To keep the running surface level with the countryside, this road was put in a trench, which created drainage problems. These problems were addressed by changes that included digging deep side ditches, making the surface as solid as possible, and constructing the road with a difference in elevation (height) between the two edges, that difference being referred to interchangeably as the road's camber or cross slope.

Thomas Telford, born in Dumfriesshire, Scotland, was a surveyor and engineer who applied Tresaguet's road building theories. In 1801 Telford worked for the Commission of Highland Roads and Bridges. He became director of the Holyhead Road Commission between 1815 and 1830. Telford extended Tresaguet's theories but emphasized high-quality stone. He recognized that some of the road problems of the French could be avoided by using cubical stone blocks.

Telford used roughly 12 in × 10 in × 6 in (30 cm × 25 cm × 15 cm) partially shaped paving stones (pitchers), with a slight flat face on the bottom surface. He turned the other faces more vertically than Tresaguet's method. The longest edge was arranged crossways to the traffic direction, and the joints were broken in the method of conventional brickwork but with the smallest faces of the pitcher forming the upper and lower surfaces.

Broken stone was wedged into the spaces between the tapered perpendicular faces to provide the layer with good lateral control. Telford kept the natural formation level and used masons to camber the upper surface of the blocks. He placed a 6-inch (15 cm) layer of stone no bigger than 2.4 in (6 cm) on top of the rock foundation. To finish the road surface he covered the stones with a mixture of gravel and broken stone. This structure came to be known as "Telford pitching." Telford's road depended on a resistant structure to prevent water from collecting and corroding the strength of the pavement. Telford raised the pavement structure above ground level whenever possible.^{[citation needed]}

Where the structure could not be raised, Telford drained the area surrounding the roadside. Previous road builders in Britain ignored drainage problems, and Telford's rediscovery of drainage principles was a major contribution to road construction. Around the same time, John Metcalf strongly advocated that drainage was in fact an important factor in road construction and astonished colleagues by building dry roads even through marshland. He accomplished this by incorporating a layer of brushwood and heather.^{[citation needed]}

John Loudon McAdam was born in Ayr, Scotland, in 1756. In 1787, he became a trustee of the Ayrshire Turnpike in the Scottish Lowlands and during the next seven years his hobby became an obsession. He moved to Bristol, England, in 1802 and became a Commissioner for Paving in 1806. On 15 January 1816, he was elected surveyor general of roads for the Bristol turnpike trust and was responsible for 149 miles of road. He then put his ideas about road construction into practice, the first 'macadamised' stretch of road being Marsh Road at Ashton Gate, Bristol. He also began to actively propagate his ideas in two booklets called Remarks (or Observations) on the Present System of Roadmaking, (which ran nine editions between 1816 and 1827) and A Practical Essay on the Scientific Repair and Preservation of Public Roads, published in 1819.