Page images
PDF
EPUB
[ocr errors]

rubble, or brick masonry, which support a top covering of flat stones, or of brick, with open joints, of about half an inch, to give a free passage-way to the water into the drain. The top is covered with a layer of straw or brushwood; and clean gravel, or broken stone, in small fragments, is laid over this, for the purpose of allowing the water to filter freely through to the drain, without carrying with it any earth or sediment, which might in time accumulate and choke it. The width and height of covered drains will depend on the materials of which they are built, and the quantity of water to which they yield a passage.

[graphic][merged small][merged small][merged small]

Besides the longitudinal covered drains in cuttings, other drains are made under the roadway which, from their form, are termed cross mitre drains. Their plan is in shape like the letter V, the angular point being at the centre of the road, and pointing in the direction of its ascent. The angle should be so regulated that the bottom of the drain shall not have a greater slope along either of its branches, than one perpendicular to one hundred base, to preserve the masonry from damage by the current. The construction of mitre drains is the same as the covered longitudinal drains. They should be placed at intervals of about 60 yards from each other.

In some cases surface drains, termed catch-water drains, are made on the side slopes of cuttings. They are run up obliquely along the surface, and empty directly into the cross drains which convey the water into the natural water-courses.

When the roadway is in side-forming, cross drains of the ordinary form of culverts are made to convey the water from the side channels and the covered drains into the natural water-courses. They should be of sufficient dimensions to convey off a large volume of water, and to admit a man to pass through them so that they may be readily cleared out, or even repaired, without breaking up the roadway over them.

The only drains required for embankments are the ordi

nary side channels of the roadway, with occasional culverts to convey the water from them into the natural water-courses. Great care should be taken to prevent the surface-water from running down the side slopes, as they would soon be washed into gullies by it.

Very wet and marshy soils require to be thoroughly drained before the roadway can be made with safety. The best system that can be followed in such cases is to cut a wide and deep open main-drain on each side of the road, to convey the water to the natural water-courses. Covered cross

drains should be made at frequent intervals, to drain the soil under the roadway. They should be sunk as low as will admit of the water running from them into the main drains, by giving a slight slope to the bottom each way from the centre of the road to facilitate its flow.

Independently of the drainage for marshy soils, they will require, when the subsoil is of a spongy, elastic nature, an artificial bed for the road covering. This bed may, in some cases, be formed by simply removing the upper stratum to a depth of several feet, and supplying its place with wellpacked gravel, or any soil of a firm character. In other cases, when the subsoil yields readily to the ordinary pressure that the road-surface must bear, a bed of brushwood, from 9 to 18 inches in thickness, must be formed to receive the soil on which the road-covering is to rest. The brushwood should be carefully selected from the long straight slender shoots of the branches or undergrowth, and be tied up in bundles, termed fascines, from 9 to 12 inches in diameter, and from 10 to 20 feet long. The fascines are laid in alternate layers crosswise and lengthwise, and the layers are either connected by pickets, or else the withes, with which the fascines are bound, are cut to allow the brushwood to form a uniform and compact bed.

This method of securing a good bed for structures on a weak wet soil has been long practised in Holland, and experience has fully tested its excellence.

722. Road-coverings. The object of a road-covering being to diminish the resistances arising from collision and friction, and thereby to reduce the force of traction to the least practicable amount, it should be composed of hard and durable materials, laid on a firm foundation, and present a uniform, even surface.

The material in ordinary use for road-coverings is stone, either in the shape of blocks of a regular form, or of large round pebbles, termed a pavement, or broken into small angular masses; or in the form of gravel.

[ocr errors]

723. Pavements. The pavements in most general use in our country are constructed of rounded pebbles, known as paving stones, varying from 3 to 8 inches in diameter, which are set in a form, or bed of clean sand or gravel, a foot or two in thickness, which is laid upon the natural soil excavated to receive the form. The largest stones are placed in the centre of the roadway. The stones are carefully set in the form, in close contact with each other, and are then firmly settled by a heavy rammer until their tops are even with the general surface of the roadway, which should be of a slightly convex shape, having a slope of about from the centre each way to the sides. After the stones are driven, the roadsurface is covered with a layer of clean sand, or fine gravel, two or three inches in thickness, which is gradually worked in between the stones by the combined action of the travel over the pavement and of the weather.

The defects of pebble pavements are obvious, and confirmed by experience. The form of sand or gravel, as usually made, is not sufficiently firm; it should be made in separate layers of about 4 inches, each layer being moistened and well settled either by ramming, or passing a heavy roller over it. Upon the form prepared in this way a layer of loose material of two or three inches in thickness may be placed to receive the ends of the paving stones. From the form of the pebbles, the resistance to traction arising from collision and friction is very great.

Pavements termed stone tramways have been tried in some of the cities of Europe, both for light and heavy traffic. They are formed by laying two lines of long stone blocks for the wheels to run on, with a pavement of pebble for the horsetrack between, the wheel-tracks. In crowded thoroughfares tramways offer but few if any advantages, as it is impracticable to confine the vehicles to them, and when exposed to heavy traffic they wear into ruts. The stone blocks should be carefully laid on a very firm bottoming, and particular attention is requisite to prevent ruts from forming between the blocks and the pebble pavement.

Stone suitable for pavements should be hard and tough, and not wear smooth under the action to which it is exposed. Some varieties of granite have been found in England to furnish the best paving blocks. In France, a very fine-grained compact gray sandstone of a bluish cast is mostly in use for the same purpose, but it wears quite smooth.

The sand used for forms should be clean and free from pebbles and gravel of a larger grain than about two-tenths of an

inch. The form should be made by moistening the sand, and compressing it in layers of about four inches in thickness, either by ramming, or by passing over each layer several times a heavy iron roller. Upon the top layer about an inch of loose sand may be spread to receive the blocks; the joints between which, after they are placed, should be carefully filled with sand.

The sand form, when carefully made, presents a very firm and stable foundation for the pavement.

Wooden pavements, formed of blocks of wood of various shapes, have been tried in England and several of our cities within the last few years, and notwithstanding they decay in a few years, yet they are extensively used in many of our large cities. The travel upon them is so free from noise, and the surface is so smooth, that, on those streets where the haulage of heavy articles is not excessive, many property holders prefer to renew a wooden pavement every eight or ten years, than be annoyed with the noise and the roughness of stone pavements. They are especially desirable upon those streets which are occupied by residences.

Asphaltic pavements have undergone a like trial, and have been found to fail after a few years' service. This material is farther objectionable as a pavement in cities where the pavements and sidewalks have frequently to be disturbed for the purposes of repairing, or laying down sewers, waterpipes, and other necessary conveniences for a city.

The best system of pavement is that which has been partially put in practice in some of the commercial cities of England, the idea of which seems to have been taken from the excellent military roads of the Romans, vestiges of which remain at the present day in a good state.

[blocks in formation]

In constructing this pavement, a bed (Fig. 226) is first prepared, by removing the surface of the soil to the depth of a foot or more, to obtain a firm stratum; the surface of this bed receives a very slight convexity, of about two inches to ten feet, from the centre to the sides of the roadway. If the soil is of a soft clayey nature, into which small fragments of

broken stone would be easily worked by the wheels of vehicles, it should be excavated a foot or two deeper to receive a form of sand, or of clean fine gravel. On the surface of the bed thus prepared, a layer of small broken stone, four inches thick, is laid; the dimensions of these fragments should not be greater than two and a half inches in any direction; the road is then opened to vehicles until this first layer becomes perfectly compact; care being taken to fill up any ruts with fresh stone, in order to obtain a uniform surface. A second layer of stone, of the same thickness as the first, is then laid on, and treated in the same manner; and finally a third layer. When the third layer has become perfectly compact, and is of a uniform surface, a layer of fine clean gravel, two and a half inches thick, is spread evenly over it to receive the paving stones. The blocks of stone are of a square shape, and of different sizes, according to the nature of the travelling over the pavement. The largest size are ten inches thick, nine inches broad, and twelve inches long; the smallest are six inches thick, five inches broad, and ten inches long. Each block is carefully settled in the form, by means of a heavy beetle; it is then removed in order to cover the side of the one against which it is to rest with hydraulic mortar; this being done, the block is replaced, and properly adjusted. The blocks of the different courses across the roadway should break joints. The surface of the road is convex; the convexity being determined by making the outer edges six inches lower than the middle, for a width of thirty feet.

This system of pavement fulfils in the best manner all the requisites of a good road-covering, presenting a hard even surface to the action of the wheels, and reposing on a firm bed formed by the broken-stone bottoming. The mortarjoints, so long as they remain tight, will effectually prevent the penetration of water beneath the pavement; but it is probable, from the effect of the transit of heavily-laden vehicles, and from the expansion and contraction of the stone, which in our climate is found to be very considerable, that the mortar would soon be crushed and washed out.

In France, and in many of the large cities of the continent, the pavements are made with blocks of rough stone of a cubical form measuring between eight and nine inches along the edge of the cube. These are laid on a form of sand of only a few inches thick when the soil beneath is firm; but in bad soils the thickness is increased to from six to twelve inches. The transversal joints are usually continuous, and those in the direction of the axis of the road break joints. In some cases

« PreviousContinue »