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wall, or as much shall be cut from the upper side as shall bring the lower to a proper level. If this consists of loose soil, it must be compressed by means of water, or shall be left through a part of the winter to receive the snows and rains; but no soft, boggy, or peat substance is on any account to be laid behind the retaining walls. Where the cutting on the upper side consists of rock rubbish, gravel, or mountain clay, it will only require to be properly levelled as the work is carried on.”
The following specification has been successfully acted upon in forming a road over a peat bog in Ireland :
“ When the line of the road has been traced out to the exact width and line of direction, main drains are to be cut on each side eight feet wide at top, four feet deep, and eighteen inches wide at bottom; the peat dug out of these drains is to be spread over the surface of the roadway in form of a ridge, taking care to previously cover all the very soft and swampy places with dried peat, sods, or brushwood: numerous drains are to be cut across the roadway from the one main drain to the other ; they are to be three feet deep at the centre of the roadway, and four feet deep at the main drains : after the whole have remained in this state for two summer months, the bed for the roadway is to be neatly formed, with the sides on the same level, and with a convexity of half an inch in the yard.
“ The carriage-way is then to be covered with six inches of clay, laid on evenly, and firmly compressed by stampers or rollers; it is to have a fall of one inch in the yard from the centre towards the sides : over the clay is to be put four inches of small gravel ; it is to be frequently rolled, and, when solid and compressed, the foundation will be formed for the reception of the road materials.”
So much depends upon proper drainage, that too great attention cannot be given to this part of the business of road-making.
This operation should be carried on at the time of forming the road. When it is to be made over flat and wet land, open main drains should be cut on the field side of the fences: these drains should communicate with the natural watercourses of the adjacent land; their size should depend upon the nature of the country and upon local circumstances.
In general, these side drains should be cut at least three feet deep below the level of the bed of the road; they should be one foot wide at bottom, and five feet wide at top.
In crossing marshy land they should be made sufficiently deep and wide to obtain earth to raise the bed of the road, from side to side, three feet higher than the natural surface, in order to compress the subsoil and reduce its elasticity.
If, in consequence of the road running along the side of a hill, or passing through a cutting of a hill, or of the intervention of buildings or other obstructions, main open drains cannot be formed, it then becomes necessary to make covered drains
on each side of the road. These should be formed of stone or brick, and should be strongly and substantially built. If built with stone, a flat stone should be laid at the bottom of the drain, the side walls should be not less than twelve inches thick, and built in regular level courses, but without mortar; they should be eighteen inches high, and twelve inches apart. Particular care must be taken that the covering stones have a bearing of at least four inches on the side walls. They should have a layer of brushwood put over them; and the drain should then be filled up with gravel, or small stones. In gravel countries, or where stone is difficult to be procured, it will be necessary to build the main side drains of brick; the side walls should be four inches thick, three bricks high, and five inches apart, and covered with brick on the flat: these covering bricks should not be laid close together; an interval of at least half an inch should be left between each, to allow the water to enter the drain from above.
In some cases it will be necessary to build circular brick drains twelve or eighteen inches in diameter, according to circumstances; but they are expensive, and require inlets, built with brick, with iron grates. In consequence of its being necessary to build these drains with mortar, they are not so good as the open-jointed drain last described, unless there is a considerable run of water. Plate II. fig. 12.
If springs rise in the site of the road, or in the slopes of deep cuttings, stone or tile drains should be made into them, so as completely to carry away all the water.
In cuttings it is necessary to make drains of small dimensions from the centre of the road to the side drains. These drains should form an angle in the centre of the road, in the shape of a V, technically called mitre drains : the angle or splay of these drains should depend upon the inclination of the road ; it should not make the inclination of the drains exceed one inch in 100; for if it be greater, the run of the water will undermine the sides, and injure them. These mitre drains should be nine inches wide at bottom, twelve inches wide at top, and ten inches deep. These drains should be placed at about sixty yards from each other, or about thirty in the mile; but if the soil be wet, this number should be considerably increased. They are to be filled with rubble stone or cleansed gravel. If gravel is used, a draining tile should be laid along the bottom before the gravel is put on. In stiff and retentive clays these drains should be twelve inches wide at bottom, sixteen inches wide at top, and eighteen inches deep. In the bottom bricks should be laid on the flat, six inches apart; these should be covered by other bricks laid across, so as to form an open channel of six inches by four, and over these gravel or small stones should be laid. Where stones can be obtained, they will answer as well as bricks.
The upper part of these mitre drains should communicate with the road materials, so as to draw the water from them.