Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of ConduitsFigure 60.—Precast concrete using beddingas support.flexible conduits, since they require deflection to develop strength in the conduitwalls, and cradles and bedding would prevent this deflection. If a flexible conduit isconstrained by a cradle, even one that extends up to the springline, the conduit couldbe overstressed beyond its design strength. Consideration for fully encasing theflexible conduit in concrete should be evaluated, and the sides of the concrete shouldbe sloped at 1H:10V or flatter. Also, the use of a filter diaphragm or collar should beconsidered.4.2 Structural design and construction considerationsA conduit must withstand internal fluid and vacuum pressures, external hydrostaticloadings and buckling pressures, embankment loads, surface surcharge loads,construction loads, operational and maintenance loadings, and combinations of theseloads. Designers should also consider the effects of horizontal and verticalmovements that may occur from settlement and spreading of the embankment andfoundation. These movements may result in loads on the conduit in excess of loadspredicted from normal static computations. Excessive movements, both vertical andlateral, can occur when conduits have foundations that are either weak orcompressible, or both. Poorly compacted embankment dams can spread from sheardeformations, which also can lead to lateral spreading of the conduit. Some shalesmay be relatively incompressible but have anisotropic shear strength conditions thatallow excessive lateral movement without much compression.85