Roads, which were formerly of the utmost importance as the only means of communication between distant places, and on which an enormous amount of labour and capital have been expended, are now becoming rapidly subordinate to railways; however this may be, it is quite clear that roads can never be superseded by the latter. The numerous streets of great towns are so many short roads, and on some of them the amount of traffic is so very great, that the utmost skill of the engineer is required to resist the consequent wear and tear.
The Roman roads, which have been the means of civilisation to the greater part of Europe, were constructed in a very solid and durable manner, by first laying down a layer of rough stone set in cement, and upon this, stones either squared or nicely fitted together and also cemented; the whole formed a solid mass of masonry, not unlike the wall of an old castle laid horizontally. Basalt (a volcanic product) was the stone used for this purpose, where it could be procured. These roads were generally raised above the surrounding ground, but excavations, bridges, and tunnels were made where these were found necessary to continue the road in a direct course.
Roads are now generally made in a less careful manner, and the same observations as to course and levelling will apply to them, that are made on these subjects in the article “Railways,” only that less cutting and filling up are required, as the gradients on common roads are allowable of much greater steepness than on railways; one in twenty-five is however about the greatest declivity that should be made, although old roads exist with slopes much greater. The surface of a road should be slightly arched and a drain made on each side, that pools of water may not collect on its surface and soften it so that it will work into holes. Ordinary roads are made by laying down a stratum of broken granite or other hard stone a few inches deep, each piece being of about half a pound weight, when this first layer is worked in by traffic, a second is laid, and so on till the surface has become quite firm and level, but in the streets of towns more care is required. In the suburbs of London, the usual plan is, to lay a deep stratum of rubbish, consisting of broken bricks, pottery and oyster-shells (obtained from the dust-bins of London) nearly a foot in thickness, and on this, broken stone or shingle (consisting of pebble stones) is laid in repeated layers as it consolidates, but in the streets having more traffic the roadway is made by a foundation of “concrete,” consisting of coarse gravel and lime made into a sort of paste, which sets into a hard cement in a day or two; on this foundation (which is carefully smoothed on the surface) is laid a paving of squared granite blocks, placed carefully side by side either in lines at right angles to the road or else diagonally. When these are all laid down and rammed to a perfectly even surface, a mixture of quick lime and sand is made into a thin paste with water, poured on and swept all over the surface with brooms, so as to fill up the interstices and cement the blocks of granite firmly together. In London the roadways are always made arched, so that the centre is a few inches higher than the sides, and a gutter or drain is placed on either side between it and the pavements which run on each side, but in many towns in the country, and on the continent, the gutter is placed in the middle of the road and each side made to slope slightly towards it.
BRIDGES.
The earliest efforts of the civil engineer were in all probability directed to the construction of roads, but it would be evident that unless they could be made to cross rivers and water-courses, they could continue but a short distance in any required direction. Bridges of some sort must therefore be constructed; small brooks were doubtless bridged over with timber laid across them, and “fords” or shallow parts of rivers taken advantage of when the stream was too wide to be spanned in this way. That these fords were once very numerous in England, is shown by the constant recurrence of such names as Stratford, Brentford, &c., and indeed fords are now in use in many places where the stream is shallow, and the traffic too little to pay for constructing a bridge. The next improvement in all probability consisted in placing stones in the fords to enable passengers to step from one to the next, and cross the stream dry-footed, which were called “stepping stones;” but the earliest bridges properly so called, were probably stones piled upon each other and united by beams of timber.
VIADUCT.