With regard to culverts, a settlement or slip of an embankment over a culvert may unequally strain, fracture, and displace portions of it, and therefore interrupt the flow of the drainage waters, which may then reach the seat of an embankment and cause it to be in a dangerous condition. They are usually necessarily placed at the deepest point of an embankment, and consequently the most difficult to make repairs. In such a situation they should be built sufficiently large to allow of the easy passage of a man, in order that due inspection may be made, and be constructed of materials of a durable character. Should a naturally firm bank exist on one or both sides of a stream, it should be stripped of all plant growth and decaying matter, and be preserved in order to form a natural wall to relieve a culvert from side pressure, but firm or hard material must be inserted between the back of the wall and the face of the stream bank so as to support the wall against the pressure it receives from the arch. This leads to a consideration of the best form of culvert. In clay soils, and those which exert pressure from expansion, especially if the culvert is surrounded by clay earth, the circular is generally considered to be the best form, and this has been proved to be so in tunnels in similar soil, with splayed wing-walls to assist and guide the flow and help to keep a clear entrance. In granular soils, such as dry sand or gravel, the earth acts differently and more in accordance with the angle of repose theory of pressure, therefore the strain upon the arch would be the greatest, and its thrust must be counterbalanced at the sides, the strain upon the invert being probably very little and due to the tendency to an overturning movement of either of the straight walls. However, in culverts as in tunnels, it is impossible to say the exact amount or direction of the strain, although it may be approximately computed.
Should it be necessary to erect a culvert upon soft ground, as much of it as practicable should be excavated and a concrete foundation be placed thereon; it is also advisable to allow an extra length to that required by the calculated slope.
A culvert should have an invert unless upon a hard rock bed, and care should be taken that there shall be no leakage at the springing of the inverted arch or under it, or beneath the sidewalls at the level of the surface of a flat stone drain or below it.
The banks of a stream or watercourse should be inspected occasionally, especially on the up-stream side of a culvert, in order to note whether they are stable or crumbling away, as then the course of the stream may be widened or diverted, and so erode the toe of an embankment and cause a slip of earth. Splayed wing-walls are a protection against such a danger besides aiding the flow of water through them, and they also lessen the chance of a damming back of the water and undermining, as they increase the discharge; for instance, a splay of 53° has been found to increase the flow about 25 per centum. The surface of the material of which a culvert is constructed should be as smooth as practicable, so as to reduce the friction of water flowing past it, which, in the case of unplaned timber, cast and wrought ironwork, ashlar masonry, brickwork, and concrete, is about the same, but is some 30 per cent. more when the surface consists of rubble masonry.
When an embankment crosses a narrow valley, in which no watercourse exists, instead of a culvert, a bed of loose stones has been placed upon the ground under the whole area of the seat of an embankment at its greatest depth, a proper fall and bed being given to it.
The toe of the slope on the up-stream side should be protected from any wash, and the stone layer be carried a few feet beyond the foot of the slope on the lower side.
The following general principles it is well to remember in designing culverts.
When a culvert is of uniform section, which is almost invariably the case, it should have the same inclination throughout.
Avoid, or ease, all bends as much as possible.
Have splayed wing-walls.