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I I I I EM 111....~2 I Mar 1969 I I I modification of alignment and gradient. It is usually more practical to construct the culvert at right angles to the roadway. However, the cost of any change in stream channel location re- quired to accomplish this should be balanced against the cost of a skewed alignment of the cul- vert and consideration should be given to hYdrological requirements in each case. (2) Grade and camber. If conditions permit, the gradient of the culvert invert should be held to approximately that of the stream bed to minimize erosion and silting. Where consider- able foundation settlement is likely to oc<:ur, camber should be employed to lI8llure posi. tive drainage. (3) Entrance a.nd outlet conditions. It is often necessary to enlarge the natural or reo located channel for a considerable distance downstream to prevent water from backing up into the culvert, and enlargement of the culvert entrance is required in many cases to prevent pond- ing above the culvert. The entrance and outlet conditions of the structure itself have an impor- tant bearing on the carrying capacity. Rounding or beveling entrance corners will materially increase carrying capacity especial. ly for short culverts of small cross section. Reasonable precautions should be taken to prevent the stream from changing its course near the entrance or outlet causing a maintenance problem due to scour. I I I 7. Pipes. a. Application. Each type of prefabricated conduit or culvert, herein called "pipe", has ad- vantages and Iimitstions which should be considered before selecting the type for a particular installation. Pipe for highway or railroad relocat ion should be designed to conform to the required standards of the affected authority. Protective coverings and coatings applied in either the shop or field should be provided where applicable. b. Types. (1) Vitrified-clall a.nd concrete pipe. Vitrified-day pipe is particularly suited for carrying industrial wastes because of its high resistance to corrosion and erosion. It is generally available. glazed or unglazed, in sizes up to 36 inches and may be obtained by special order up to 42 inches. Extra strength pipe will be required for most installations. The pipe should be encased in a con- crete shell to give additional strength. Methods of embedment are discussed in reference (11). A chemically resistant watertight nonrigid joi'nt should be used. Reinforced concrete pipe is subject to corrosion by sewer acids and soils which are highly acid or sulfate-alkaline. It is available up to aoou t 120 inches in diameter. Care should be taken to provide joints of sufficient flexibility, watertightness, and ability to accommodate differential set- tlement. Watertightness is especially important in pressure pipes and provision must be made for differential settlement in foundations where substantial settlement is anticipated. For reinforced concrete pipe under levees, a pressure type joint with collars should be used. For other locations, pressure type joints with gaskets should be used and collars may be omitted unless foundation settlement conditions are severe. Clay and concrete pipe for highway or railroad work should be designed to conform to the required standards of the affected authority. (2) Cast iron and steel pipe. Cast iron pipe is used under levees and for water mains and other installations where fluids are carried under pressure. It is also suitable for pressure sewers and for gravity sewers where watertightness is essential. It can resist relatively high intemal and external pressures and corrosion in most soils. However, it is subject to corrosion by acids. highly septic sewage and acid soils. It is generally available in sizes up to about 48 inches. Flexible bolted joints are required under levees and in other loca tions where differential settlement is anticipated. I I I I I I I I 11 I