CULVERTS 235. The term culvert is usually applied to a small waterway which passes under an embankment of a railroad or a highway. The term is confined to waterways which are so small that standard plans are prepared which depend only on the assumed area of waterway that is required. Although the term is sometimes applied to arches having a span of 10 or 15 feet, or even more, the fact that the struc tures are built according to standard plans justifies the use of the term culvert as distinguished from a structure crossing some perennial stream where a special design for the location is made. The term culvert therefore includes the drainage openings which may be needed to drain the hollow on one side of an embankment, even though the culvert is normally dry.
239. Various Types of Culverts. Culverts are variously made of cast iron, wrought iron, and tile pipe, wood, stone blocks with large cover-plates of stone slabs, stone arches, and plain and reinforced concrete; still another variety is made by building two side walls of stone and making a cover-plate of old rails.
240. Culverts made of wood should be considered as temporary, on account of the inevitable decay of the wood in the course of a few years. When wood is used, the area of the opening should be made much larger than that actually required, so that a more permanent culvert of sufficient size may be constructed 'inside of the wooden culvert before it has decayed. For present purposes, the discussion of the subject of Culverts will be limited to those built of stone and concrete.
241. Stone Box Culverts. The choice of stone as a material for culverts should depend on the possibility of obtaining a good quality of building stone in the immediate neighborhood. Frequently temporary trestles are used when good stone is unobtainable, with the idea that after the railroad is completed, it will be possible to transport a suitable quality of building stone from a distance and build the culvert under the trestle. The engineer should avoid the mistake of using a poor quality of building stone for the construction of even a culve-4 simply because such a stone is readily obtainable.
Since a culvert always implies a stream of water which will have a scouring action during floods, it is essential that the side walls of culverts should have an ample foundation, which is sunk to such a depth that there is no danger that it will be undermined. There are
cases where a bed of quicksand has been encountered, and where the cost of excavating to a firmer soil would be very large. In such a case, it is generally possible to obtain a sufficient foundation by con structing a platform or grillage of timber which underlies the entire culvert, beneath the floor of the culvert. Of course, timber should not be used for the foundation, except in cases where it will always be underneath the level of the ground-water and will therefore always be wet. If the soil has a charAter such that it will be easily scoured, the floor of the culvert between the side walls should be paved with large pebbles, so as to protect it from scouring action. At both ends of the culvert, there should always be built a vertical wall which should run from the floor of the culvert down to a depth that will certainly be below any possible scouring influence, in order that the side walls and the flooring of the culvert cannot possibly be undermined.
The above specifications apply to all forms of stone culverts, and even to arch culverts, except that in the case of the larger arch cul verts the precautions in these respects should be correspondingly observed. When stone culverts are built with vertical side walls which are from 2 to 4 feet apart, they are sometimes capped with large flagstones covering the span between the walls. The thickness of the cover-stone is sometimes determined by an assumption as to the transverse strength of the stone, and by applying the ordinary theory of flexure. The application of this theory depends on the assumption that the neutral axis for a rectangular section is at the center of depth of the stone, and that the modulus of elasticity for tension and com pression is the same. Although these assumptions arc practically true for steel and even wood, they are far from being true for stone. It is therefore improper to apply the theory of flexure to stone slabs, except on the basis of moduli of rupture which have been experi mentally determined from specimens having substantially the same thickness as the thickness proposed. Also, on account of the varia bility of the actual strength of stones though nominally of the same quality, a very large factor of safety over the supposed ultimate strength of the stone should be used.