Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 7—Potential Failure Modes Associated with ConduitsInternal erosion could occur if a preferential flow path (like a hydraulic fracture)develops that is connected to the conduit defect. If internal erosion occurs in soilssurrounding a conduit defect in this failure mode, the potential for eventual failure ishigh, because all soils when subjected to continued flow along a preferential flowpath are erosive over time. Highly erodible soils, such as nonplastic silts, broadlygraded silty coarse-grained soils, and dispersive clays, could develop erosion featuresmore quickly. The most likely manifestation of erosion in this failure mode is asinkhole that develops on the embankment surface.As previously discussed, Failure Mode No. 1 may involve either a backward erosionpiping or internal erosion mechanism of particle erosion. The Introduction of thisdocument includes extensive discussions of factors that should be evaluated todetermine which of these mechanisms is likely for a specific situation.The sequence in which this failure mode could develop is illustrated in figure 99 anddescribed in the following steps. Note that the following description specificallyinvolves the development of backward erosion piping in a situation where theconduit is surrounded by soils susceptible to this failure mechanism. A similar set ofsteps could be described for a scenario involving internal erosion rather thanbackward erosion piping, when the surrounding soils are not susceptible to backwarderosion piping. For the sake of brevity, this description of similar steps in an internalerosion scenario is not repeated.1. As water is impounded in the reservoir, seepage develops through theembankment dam. The time for this to occur varies with the permeability ofthe embankment zones. A phreatic line develops, and seepage forces are activein the saturated soils around the conduit.2. Seepage can enter any defects in the conduit, if the conduit has an interiorpressure lower than the water in the soil pores. If the seepage discharging intothe nonpressurized conduit has sufficient gradient and soils are susceptible tobackward erosion piping, soil particles may be carried with the water.3. Backward erosion piping of the soils in the embankment dam will cause atunnel to develop for soils that can support a tunnel. If the soils cannotsupport a tunnel, a sinkhole may occur instead. A failure can occur if the defectin the conduit is large enough to allow most of the reservoir water to escape.4. If soils between the reservoir and the defect in the conduit are not susceptibleto backward erosion piping, and no preferential flow paths occur in thesurrounding soils, the defect in the conduit may not result in immediateproblems.161