Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/607

Rh ROADS 583 iiick- fnr roads of importance near towns. The side spaces may be from 4 or 5 to 8 or 10 feet wide ; wide sides give the sun and air access to the road, and tend to keep it dry, and also afford space for the deposit of road materials and scrapings. In cuttings or on em- bankments the transverse section has of course to be modified. The road surface should have just enough convexity to throw the wet off freely, and a very moderate amount is sufficient when a good surface is maintained. On a too convex road the traffic keeps to the middle, and wears ruts which retain the water, so that the surface is not so dry as with a natter section which allows the traffic to distribute itself over the whole width. Telford used a cross section differing slightly from an arc of a circle in being more convex in the middle than at the sides. Walker recommended two straight lines joined in the middle of the road by a curve, and inclined about 1 in 24 towards the sides, the objection to which is that the flat sides are liable to wear hollow. An arc of a circle is often used, and is a good form, but on the whole a curve more convex at the centre than towards the sides is the best. The rise in the curve from the sides to the centre need not exceed one- fortieth of the width, and one -sixtieth is generally enough on well-kept roads, and if seven-eighths of the total rise are given at one-fourth the distance from the centre to the sides and five-eighths at half that distance a curve of suitable form will be obtained. It is generally best to obtain the requisite convexity by rounding the formation surface or seat of the road and giving a uniform thickness to the coating of stone. When there is not a kerb there should be a " shouldering" of sods and earth on each side to keep the road materials in place, and to form with the finished surface the water tables or side channels in which the surface drainage is collected, to be conveyed by outlets at frequent intervals to the side ditches. The outlets are open cuts through the sides or drains beneath the footpaths. The side ditches should be deep enough to thoroughly drain the foundation of the road, and cross or mitre drains under the road communicating with the side ditches may be required in wet soil. A thorough drainage of the subsoil is of the greatest importance, and it is economical in the end to go to considerable expense to secure it. In a cutting, or where there are no side ditches, the surface water may be taken off by gratings and under drains beneath the side channels. The thickness to be given to a road made altogether of broken drained soil a thickness of 6 inches will make an excellent road for ordinary traffic, and Macadam's opinion that 10 inches of well-con- solidated material was sufficient to carry the heaviest traffic on any substratum if properly drained has proved to be generally correct. In a new road the loss of thickness during consolidation must be allowed for, and the materials should be laid about one-half thicker than the coating is intended to be. When the materials are not rolled, a thickness of 3 to 6 inches should be laid first, and when that has partly consolidated under the traffic other coats may be added to make up the full thickness. There is great wear and waste of the materials in consolidating if they are laid too thickly at once. Inferior material is sometimes used in the lower part of the road coating, especially when the surface is to be of granite or other hard expensive stone. Thus flints or gravel may be used for the lower 5 or 6 inches of a road to be coated with 3 or 4 inches of granite, liudiug. Telford covered the broken stone of new roads with 1^ inches of gravel to act as a binding material. Macadam absolutely inter- dicted the use of any binding material, leaving the broken stone to work in and unite by its own angles under the traffic. An un- sound road may be made by the improper use of a binding material, but there is no doubt that broken stone consolidates more quickly, and without losing its angular form, when binding is moderately used. About one half the volume of broken stone is void space, and, as the results of examinations into the composition of the coat- ings of roads when thoroughly consolidated prove that a very large proportion must necessarily be small stones and detritus, it is much better to give some portion of this at first rather than to obtain it by the crushing and grinding of the materials by the traffic. The binding material fine gravel, saud, or road scrapings should be spread over the surface after the broken stone is laid and not be mixed with it. Uniform consolidation is much aided by raking. Whenever it is possible a new road should be finished with a roller. The materials are consolidated with less waste, and wear and tear of vehicles and horses is saved. Horse-rollers, if heavy enough to be efficient, require a number of horses to draw them and are cumbersome to use. A ton or a ton and a half weight per foot of width is desirable, and to obtain it a roller 4 feet wide must be loaded to 5 or 6 tons, and will require as many horses to draw it. In Great Britain horse-rollers have to a great extent been super- seded by steam road rollers in consequence of the superiority and economy in the work done. A 15-ton roller, 7 feet wide, giving upwards of 2 tons weight per foot, can thoroughly consolidate 1000 to 2000 square yards of newly-laid materials per day. The materials should be formed to the proper section, and not more than 4 or 5 inches in thickness ; if a greater thickness is required tolling. it is better to roll two coats separately. After several passages of the roller any hollows must be filled up with small materials, and the rolling must be continued until it causes no motion among the stones. When -this result has been attained the binding material may be added. It should be spread dry and uniformly in moderate quantities and should be rolled into the interstices with the aid of watering and sweeping. Provided that all the interstices in the upper stratum of stones are filled after the stones are thoroughly consolidated, the less binding that is used the better. By using binding in larger quantity, and before the stone is thoroughly con- solidated, the amount of rolling required is lessened, but at the expense of durability in the road. Watering is necessary from the commencement of the rolling unless the weather is wet, but excessive watering, especially in the earlier stages, tends to soften the foundation. A pitched foundation like that used by Telford is always desir- Founda- able for a road that is siibject to heavy traffic. It consists of flat tion. stones carefully set on edge in courses across the road with the broadest edges downwards. The upper edges should not exceed 4 inches in breadth, to hold the broken stone well. All inequalities must be knocked off, and small stones and chips must be firmly pinned into the interstices with a hammer, so as to form a regular convex surface, with every stone firmly fixed in place. The thick- ness of the pitching is generally 6 or 7 inches ; it should not be less than 4, and it may generally be thicker without any sensible increase of cost. At least 4 inches of broken stone are required over the pitched foundation, and, when consolidated, 6 are always suffi- cient. A foundation of cement concrete 6 inches thick was used by Sir J. Macneill on the Highgate Archway (London) road on a bad clay bottom, and common lime concrete was subsequently used elsewhere. A bed of lias lime concrete 12 inches thick was laid as a foundation in Southwark Street and on the Thames Embankment, but it is too expensive for a macadamized road under ordinary cir- cumstances. Burned clay, gravel, or even sand may be usefully employed as a foundation oil a clay bottom, to cut off the road material from the clay. The qualities required in a good road stone are hardness, tough- Material ness, and ability to resist the action of the weather, and these are not always found together in the same stone. Limestones possess another quality, that of furnishing a mortar-like detritus which binds the stone together, and enables it to wear better than a harder material that does not bind. For heavy traffic the best materials are traps, basalts, greenstones, and syenite ; quartzose grits and cherty sandstones are also excellent materials. For moderate traffic the harder limestones are sufficiently durable and make the smoothest and pleasantest roads. Coefficients of quality for various road materials have been obtained by the engineers of the French " Administration des Fonts et Chaussees." The quality was assumed to be in inverse proportion to the quantity consumed on a length of road with the same traffic, and measurements of traffic and wear were systematically made to arrive at correct results. These processes requiring great care and considerable time, direct experiments on resistance to crushing and to rubbing and collision have also been made on 673 samples of road materials of all kinds. The coefficients obtained by these experiments, which were found to agree fairly well with those arrived at by actual wear in the roads, are summarized in the following table. The coefficient 20 is equiva- lent to "excellent," 10 to "sufficiently good," and 5 to "bad." Materials. Coefficient of Wear. Coefficient of Crushing. Basalt ... 12-5 t 14-1 10-3 7-3 11-6 14-5 13-8 14-3 12-9 9'8 3-5 6-6 024-2 22-9 19 18 12-7 15-3 30 26-2 17-8 21-3 16-8 15-7 12-1 t 8-3 13-4 7-7 12-4 7-2 12-3 9-9 12-3 14-2 17-8 6-5 o!6 16-3 14-8 15-8 13 11-1 21-6 16-6 13-2 17-6 25-5 13-5 Porphyry Granite Slag Quartzite Quartzose sandstone Silex Chalk flints Limestone Stone for a new road should be evenly broken to a size that will pass every way through a ring 1 inches in diameter. For repairs, especially when the material is tough, a gauge of 2J or 2 inches may be used with advantage, as the stone covers a larger surface, consolidates sooner, and makes a smoother surface. Stone is best broken by hand, but stone-breaking machines have been introduced which supersede hand-breaking to some extent, especially where large quantities of hard stone are to be broken. There is always a certain amount of crushing in breaking by a machine, from which softer stones suffer more, and machine-broken stone is never nearly so cubical, uniform in size, or durable as stone well broken by hand. Broken road material contains about 55 per cent, of solid stone to 45 of void space. In a well-consolidated road the void is filled up by small fragments, detritus, and mud, the result of wear, and specimens of good road surfaces weigh from 93 to 95 per cent, of the weight of the solid stone of which they are made. In the
 * ss. stone will depend on the traffic it is intended for. On a good well-