DRAINAGE OF ROADS AND STREETS.
The ability of earth to sustain a load depends in a large measure upon the amount of moisture contained by it. Most earths form a good firm foundation so long as they are kept dry, but when wet they lose their sustaining power, becoming soft and incoherent. When softened`' by moisture the soil may be easily displaced by the settling of the foundation of the road, or forced upward into any interstices that may exist in its superstructure.
In cold climates the drainage of a road is also impor tant because of the danger of injury from freezing. Frost has no disturbing effect upon dry material, and hence is an element of danger only in a road that retains water.
In order, therefore, that the loads may be uniformly sustained, and the surface of the road kept firm and even, it is evidently of first importance that the road bed be maintained in a dry condition. The improve ment and maintenance of a road are therefore largely questions of drainage, the object being to prevent water from reaching the road and to provide means for immediately removing such as does reach it before the soil becomes saturated and softened.
Surface drainage is always necessary if the body of the road is to be kept In a dry condition, and is accom plished by having the surface of such form that water falling upon it will quickly run into the gutters.
The necessity for underdrainage in any case depends upon local conditions, the nature of the soil and the tendency of the site to dampness. Underdrains are for the purpose of lowering the level of ground water in wet weather and preventing water from underground sources reaching the 'road bed and softening it. A careful examination of local conditions is necessary in any case to determine the advisability of constructing underdrains. Where the soil upon which the road is constructed is so placed that the ground water is at any time likely to stand close to the surface and become soft immediately under the road-bed, underdrainage is necessary to good results in the maintenance of the road. In any case in which the level of ground water stands within about 3 feet of the surface, the road will be benefited by sub-surface drainage, although it may not be altogether necessary to the maintenance of the road. Underdrainage is of little use for the removal of water from depressions in the surface of the road.