CONCRETE PIPE Determination of Loads Three types of loads must be considered in the design of precast concrete pipe: • Earth loads • Live loads from trucks, aircraft, and trains • Surcharge loads or loads from an additional earth fill or building over an installed pipe Earth Loads (Dead Loads) The earth load is the weight of the earth that must ultimately be carried by the pipe. This weight varies with the soil characteristics. More importantly, however, it varies with the installation conditions. The method for determining the earth loading is best approached by considering the two major classes of construction, trench and embankment. Trench Installations Trench installations are made in relatively narrow excavations and the pipeline covered with earth backfill which extends to the original ground surface. Sewers, drains and water mains are usually constructed in trenches. Embankment Installations The essential features of embankment installations are illustrated to the right. Highway and railroad culverts are typically installed under fills or embankments. As shown in the adjacent figure, embankment installations are subdivided into three groups: • Positive projection pipe is installed with the top of the pipe projecting above the surface of the natural ground, or compacted fill, and then covered with earth fill. This type also includes pipe installed in extremely wide trenches. • Negative projection pipe is installed in relatively shallow trenches of such depth that the top of the pipe is below the level of the natural ground surface or compacted fill, and then covered with earth fill to a height appreciably greater than the distance from the natural ground surface or original compacted fill surface to the top of the pipe. • Induced trench pipe is initially installed as positive projection. When the embankment fill has been placed to an elevation of at least one pipe diameter over the proposed top of the pipe, a trench is excavated over the pipe and backfilled with a more compressible material, simulating a negative projection installation. PAGE 30 LAFARGE PIPE eMANUAL