Culverts are employed for conveying under a railroad, highway, or canal the small streams crossed. They may be of stone, brick, con crete, earthenware, or iron pipe or any of these in combination. Two general forms of masonry culverts are in use, the box and the arch.
Box Culverts. The box consists of vertical side walls of masonry with flagstones on top extending from one wall to another.
The foundation consists of large stones and the side walls may be laid dry or in mortar.
The paving should be laid independent of the walls and should be set in cement mortar. The end walls are finished either with a plain wall perpendicular to the axis of the culvert and may be stepped, or provided with wing walls as the circumstances of each case may require.
The thickness of the cover stone may be determined by con sidering it as a beam supported at the ends and loaded uniformly.
Figs. 36 to 39 show the form of this class of culverts and the dimensions given in Table 15 will serve as an approximate guide for general use.
Arch Culverts. The dimensions of arch culverts are deter mined in the manner described herein under arches, attention, how ever, being given to the following points: Wing Walls. There are three common ways of arranging the wing walls at the end of arch culverts: (1) The culvert is finished with straight walls at right angles to the axis of the culvert. (2) The wings are placed at an angle of 30 degrees with the axis of the culvert. (3) The wing walls are built parallel to the axis of the culvert, the back of the wing and the abutment being in a straight line and the only splay being derived from thinning the wings at their outer edge. The most economical and better form for hydraulic considerations is the second form.
Designing Culverts. In the design of culverts care is required to provide an ample way for the water to be passed. If the culvert is too small, it is liable to cause a washout, entailing interruption of traffic and cost of repairs, and possibly may cause accidents that will require the payment of large sums for damages. On the other hand, if the culvert is made unnecessarily large, the cost of construction is needlessly increased.
The area of waterway required, depends (1) upon the rate of rainfall; (2) the kind and condition of the soil; (3) the character and inclination of the surface; (4) the condition and inclination of the bed of the stream; (5) the shape of the area to be drained, and the position of the branches of the stream; (6) the form of the mouth and the inclination of the bed of the culvert; and (7) whether it is permissible to back the water up above the culvert, thereby causing it to discharge under a head.
(1) The maximum rainfall as shown by statistics is about one inch per hour (except during heavy storms), equal to 3,630 cubic feet per acre. Owing to various causes, not more than 50 to 75 per cent of this amount will reach the culvert within the same hour.
Inches of rainfall X 3,630 = cubic feet per acre.
Inches of rainfall X 2,323,200 = cubic feet per square mile.
(2) The amount of water to be drained off will depend upon the permeability of the surface of_the ground, which will vary greatly with the kind of soil, the degree of saturation, the condition of JO cultivation, the amount of vegetation, etc.
(3) The rapidity with which the water will reach the water course depends upon whether the surface is rough or smooth, steep or flat, barren or covered with vegetation, etc.
(4) The rapidity with which the water will reach the culvert depends upon whether there is a well-defined and unobstructed chan nel, or whether the water finds its way in a broad thin sheet. If the watercourse is unobstructed and has a considerable inclination, the water may arrive at the culvert nearly as rapidly as it falls; but if the channel is obstructed, the water may be much longer in passing the culvert than in falling.
(5) The area of the waterway depends upon the amount of the area to be drained; but in many cases the shape of this area and the position of the branches of the stream are of more importance than the amount of the territory. For example, if the area is long and narrow, the water from the lower portion may pass through the culvert before that from the upper end arrives; or, on the other hand, if the upper end of the area is steeper than the lower, the water from the former may arrive simultaneously with that from the latter. Again, if the lower part of the area is better supplied with branches than the upper portion, the water from the former will be carried past the culvert before the arrival of that from the latter; or, on the other hand, if the upper portion is better supplied with branch watercourses than the lower, the water from the whole area may arrive at the culvert at nearly the same time. In large areas the shape of the area and the position of the watercourses are very important considerations.