Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of ConduitsIf the joints between the ends of conduit sections separate or develop other defects,the conduit may develop leaks. This leakage can lead to the development of internalerosion or backward erosion piping failure mechanisms. The designer shouldcarefully consider the important parameters related to watertightness, such as:• Conduit joints• Barriers within jointsGuidance pertaining to these parameters is discussed in the following sections.4.3.1 Conduit jointConduit designers should be aware that foundation conditions are usually nothomogenous along the alignment of the conduit. Variable foundation conditions canresult in abrupt changes in the foundation settlement of a conduit beneath anembankment dam, causing large relative movements and failure of the conduit. Aproperly designed joint will limit vertical and transverse displacement of conduitsections relative to each other as the embankment dam settles. A properly designedjoint will also accommodate rotation and longitudinal movement while retainingwatertightness in the conduit. Figure 70 shows a joint within an outlet worksconduit that has settled differentially and is allowing seepage to enter through thejoint. Designers must estimate the maximum joint elongation that may occur fromthe compressibility of the foundation as accurately as possible. The predicted jointelongation depends on the shear strength of the foundation, the estimated settlementof the foundation, the configuration of the embankment dam, and the lengths ofconduit joints used in the design. If the predicted elongation is greater than thedesigned joints can accommodate, changes to the design are necessary. Designchanges may involve using shorter lengths of conduit, removing compressiblefoundation soils and replacing them with compacted backfill, and flattening theslopes of the embankment dam.Embankment dam settlement may not always be uniform, as predicted by analyses,and is often be erratic and can result in abrupt joint displacements in certainsituations. Abrupt joint displacements may be the result of localized jointmovement, and settlement can be more extensive than predicted by theoreticalanalysis. Figure 71 illustrates how actual settlement can differ from theoreticalsettlement. Abrupt joint displacements may be more likely for conduits that areconstructed using precast concrete pipe than for conduits constructed withreinforced cast-in-place concrete. The reason for this difference is that reinforcedcast-in-place concrete conduits are constructed with longitudinal reinforcementextending through the joint (control joint), which allows the conduits to bridge overa weak foundation and spread the load more uniformly over more of the conduitfoundation.99