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CONDUITS Types of Conduits.—Conduits for carrying water may he de signed either for gravity flow or for internal pressure. Brick masonry was formerly largely used in the construction of gravity conduits, particularly for larger sewers, but is now being replaced for the most part by the use of concrete. For conduits to carry water under pressure, reinforced concrete or steel pipe is usually employed.

A conduit consists essentially of two parts, the invert, which forms the channel for the water, and the top, usually arched, which covers the channel and carries the weight of earth or other loads which may come upon it. The shape of the invert depends upon the require ments of the service. In sewers, special forms of invert are frequently needed to prevent deposits at times of minimum flow. The designs of sections for various uses may be found in works upon water supply, irrigation, and sewerage.

Sewage may sometimes cause disintegration of concrete, and the inverts of conduits intended to carry sewage are therefore commonly lined with vitrified brick—a method particularly desirable where the sewage is stale or impregnated with chemicals from manufacturing plants. In conduits carrying water for irrigation, injury to concrete may result from alkalis in the soil unless special precautions are taken.

The inverts of carefully constructed concrete conduits usually resist the abrasion of flowing water fully as well as those with brick or stone lining—such resistance depending upon the alignment of the conduit and the amount of sediment carried by the water. With clear water and an undisturbed flow, very high velocities may pro duce no appreciable damage, while the impact caused by changes in the direction of flow cause rapid wear, particularly when sand and gravel are carried by the stream.

No conduit is absolutely water-tight, and careful attention should always be given to reducing leakages to a minimum. Usually the most serious leakage occurs at joints where one section joins another, although there will generally be some porous spots through which small quantities of water may pass. The leakage may commonly be reduced to very small proportions by careful design and construe tion, reinforcing so as to prevent cracks and using dense and uniform mixtures of concrete. This subject is discussed in Art. 2:3.

Conduits of small size are sometimes made rectangular in section and designed in the same manner as rectangular culverts. Larger conduits are usually of curved form with arched tops.

183. Design of Gravity determining the size and general shape of conduit required for a given service, the design depends upon the character of the soil upon which it is to be placed and the external loads that it must carry. When the invert rests upon a firm foundation, capable of supporting the structure without sen sible yielding, the invert may be considered as fixed in position and the arch may he designed with ends fixed upon the sides of the invert. The design of such arches may be made by the ordinary method used for arch bridges or culverts. Actual loads, in so far as they can be determined, should be used in such designs. Where the loads are light, such conduits may often he built of plain concrete; usually, however, it is preferable to reinforce arches of more than 4 or 5 feet span. Fig. 102 shows typical fortes of standard sewer conduits.

The horizontal earth pressure to which the side of a conduit may he exposed cannot be accurately determined. It is customary to use Rankine's minimum value, in which w is the unit vertical pressure and ˘ is the angle of friction of the earth. Taking ˘=30° for ordinary earth, this makes the unit horizontal pressure at any point equal to one-third of the unit vertical pressure at the same point. In some instances it may be necessary to consider the possible effect of variations in horizontal pressures.

As the tendency of such a structure under vertical loading is to deflect outward upon the sides, it is reasonable to assume that at least this minimum horizontal pressure may always be depended upon, or a greater passive pressure if needed. In case of soft, wet earth, the horizontal pressure will be much greater, reaching a maximum when it is practically fluid and exerts normal pressure at all points.

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