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Grade and Cross Section

road, width, surface, roads, crown, country and water

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GRADE AND CROSS SECTION.

As already explained in Art. Io, the drainage of the surface of a road is accomplished by crowning the sur face and giving it a proper longitudinal slope. Under drains will not drain water from the surface of the road, and unless the crown is at all times maintained and the surface kept smooth, water is likely to stand upon the surface and soften it.

Grade. The surface of a country road should not be level in the direction of its length, but should have a sufficient longitudinal slope to drain any water from its surface which might otherwise be held by small ruts or depressions. The minimum grade for an earth road should be at least foot in too feet, and for a broken-stone road nearly as much. The grade, except in very rough country, should not exceed 4 or 5 feet per z00, and when steeper grades are necessary, they should be made as light as may be feasible. The effect of changing the rate of grade is discussed in Art. 19.

Width. Too great width of roadway upon country roads causes an unnecessary expense in the con struction and maintenance of the road, and the width should be only sufficient to provide space for the easy conduct of the traffic. For roads of ordinary traffic, this requires only that there be room for teams moving in opposite directions to freely pass. A width of 20 feet is ample for most country roads; and for roads of lighter traffic i6 feet is often sufficient. Outside of this width, side ditches must be formed for carrying the surface drainage.

In the construction of gravel or broken-stone roads, the paved portion of the road does not usually extend to the full width of the roadway, a shoulder of earth being left on each side, as shown in Fig. 13. The width of broken stone on ordinary country roads may vary from about 12 feet to 16 feet. A greater width than this need only be employed on important roads which convey large traffic, or on city streets.

While the improved portion of the road should be as small as is consistent with the proper discharge of the duty required of it, the available right of way need not be so restricted, but should be laid out wide enough to permit of the widening of the used portion when necessary, and allow room at the sides for pedes trians, with a grass border and line of trees. When

trees are planted along the roadway they should not be placed so as to form a dense shade over any portion of the traveled road, although a moderate shade is not a disadvantage, and care should be used that they are not near enough to a covered drain to permit the roots to grow into the drain and choke it.

Crown. The surface of a road must be crowned sufficiently to cause the water which falls upon it to run at once into the gutters. The height of crown required depends upon the character of the surface and upon the grade of the road. A high crown is objec tionable because it concentrates the travel in the middle of the road, which tends to wear hollows longitudinally along the road into which water may settle; but if the crown be too low, small depressions worn into the surface by the traffic may hold water and cause the road to become soft. The slope from the center to the side of an earth road should not be less than one in twenty nor greater than one in ten, corresponding to a height of crown from one-fortieth to one-twentieth of the width of the road. For roads upon which the surface can be kept in smooth condition and on moderate grades, the lower limit may be used, but on the average country road a steeper slope is desirable and a crown of one-twentieth the width is not too great to secure efficient drainage.

Gravel and macadam roads should be crowned somewhat less than earth roads. On wide macadam streets kept in smooth condition, the side slopes may be as low as one in thirty, or a crown equal to one sixtieth of the street width. On such work, however, it is more common and probably better to use a crown of one-fiftieth, or even one-fortieth the width; while on ordinary graveled or macadamized country roads the crown should be from one-thirtieth to one-twenty fourth of the width. When constant attention and careful maintenance can be relied upon to keep the surface in smooth condition a less crown may be used than would be allowable if the road is likely to be subjected to considerable wear between periods when repairs are made.

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