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the blocks are so laid that the joints make an angle of 45° with the axis of the roadway, one set being continuous, the other breaking joints with them. By this arrangement of the joints, it is said that the wear upon the edges of the blocks, by which the upper surface soon assumes a convex shape, is diminished. It has been ascertained by experience that the wear upon the edges of the blocks is greatest at the joints which run transversely to the axis when the blocks are laid in the usual manner. From the experiments of M. Morin, to ascertain the influence of the shape of stone blocks on the force of traction, it was found that the resistance offered by a pavement of blocks averaging from five to six inches in breadth, measured in the direction of the axis of the roadway, and about nine inches in length, was less than in one of cubical blocks of the ordinary size.

Pavements in cities must be accompanied by sidewalks and crossing-places for foot-passengers. The sidewalks are made of large flat flagging-stone, at least two inches thick, laid on a form of clean gravel well rammed and settled. The width of the sidewalks will depend on the street being more or less frequented by a crowd. It would, in all cases, be well to have them at least twelve feet wide; they receive a slope, or pitch, of one inch to ten feet, towards the pavement, to convey the surface-water to the side channels. The pavement is separated from the sidewalk by a row of long slabs set on their edges, termed curb-stones, which confine both the flagging and paving stones. The curb-stones form the sides of the side channels, and should for this purpose project six inches above the outside paving stones, and be sunk at least four inches below their top surface; they should, moreover, be flush with the upper surface of the sidewalks, to allow the water to run over into the side channels, and to prevent accidents which might otherwise happen from their tripping persons passing in haste.

The crossings should be from four to six feet wide, and be slightly raised above the general surface of the pavement, to keep them free from mud.

724. Broken-stone Road-covering. The ordinary roadcovering for common roads, in use in this country and Europe, is formed of a coating of stone broken into small fragments, which is laid either upon the natural soil, or upon a paved bottoming of small irregular blocks of stone. In England these two systems have their respective partisans; the one claiming the superiority for road-coverings of stone broken into small fragments, a method brought into vogue

some years since by Mr. McAdam, from whom these roads have been termed macadamized; the other being the plan pursued by Mr. Telford in the great national roads constructed in Great Britain within about the same period.

The subject of road-making has within the last few years excited renewed interest and discussion among engineers in France; the conclusion, drawn from experience, there generally adopted is, that a covering alone of stone broken into small fragments is sufficient under the heaviest traffic and most frequented roads. Some of the French engineers recommend, in very yielding clayey soils, that either a paved bottoming after Telford's method be resorted to, or that the soil be well compressed at the surface before placing the road-covering.

The paved bottom road-covering on Telford's plan (Fig. 225), is formed by excavating the surface of the ground to a suitable depth, and preparing the form for the pavement with the precautions as for a common pavement. Blocks of stone of an irregular pyramidal shape are selected for the pavement, which, for a roadway 30 feet in width, should be seven inches thick for the centre of the road, and three inches thick at the sides. The base of each block should not measure more than five inches, and the top not less than four inches.

The blocks are set by the hand, with great care, as closely in contact at their bases as practicable; and blocks of a suitable size are selected to give the surface of the pavement a slightly convex shape from the centre outwards. The spaces between the blocks are filled with chippings of stone compactly set with a small hammer.

A layer of broken stone, four inches thick, is laid over this pavement, for a width of nine feet on each side of the centre; no fragment of this layer should measure over two and a half inches in any direction. A layer of broken stone of smaller dimensions, or of clean coarse gravel, is spread over the wings to the same depth as the centre layer.

The road-covering, thus prepared, is thrown open to vehicles until the upper layer has become perfectly compact; care having been taken to fill in the ruts with fresh stone, in order to obtain a uniform surface. A second layer, about two inches in depth, is then laid over the centre of the roadway; and the wings receive also a layer of new material laid on to a sufficient thickness to make the outside of the roadway nine inches lower than the centre, by giving a slight convexity to the surface from the centre outwards. A coating of

clean coarse gravel, one inch and a half thick, termed a binding, is spread over the surface, and the road-covering is then ready to be thrown open to travelling.

The stone used for the pavement may be of an inferior quality, in hardness and strength, to that placed at the surface, as it is but little exposed to the wear and tear occasioned by travelling. The surface-stone should be of the hardest kind that can be procured. The gravel binding is laid over the surface to facilitate the travelling, whilst the under stratum of stone is still loose; it is, however, hurtful, as, by working in between the broken stones, it prevents them from setting as compactly as they would otherwise do.

If the roadway cannot be paved the entire width, it should, at least, receive a pavement for the width of nine feet on each side of the centre. The wings, in this case, may be formed entirely of clean gravel, or of chippings of stone.

For roads which are not much travelled, like the ordinary cross roads of the country, the pavement will not demand so much care; but may be made of any stone at hand, broken into fragments of such dimensions that no stone shall weigh over four pounds. The surface-coating may be formed in the manner just described.

725. In forming a road-covering of broken stone alone, the bed for the covering is arranged in the same manner as for the paved bottoming: a layer of the stone, four inches in thickness, is carefully spread over the bed, and the road is thrown open to vehicles, care being taken to fill the ruts, and preserve the surface in a uniform state until the layer has become compact; successive layers are laid on and treated in the same manner as the first, until the covering has received a thickness of about twelve inches in the centre, with the ordinary convexity at the surface.

726. Gravel Roads. Where good gravel can be procured the road-covering may be made of this material, which should be well screened, and all pebbles found in it over two and a half inches in diameter, should be broken into fragments of not greater dimensions than these. A firm level form having been prepared, a layer of gravel, four inches in thickness, is laid on, and, when this has become compact from the travel, successive layers of about three inches in thickness are laid on and treated like the first, until the covering has received a thickness of sixteen inches in the centre and the ordinary convexity.

The Superintending Engineer of Central Park, of New York City, Mr. W. H. Grant, made experiments upon Telford,

McAdam, and gravel roads in the Park, and he came to the conclusion that the gravel roads, as there constructed, were better for the purposes of park roads than either of the others. (Journal of the Franklin Institute, 1867. Vol. 84, p. 233.)

The gravel roads which were constructed by him had a rubble, or broken-stone foundation, over which was passed a very heavy roller; and upon which was placed layers of gravel which were thoroughly rolled. In some cases screened gravel was used, and in others gravel directly from the bed. Paved foundations for receiving the gravel make the road much more durable, although the original cost is considerably increased thereby. Roads of this kind, which are constantly used, should be frequently repaired, and the additional layers of gravel should be thoroughly pressed with a heavy roller. For detailed information, see Journal of the Franklin Institute, 1867. Vol. 83, pp. 100, 153, 233, 297 and 391, and Vol. 84, pp. 233 and 311.

727. As has been already stated, the French civil engineers do not regard a paved bottoming as essential for broken-stone road-coverings, except in cases of a very heavy traffic, or where the substratum of the road is of a very yielding character. They also give less thickness to the road-covering than the English engineers of Telford's school deem necessary; allowing not more than six to eight inches to road-coverings for light traffic, and about ten inches only for the heaviest traffic.

If the soil upon which the road-covering is to be placed is not dry and firm, they compress it by rolling, which is done by passing over it several times an iron cylinder, about six feet in diameter, and four feet in length, the weight of which can be increased, by additional weights, from six thousand to about twenty thousand pounds. The road material is placed upon the bed, when well compressed and levelled, in layers of about four inches, each layer being compressed by passing the cylinder several times over it before a new one is laid on. If the operation of rolling is performed in dry weather, the layer of stone is watered, and some add a thin layer of clean sand, from four to eight tenths of an inch in thickness, over each layer before it is rolled, for the purpose of consolidating the surface of the layer, by filling the voids between the broken-stone fragments. After the surface has been well consolidated by rolling, the road is thrown open for travel, and all ruts and other displacement of the stone on the surface are carefully repaired, by adding fresh material, and levelling the ridges by ramming.

Great importance is attached by the French engineers to the use of the iron cylinder for compressing the materials of a new road, and to minute attention to daily repairs. It is stated that by the use of the cylinder the road is presented at once in a good travelling condition; the wear of the materials is less than by the old method of gradually consolidating them by the travel; the cost of repairs during the first year is diminished; it gives to the road-covering a more uniform thickness, and admits of its being thinner than in the usual method.

The iron roller is now moved by a locomotive, to which it is attached by a suitable gearing, that admits of reversing, so as to travel backward and forward over the road surface.

728. Asphaltic Roadways and Sidewalks. In preparing roadways with an asphaltic surface, the ground or subsoil is first made level crosswise, and very compact, by rolling it with a heavy cylinder. Upon this a bed of hydraulic concrete, consisting of one part in volume of hydraulic mortar, to two and a quarter parts in volume of gravel, is laid to the thickness of two and a half inches. This foundation is allowed to become perfectly hard and dry before the asphalt is laid over it.

The asphaltic rock reduced to powder by the ordinary process is uniformly spread over the concrete bed, the surface of which should be thoroughly dry before receiving the mastic, to the depth of two to two and a half inches. This will produce a layer of packed material varying from one and threequarters to two inches in thickness.

The packing is done with hot irons or pestles, worked by hand, and applied lightly, so as to produce a uniform smooth. surface. After the upper bed is compressed in this manner to a proper thickness, a thin coat of fine dry powder, the siftings of earth or of mineral coal ashes, is spread over the surface to fill up inequalities, and the surface is again smoothed over by a flat-iron, heated nearly to a red heat; and, whilst the asphalt is still hot, it is rolled with polished iron rollers, the lighter, weighing four hundred and forty pounds, being first applied, and then a heavier, weighing three thousand pounds.

In recommencing work on an unfinished portion, the part to which the fresh material is to be joined is first thoroughly cleansed from dust, and hot asphalt poured over it.

For sidewalks the asphaltic rock is reduced to a powder, either by crushing it under rollers or by roasting; this is then sifted through wire gauze, with meshes of one-tenth of

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