CONSTRUCTION - THE GRAVEL ROAD The subgrade for a gravel road should be prepared in sub stantially the same manner as an earth road (see Art. 1, Chapter III). Indeed a first-class earth road is the best foundation for a gravel road.
The tile should be placed under the side ditches—as de scribed for earth roads (§ 109). Some writers recommend that a tile be laid under the middle of the gravel or broken stone, with the earth sloping both ways to the tile. There are several objec tions to this construction: (1) sloping the earth is not of much advantage, and (2) it needlessly increases the depth of the gravel or broken stone; and (3) if the road is otherwise well made, the surface should be practically impervious to water. See § 109.
Some writers advocate a tile each side of the graveled portion, with short lines of tile running each way from the center of the roadway obliquely to the side tile, these "miter drains" to be placed 15 feet apart in wet places. Clearly this construction is based upon a misapprehension of the source of the water reaching a drain tile. The water that enters a tile comes from below and not directly down from above. It is abundantly proven that in an earth road needing underdrainage, little or no water penetrates the surface; and with good gravel or broken stone roads there will be still less. Therefore " miter underdrains " below the graveled portion of the roadway are absolutely worthless, and tiles at the edges of the hardened way are no better than tiles under the side ditches.
Crown. The same general principles concerning the crown apply in gravel roads as in earth roads—see § 115-16. The slope of the gravel surface from the center to the side should be at least one quarter of an inch per foot, and it should not be more than three quarters of an inch per foot. The first is about right for park drives, which have light traffic and are well cared for; if the drive is narrow, the crown may be a little greater than this; but if it is broad, the crown should be less, to prevent the surface from being gullied out near the gutters by the water running from the center to the sides. The maximum crown, as above, would be about right for a country gravel road with heavy traffic, or for a street. If the gravel contains an excess of clay, the crown should be greater than the above maximum, as the surface will be liable to rut up.
Frequently gravel roads have an excessive crown, which forces traffic to use a narrow strip in the center—see § 114. This results from the fact that the gravel is placed thicker at the center than at the edges, upon an earth road which already has some crown; and thus the surface of the gravel is given a greater crown than the original earth road, while a gravel road should have a less crown than an earth one.
For a discussion of the mathematical form of the transverse profile, see § :309-12.