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. The upper part of these mitre drains should communicate with the road materials, so as to draw the water from them. According to the inclinations of a road, and the form and wetness of the country through which it passes, cross drains of good masonry should be built under the road, having their extremities carried under the road fences. One of these drains should be made wherever the water would lie on one side of the road, and can only be got rid of by carrying it to the other side. When the road passes along the slope of a hill or mountain, a great number of these drains are necessary to carry off the water that collects in the channel of the road on the side next the high ground. They should be placed at from 50 to 100 yards' distance from each other, according to the declivity of the hill; so that the side channels may not be cut by carrying water too far. In these situations inlets should be built of masonry, to carry the water from the side channel of the road into the cross drains. The manner of building an inlet will be described in the chapter on Road Masonry. Numerous outlets should also be made from the side channels of the road, under the footpaths, or wastes and fences, into the field ditches. In mountainous countries, where the road passes along the slopes of the hills, it is necessary to carry open or catchwater drains, branching from the upper ends of the cross drains, in an inclined direction, so as to catch the surface water before it can reach the road. After all these precautions have been taken, the preservation of the surface of the road from injury by water should be further secured, by giving to the surface of it a proper convexity in its cross section, and by making regular side channels. These side channels will be formed by the angle where the slope of the side parts of the surface of the road abuts against the edge of the footpath, or other defining bounds of the H roadway. They will be capable of carrying off a great quantity of water, without being made into the form of a square-sided drain. Attention to make the surface of a road of a proper convex form is particularly necessary on hills, in order that the water may have a tendency to fall from the centre to the sides, in place of running from the sides to the middle part of the road, which it certainly will do unless the side channels are kept below the centre of the road, in the manner hereafter described. On all hills the greatest care should, also, be taken to keep the side channels always open; for, if they are obstructed with dirt, the water will find its way over the middle of the road, and cut channels in it. The side channels of a road should be all thoroughly repaired as well as all the road drains before the approach of winter, and again after the winter is over; but, besides these repairs at fixed periods, daily attention should be given to take care that no obstruction gets into them. Whenever a branch or field road joins a main road, it should not be allowed to interfere with the side channel: in order to secure this object, the point of junction should always be on the field side of the side channel; unless this is the case, the branch or field road will, when on a higher level than the main road, carry its surface water upon the main road. In addition to all these means recommended to be adopted for securing the drainage of a road, it is of the utmost importance that evaporation should have full effect in drying up the surface of a road, by allowing the sun and wind to act upon it in the freest manner. The necessity of giving a road a good exposure has already been mentioned under the head of "Laying out a Road;" and the value of a rapid evaporation will be more fully explained when the repairing of roads is brought under consideration. If roads be kept dry, they will be maintained in a good state, with proportionally less expense. It has been well observed, that the statuary cannot saw his marble, nor the lapidary cut his jewels, without the assistance of the powder of the specific materials on which he is acting: this, when combined with water, produces sufficient attrition to accomplish his purpose. A similar effect is produced on roads, since the reduced particles of the materials, when wet, assist the wheels in rapidly grinding down the surface. |
