Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams4.1.3 Conduit shape EFigure 57.—Concrete saddles are used to supportsteel pipe located within a larger access conduit.Conduit shape E (figure 50) is formed by separating two semicircular sections byshort side sections. Figure 58 shows an example of the interior of conduit shape E.Conduit shape E generally achieves maximum economy of materials by mobilizingmore of the relieving fill pressure. The sides of the conduit should be sloped to1H:10V or more through the im pervious zone to allow equipment to compact theearthfill directly against the conduit. Contractors may use straight cords to avoidusing curved forming techniques for the exterior of this shape. If straight cords areused, the designer must ensure that stress concentrations do not change andadequate concrete thickness is provided.4.1.4 Conduit shape FAn externally shaped circular conduit is more adaptable to changes in loading andstresses that may be caused by unequal fill or foundation settlement (i.e., betterdistribution of loads). As a flow surface, a circular internal cross section is usedprimarily for pressure flow conditions, since it is a very hydraulically efficient shape.The interior of a circular conduit would be similar to figure 54. The use of anexternally shaped circular conduit through an embankment dam should be carefullyevaluated due to concerns with the difficulty or inability to uniformly compact theearthfill around the conduit. Precast concrete pipe is the most often used externallyshaped circular conduit. The earthfill beneath the haunches of the conduit cannot beadequately compacted with pneumatic tired equipment and requires compaction withhand held tampers. Efforts to obtain proper compaction using hand tampers couldcause movement or displacement of smaller conduits. Improper compaction of theearthfill around the conduit and movement of the conduit can result in differential80