Form of Section. There is considerable difference of opinion amongst road builders as to the best form to give the surface of a road. Some use a section com posed of two planes of equal inclination rounded off in the middle and sloping uniformly to the sides as shown in Fig. 14. Others prefer to use a convex curve, approximately the arc of a circle, or more commonly, a parabolic curve, which is practically identical with the circular arc. The exact form is not a matter of importance on a country road, and either of them, or some intermediate form, may give good results in practice. It is not desirable to insist upon great accuracy in the form of section provided a proper crown be given and the surface be properly smoothed. Where a smooth pavement is used, however, it is desirable to place it accurately to a uniform section.
Gutters. At the side of the road longitudinal ditches must be provided for the purpose of carrying the water drained from the surface of the road to some point where it may be turned into a natural drainage channel. In many instances these side ditches also carry the drainage from land adjacent to the road. The size and form of the gutters will naturally depend upon the quantity of water to be carried and the slope of the gutters. In some instances the extension of the road surfaces, as shown in Fig. 13, will be sufficient and no special gutters will be required. In deep cuts where the excavation necessary to form side ditches would be expensive, a tile may be placed under each side of the road, as shown in Fig. 16, into which the drainage
from above the cut and from the small gutters may be carried.
Broad, shallow gutters are, in general, to be pre ferred to deep and narrow ones. The side slopes should not, in any case, be less than 2 horizontal to z vertical, and 4 or 5 to i, on the side next to the road way, is better. Shallow gutters are easier to form and keep clean, and are not so likely to wash out at times of heavy rainfall. It is not desirable to use deep side ditches for the purpose of under drainage, and water will not be drawn from the surface of a hollow roadway into such ditches. Fig. 17 shows a common form where it is intended to use the side ditches to prevent any seepage of water from the sides to the road-bed.
This is the standard section given for state aid roads in New York, using a ditch two feet deep with side slopes two to one. This form is also used by the Massachusetts Highway Commission, who recommend ditches .3 feet deep, I foot wide at bottom and with slopes 2 to i. Fig. i8 shows section recommended by the Illinois Highway Commission for use in level country, where the roadway is formed by material excavated from the side ditches.
On the average country road, surface drainage will be amply secured by gutters i8 inches to 2 feet below the crown of the roadway, and side ditches of greater depth are a source of unnecessary expense. Where under drainage is necessary it should be accomplished by tile or other covered drains.