Wherever the substratum of a road is wet and soft, great care is necessary to make the bed Polish If the ground be boggy or marshy, it is desirable to form an embankment of sufficient thickness to com press the elastic foundation ; such embankments being sometimes supported by faggots. Telford and most engineers recommend that ramming with stone-chips should be resorted to where the bed is wet and spona ; and that where soft clay occurs, a stratum of earth or of sand should be laid between it and the road materials, a precaution which tends to diminish the injurious effect of frost on a road with a clay bottom. Though great care is usually considered necessary in order to obtain a firm foundation, McAdam and seine others have not only contended against its importance, but actually preferred, in cer tain eases, a yielding substratum to ono of rock, on the supposition that the wear of the road is diminished by elasticity. Careful observa tions on the repairs of a road in Somersetshire, of which about seven miles are supported by a morass, and five or six by limestone rock, indicated a difference of expense in repair of about five to seven in favour of the morass, though it was so soft that the vibration caused by a carriage passing was sufficient t3 break the young ice in the side ditches; but extended experience seems to confirm the more general opinion in favour of a hard unyielding foundation.
Deep ditches should be cut for the efficient drainage of the road, which is of paramount importance; and these should be on the field side of the fences. They should extend to a depth of from 2 feet 6 inches to 4 feet below the bed of the road, according to the nature of the ground. The earth thrown out from them is commonly used in forming banks for the hedges ; but in wet soils, where the ditches are made larger and deeper than usual, the additional earth excavated is applied to raising the bed of the road. Where brick or stone-covered drains are substituted for open channels, it is usual to build them with open joints, to allow the passage of water through the sides. Cross drains of masonry are introduced at intervals to connect the side channels, and numerous minor drains filled with rubble stones or cleau gravel are formed in the bed of the road. The latter are frequently of the kind called mitre-drains, which are made V shaped in plan, diverg ing from the centre of the bed, and extending diagonally to the sides, their angle regulated by the longitudinal slope of the road, so that their inclination may not exceed 1 in 100. These may be placed about sixty yards apart, or closer in wet soils, and they receive the water that filters through the surface materials. In cuttings or exca vations it is advisable to make drains to catch the water descending from the sides, and prevent its reaching the surface of the road.
In treating of the choice and application of the hard materials which compose the surface of the road, the formation of metalled roads, or those made of broken stone and similar materials, will be first considered; and afterwards that of the principal varieties of pavement.
Metalled or Broken-Stone R.ads.—In the formation of metalled roads the system adopted in the great works of Telford and his followers, is the one most generally adopted by English engineers. The distin guishing characteristic of this system is the use of a rough pavement of hand-laid stones on the bed of the road, to support the small broken stone of which the surface is composed. In the very imperfect mode of road-making formerly practised (which scarcely deserves the name of a system), it was very common to cover a bad road with a large quantity of stones, often unbroken, and generally of very irregular dimensions. These stones, owing to their rounded form and the soft ness of the substratum, never consolidated into a hard surface, and in course of time sunk into the soft earth beneath, which worked up among the stones in the form of mud. Thus enormous quantities of stone were used without producing a good road, the stone sinking into the earth to a surprising extent. This evil is greatly diminished by good drainage, and by the use of stones of uniform size broken into angular pieces, which have a tendency to lock together into a hard and compact mass,—a fact of the highest importance in the science of road-making, and which appears to have been first prominently brought forward by the late Mr. McAdam. A great extent of excellent road has been made on the plan advocated by McAdam, who considered paving unnecessary, and laid the broken stone immediately on the surface of the earth, depending on its forming a hard crust impervious to water, so that the earth, being always kept dry, may have no ten dency to work up among the metal or broken stone. McAdam used no stones exceeding six ounces in weight, and me the prefereuce to those of about one ounce, or an inch diameter, which he spread over the road in thin layers, each being worked over by carriages till in some degree consolidated ; and he objected to the use of chalk or earth mixed "svith the stoup for the purpose of binding it together. De considered a thickness of ten inches of broken stone, well consolidated, to be sufficient for bearing any load, even where the foundation is a morass, in which mac he considered no intermediate substance neces sary. Near Bristol, a road in which the metalling had worn down to a thickness of only four inches, was found to have kept the substratum of earth perfectly dry. But, satisfactory as this plan of road-making has proved in some cases, there are others in which it has failed, and some in which a very Large quantity of stone has been applied before a firm road could be obtained. A road from Lewes to Eastbourne, made on Mr. .MeAclasu's principle, is said to have required three feet of materials in many parts before it was consolidated, though it was ultimately brought into a good state.