GENERAL. FORM of CULVERT. Splay of Wings. There are three common ways of disposing the wing walls at the end of the arch culvert, which are the same as discussed in Art. 2, Pipe Cul verts—see * 1126-29.
When culverts were made of stone-block masonry, the form shown in Fig. 166 was very common. Apparently the inner end of the wings was set back from the side of the waterway, so that the wing could be carried up outside of the arch ring without the latter inter fering with the bonding of the wing to the head wall (see Fig. 168) ; but the corners thus formed at a and b are very objectionable, since they reduce the capacity of the culvert and add to its cost. The angles at a and b, Fig. 166, materially decrease the amount of water which can enter under a given head and also the amount which can be discharged. It is a well-established fact in hydraulics that the discharging capacity of a pipe can be increased 200, or even 300, per cent simply by giving the inlet and outlet forms some what similar to Fig. 167. Although nothing like this increase can be obtained with a culvert, one finished at both the upper and the lower end like Fig. 167 will discharge considerably more water than one like Fig. 166. The capacity of Fig. 167 decreases as the angle between the wing and the axis increases; hence, the less splay the better, provided the outer ends of the wings are far enough apart to accommodate the natural width of the stream at high water. Also the less the splay, the less the probability of the culvert's being choked with drift. Fig. 166 is very bad for both the admission and the discharge of water, and also on account of the great liability that drift and rolling stones will catch in the angles between the wings and the end walls. With a culvert having a ground plan like Fig.
166, the wings sometimes had a vertical face and sometimes a bat tered face; and with the latter form of wings, the arrangement shown in Fig. 166 was sometimes slightly (but only slightly) improved by moving the wing forward as shown in Fig. 168.
Four methods have been employed to eliminate the corners a and b, Fig. 166, in an arch culvert with flared wings. 1. When culverts were built of coursed masonry, the face of the wing was built vertical at its intersection with the vertical face of the side wall and battered elsewhere; or in other words, the face of the wing below the springing line of the arch was warped. With rock
face masonry this would not be very objectionable; but with con crete it would be quite objectionable, since it would complicate the building of the forms. 2. Occasionally the wing was moved inward until the battered face intersected the face of the head wall at the springing line of the arch, and then the corner of the wing which would otherwise project into the waterway was rounded off to a gentle curve, as shown in Fig. 169. This solution is better for masonry than for concrete. 3. A solution somewhat similar to the last consists in placing the wing as in Fig. 169 and cutting off the portion that would project into the waterway by extending the plane of the inside face of the vertical side wall. The surface of the portion so cut away is shaded in Fig. 170. For an illustration of this method as applied in practice, see Fig. 184, page 604, in which illustration the portion of the wing that is cut away is shown by the shaded portion abed. This method complicates a trifle the construction of the concrete forms. 4. Not infrequently the face of the side wall is battered and intersects the battered face of the wing wall in a right line which passes through the springing line. For an example of this form of construction, see Fig. 173, page 597, and Fig. 175, page 599.
At present, concrete arch culverts are usually built of the general form shown in Fig. 167, except that either the face of the wing is vertical or the face of the wall at the. side of the waterway has the same batter as the face of the wing.
The segmental arch gave the greatest waterway for a given quantity of arch masonry; or for the same width of waterway re quired less arch masonry. Theoretically, the segmental arch should be the thicker; but the thickness of the arch ring of culverts is usually greatly in excess of that required for strength, and conse quently the relative thickness was not considered.
With concrete arches, the concrete in the arch costs no more per cubic unit than that in the remainder of the ture, and hence there is little or no difference between semicircular and mental arch culverts. However, it is sometimes claimed that the center for the semicircular arch is more easy for an ordinary penter to comprehend and construct than that for a segmental arch.