The drainage of the subsoil under a road is intended to lower the level of ground water in wet weather and prevent water from sub-surface sources reaching the road-bed.
The necessity for subdrainage, and the method to be employed in any case, depends upon whether the soil over which the road is being constructed is natu rally wet or dry, and whether the road-bed is so situ ated and formed as to give it natural drainage.
The material of which a road-bed is composed is important because it determines to a large extent whether artificial drainage is necessary, and also what method should be adopted for securing drainage.
Soils differ in their power to resist the percolation of water through them, in the rapidity and extent of their absorption of water with which they come in con tact, in the extent to which moisture renders them soft and unstable, and in their power of retaining moisture.
A light soil of a sandy nature usually presents little difficulty in the matter of drainage, as, while it is easily penetrated by water, it is not retentive of moisture, which passes freely through it without saturating it unless prevented from escaping.
If the natural drainage, therefore, have a fall away from a road-bed formed of such material, it will usually need no artificial drainage, and where subdrains are nec essary they may be relied upon to draw the water from the soil to a consideible distance each side of the drain.
A nearly pure sand is more firm and stable, under loads, when quite damp than if dry, although a fine sand saturated by water which is unable to escape may become unstable and treacherous.
Clays usually offer considerable resistance to the passing of water through them, and are very retentive of moisture. As a rule, however, a clay soil does not absorb water readily, and requires that water be held for some time in contact with it in order that it may become saturated, although when saturated it is the most unstable of soils. A clay that when dry will stand in a vertical wall and support a heavy weight, when wet may lose all coherence and become a fluid mass. When water comes in contact with a bed of
such clay, the outside becomes saturated and semi fluid before the moisture penetrates into it sufficiently to even moisten it a few inches from the surface.
A clay soil is, therefore, always difficult to drain by removing the water after it has soaked in, or by per mitting it to pass through the road-bed to the subdrains beneath. Drainage, in such cases, may often be so arranged as to prevent water from standing against the road and thus prevent it from becoming saturated. As the clay is comparatively non-absorptive, the water which may come upon its surface, if allowed to escape at once, will not penetrate into it, and hence will not cause softening.
A heavy silt formation is sometimes met with which is even more difficult to drain than a true clay. It is nearly as retentive of moisture as a clay, strongly resisting the passage of water through it, but at the same time absorbs water quite freely when in contact with it.
Between the extremes mentioned above there are a great number of varieties of soil which possess to a greater or less extent the characteristics of either or both, and gradually merge the one into the other. In applying a system of drainage in any case, careful attention should always be given to the characteristics of the soil, as determining very largely the treatment to be used.
Where artificial subdrainage is necessary the drains should be located, in so far as possible, with a view to cutting off the supply of water before it reaches the road-bed. To accomplish this to the best advantage the local conditions must be observed, the sources of this supply determined, and the nature of the under flow, if any exist, considered. In most instances on roads over soil commonly met upon country roads a single line of tile under one side of the road will lower the ground water sufficiently to prevent it reaching the road-bed.