Remarks on the Present System of Road Making / With Observations, Deduced from Practice and Experience, With a View to a Revision of the Existing Laws, and the Introduction of Improvement in the Method of Making, Repairing, and Preserving Roads, and Defending the Road Funds from Misapplication. Seventh Edition, Carefully Revised, With an Appendix, and Report from the Select Committee of the House of Commons, June 1823, with Extracts from the Evidence

by John Loudon McAdam

It is well known to every skilful and observant road-maker, that if strata of stone of various sizes be placed as a road, the largest stones will constantly work up by the shaking and pressure of the traffic, and that the only mode of keeping the stones of a road from motion, is to use materials of a uniform size from the bottom. In roads made upon large stones as a foundation, the perpetual motion, or change of the position of the materials, keeps open many apertures through which the water passes.

It has also been found, that roads placed upon a hard bottom, wear away more quickly than those which are placed upon a soft soil. This has been apparent upon roads where motives of economy, or other causes, have prevented the road being lifted to the bottom at once; the wear has always been found to diminish, as soon as it was possible to remove the hard foundation. It is a known fact, that a road lasts much longer over a morass than when made over rock. The evidence produced before the Committee of the House of Commons, shewed the comparison on the road between Bristol and Bridgwater, to be as five to seven in favour of the wearing on the morass, where the road is laid on the naked surface of the soil, against a part of the same road made over rocky ground.

The practice common in England, and universal in Scotland, on the formation of a new road, is, to dig a trench below the surface of the ground adjoining, and in this trench to deposit a quantity of large stones; after this, a second quantity of stone, broken smaller, generally to about seven or eight pounds weight; these previous beds of stone are called the bottoming of the road, and are of various thickness, according to the caprice of the maker, and generally in proportion to the sum of money placed at his disposal. On some new roads, made in Scotland, in the summer of 1819, the thickness exceeded three feet.

That which is properly called the road, is then placed on the bottoming, by putting large quantities of broken stone or gravel, generally a foot or eighteen inches thick, at once upon it.

Were the materials of which the road itself is composed, properly selected, prepared, and laid, some of the inconveniences of this system might be avoided; but in the careless way in which this service is generally performed, the road is as open as a sieve to receive water; which penetrates through the whole mass, is received and retained in the trench, whence the road is liable to give way in all changes of weather.

A road formed on such principles has never effectually answered the purpose which the road-maker should constantly have in view; namely, to make a secure, level flooring, over which carriages may pass with safety, and equal expedition, at all seasons of the year.

If it be admitted, as I believe it is now very generally, that in this kingdom an artificial road is only required to obviate the inconvenience of a very unsettled climate; and that water with alternate frost and thaw, are the evils to be guarded against, it must be obvious that nothing can be more erroneous than providing a reservoir for water under the road and giving facility to the water to pass through the road into this trench, where it is acted upon by frost to the destruction of the road.

As no artificial road can ever be made so good, and so useful as the natural soil in a dry state, it is only necessary to procure, and preserve this dry state of so much ground as is intended to be occupied by a road.

The first operation in making a road should be the reverse of digging a trench. The road should not be sunk below, but rather raised above, the ordinary level of the adjacent ground, care should at any rate be taken, that there be a sufficient fall to take off the water, so that it should always be some inches below the level of the ground upon which the road is intended to be placed: this must be done, either by making drains to lower ground, or if that be not practicable, from the nature of the country, then the soil upon which the road is proposed to be laid, must be raised by addition, so as to be some inches above the level of the water.

Having secured the soil from under water, the road-maker is next to secure it from rain water, by a solid road, made of clean, dry stone, or flint, so selected, prepared, and laid, as to be perfectly impervious to water: and this cannot be effected, unless the greatest care be taken, that no earth, clay, chalk, or other matter, that will hold or conduct water, be mixed with the broken stone; which must be so prepared and laid, as to unite by its own angles into a firm, compact, impenetrable body.