1. Field of the Invention
This invention relates to an arch-beam-culvert structure and in particular to an arch-beam structure for use in the construction of culverts or the like.
2. Discussion of the Prior Art
At present, metal culvert type structures are constructed using arcuate metal plates forming the basic structure, and sometimes including buttresses. Such structures are described in, i.e., Canadian Pat. Nos. 749,630; 804,292; and 862,492, which issued to C. L. Fisher on Jan. 3, 1967; Jan. 21, 1969; and Feb. 2, 1971, respectively.
In general, existing culvert type structures rely on passive earth resistance of the soil adjacent to the flexible metal structure or buttresses to strengthen and support the structure. Where shallow depth of cover is encountered, the weakest point of structures of this type is often in the area of the top arch of the structure and in the areas of the upper portions of the sides. The frictional resistance of the soil, and soil overburden confining pressure may be low in such areas because of the relative closeness to free ground surface, and the outward thrusting arching reaction forces may be large. Assuming that the flexible plates used in the structures are sufficiently strong, the weakness in such structures is in yielding or movement of the backfill material and/or in the adjoining natural ground. Failure surfaces may develop from an area of highly stressed backfill, usually where the radius of curvature of the structure is small, to a point on the surface of the fill or overburden along a failure plane or curve determined by a variety of parameters, including soil properties and the geometry of the structure. The flatter or more horizontally ellipsed the structure or the shallower the overburden, the greater the problem, since the side radii of such a structure are necessarily small and consequently the pressures tending to cause failure are larger, these pressures being related inversely to the radii.
Failure in the described area may result in a lack of sufficient support for the arching of the structure roof, thus allowing the roof to excessively deform or collapse.
However, there is a definite need for flatter structures and for structures with shallower cover or overburden because wide, low structures permit the flow of larger volumes of water at any given headwater elevation at the entrance to the structure. Thus, wide, low structures reduce the likelihood of flooding upstream of the structures; lower the overhead gradeline and overburden required; and, assuming that the structural strength problems are solved, at reasonable cost, lead to more economical construction. By the same token, a flatter structure often constitutes the best configuration for vehicle underpasses, utility conduits, pedestrian walkways, etc.
The solutions to the problems inherent with flat structures offered by the patents mentioned hereinbefore are in some instances unnecessarily complicated and in others are not of sufficient strength since they do not confine the soil adjacent to the principle reactions. Therefore, they do not provide for construction and operation under extremely shallow covers, as does the present invention.
The object of the present invention is to alleviate at least partially the problems encountered in the construction of culverts having a planar or arcuate top surface, i.e., metal culvert-type structures, by providing a relatively simple, strong arch-beam structure which provides substantial confining means adjacent to the edge or side of the structure.