The discharging capacity of a culvert will depend upon the slope of the water surface passing through the culvert. Increasing the slope of the .bed of the culvert will increase its carrying capacity, provided the water can flow freely away below the culvert. If a culvert be so constructed as to permit the water to dam up above it, causing the culvert to flow full and under pressure, the effect is the same as increasing the slope and increases the capacity of the culvert. The velocity through a culvert is approximately propor tional to the square root of the head of water, the head being the difference of elevation of the water surface at the entrance to the culvert and that where the water leaves the culvert.
There are three types of culverts in common use for road purposes: stone box culverts, pipe culverts, and concrete culverts. In some localities wooden box cul verts are also used for this purpose; these are very un economical on account of their perishable nature 'and their use should be abandoned.
Stone Culverts. Culverts of stone may be either arch culverts or box culverts. Box culverts are usually formed of two side walls and a cover. The side walls consist usually of rubble stonework laid up dry or in mortar, as the case may be. Where the stream to be carried is of small importance, and the capacity of the culvert not greatly taxed, dry walls may give satis factory results, but when the culvert is likely to flow full at certain times it should be laid up in hydraulic cement mortar, and in any case the greater stability given by the mortar would be well worth the small additional cost. Fig. 8 shows a section of the ordi nary form of box culvert. The use of head walls and paving the waterway for a short distance is necessary for these, as for pipe culverts.
Where suitable stone is available, box culverts are easily constructed and economical. They are corn monly used for openings 2 to 4 feet in width and 2 to 5 feet in height. The width that may be used depends upon the available cover stones. Where the allowable width is not sufficient to give the needed area of waterway, a double culvert may sometimes be used to advantage. This consists of two openings with a middle wall to support the covers.
The culvert's opening should always be large enough to admit of a man passing through it for the purpose of cleaning it — at least i8 by 24 inches. The side walls should extend downward below the bottom of the culvert sufficiently to obtain a good foundation, and the thickness required for the side walls usually varies from one-half to three-fourths the height, depending upon the pressure likely to come against them.
In many cases for small work the side walls, instead of extending downward, rest upon the paving which is extended under them. This gives a somewhat less expensive construction, and is often satisfactory on good ground.
The cover stones may be from to the span in thickness, 'and should be long enough to have a bear ing upon each side wall of at least one-half the thick ness of the wall.
Pipe Culverts. Pipe culverts may be constructed either of salt-glazed vitrified sewer pipe, or of iron water pipe. For culverts of sizes up to about 30 inches diameter, vitrified pipe is often the most economical material to use provided it is placed on good founda tion and sufficiently covered not to be subject to shock from the traffic. The iron pipe possesses greater
strength, and is preferable where a firm foundation is not easily obtained, or where a sufficient covering can not be had for the vitrified pipe, as it is not so easily broken by a slight settlement or by shocks. It is somewhat expensive and not economical for ordinary use.
In laying pipe culverts, they should be placed on a solid bed, and the earth be well tamped about them. It is desirable to have the bottom of the trench exca vated to fit the lower part of the pipe, depressions being formed for the sockets. It is necessary in every case that the pipe be firmly and uniformly supported from below, in order that the culvert may not be broken by settlement, which is especially likely to occur in new work.
The joints in the pipe should be made water-tight, especially where the culvert is likely to flow full or under pressure, as any water escaping through the joints will tend to cause a wash beneath the pipe and undermine the culvert. Joints are commonly filled with clay, but where strength is needed the use of hydraulic cement mortar is preferable. The cost of filling joints is small and adds much to the security of the culvert.
Care should be taken that the culvert have sufficient slope and be so placed that water may not stand in it, in order to prevent injury from freezing. When this is not feasible, iron pipe should be used. The top of the culvert pipe should be at least 18 inches below the road surface to avoid crushing, and for the larger sizes of pipe (24 to 36 inches), at least two feet.
The ends of pipe culverts should be set in masonry walls to give protection against the washing of the face of the embankment, hold the ends firmly in place, and prevent the entrance of water into the earth on the outside of the pipe.
These walls to give efficient protection must be of substantial construction, going down to a solid founda tion below the bed of the stream. They may be built of rubble masonry, and should be laid up in hydraulic cement mortar. Such construction is represented in Fig. 9. The wall must extend far enough on the side to sustain the earth of the embankment from the waterway, or wing walls may be used extending up stream for this purpose. The waterway should be paved above the culvert far enough to prevent scour ing at the base of the wall.
For quite small streams the walls may sometimes be omitted if the face of the embankment about the entrance to the pipe and the waterway for some dis tance above and below be riprapped. Where it is necessary to economize in the cost of construction, this method is preferable to the use of very light end walls.
On streams too large for a single pipe it is often economical to lay two or three pipes side by side, rather than to construct an arch or the open way of a bridge. In laying large pipes it is usually advisable to place -a broken-stone or concrete foundation under the pipes throughout their lengths to insure uniform support.