Gimbal Design of Culverts

GIMBAL DESIGN OF CULVERTS. Any culvert consists of two distinct parts, the trunk and the head-walls or wings at the ends of the trunk; and consequently the design of any culvert consists of (1) the arrangement of the head-walls or wings so as to protect the embankment and facilitate the flow of the water through the culvert, and (2) the proportioning of the cross section of the trunk or barrel of the culvert. The first is substantially the same for all forms of culverts.

Design of Ends.

There are three methods of finishing the ends of culverts—either box or arch. 1. The culvert is finished with a straight wall at right angles to the axis of the culvert (see Fig. 151). 2. The wings are placed at an angle of 30° with the axis of the culvert (see Fig. 152). 3. The wing walls are built parallel to the axis of the culvert, the back of the wing and the abutment be ing in a straight line and the only splay or flare being derived from thinning the wings at their outer ends (see Fig. 153). The first method is shown to a larger scale in Fig. 154, page 573, and Fig. 156, page 584; the second at the up-stream end of Fig. 157, page 585; and the third at the down-stream end of Fig. 157, and in Fig. 161, page 587.

The quantity of masonry required for these three forms of wings does not differ materially, Fig 153 requiring the least and Fig. 151 the most. The most economical angle for the wings of Fig. 152 is about 30° with the axis.

The position of the wings shown in Fig. 152 is by far the most common and is better than either of the others. Fig. 151 is objection able for hydraulic considerations, and also because it is more likely to become choked than either of the others. Fig. 153 does not have splay enough to admit the natural width of the stream at high water, and does not give sufficient protection to the toe of the embankment. However, if the culvert is ever to be extended to accommodate another track, the straight wing has a decided advantage.

The wings, both straight and splayed, are usually stepped or sloped to conform to the side slope of the embankment; but occa sionally the straight wings are built with a level top surface under the belief that with such wings the culvert is less likely to become clogged by drift than if the wings were sloped or stepped, since even though drift may partially or entirely stop the flow of the water between the ends of the wings the water may pour over the drift into the well thus formed and still find its way into the culvert. The need of this construction is usually not very great, nor is the benefit of it certain; but the extra cost is not very great, since it is a little cheaper to con struct a wing with a level top than one with a stepped or sloped upper surface.

Sometimes posts are set in the channel a little above the mouth of the culvert, to catch the drift, which accomplish somewhat the same purpose as building the wing wal:s up square.

Usually the two ends of the culvert are finished alike; but sometimes the up-stream end has wings as in Fig. 152, and the down-stream end wings as in Fig. 153, which is very good practice. For examples of this construction, see Fig. 157 (page 585), and Fig. 174 (page 598).

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CULVERTS. The simplest form of a culvert is a pipe of burned clay, cast iron or concrete. Owing to the undesirability of openings through an embankment, very small openings are not made, the water being conducted along the side of the roadway until it can be discharged into a proportionally large stream through the embankment. Therefore, the smaller sizes of drain and sewer tiles and cast-iron pipes are not used for culverts.

Pipe culverts are durable, and on account of the smoothness of their inner surface are hydraulically efficient. They are also com paratively cheap, and are readily put into place—particularly in an opening that has temporarily been lined with wood—without dis turbing the roadbed.