Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsfor failures in the earthfill above a conduit is that most embankment dams are notconstructed of soils susceptible to backward erosion piping without proper designfeatures to prevent the backward erosion piping. The Introduction of this documentcontains extensive discussions of factors that are important in distinguishing betweeninternal erosion and backward erosion piping mechanisms of particle erosion.Figure 7 illustrates the sequence in development of Failure Mode 4.Figure 104 shows a small embankment dam that failed by internal erosion. Theconduit created differential settlement in soils above the conduit that resulted inhydraulic fracture. The embankment soils were highly dispersive clays.If the erosion tunnel widens enough, the tunnel can collapse, and a tunnel-shapedfailure surface is not observed after the failure. The failure is simply a breach in theembankment dam.The near failure of the USACE’s Wister Dam is a good example of this scenario ofinternal erosion. The failure of the embankment dam during first filling wasnarrowly averted by quickly lowering the pool and employing other interventionmeasures. The embankment dam was constructed of highly dispersive clays withouta chimney filter. The problems occurred in a closure section of the embankmentdam. See the Wister Dam case history in appendix B. Sherard (1986, p. 911)provides further details on this interesting case history.Another example of internal erosion resulting from the existence of hydraulicfracture cracks within an embankment dam is the Upper Red Rock Site 20 Dam.See appendix B for a detailed discussion of this case history.Before the NRCS gained an understanding of the behavior of dispersive clay soils,over 15 embankment dams constructed of dispersive clays failed. Most of thefailures occurred near the conduits through the embankment dam. The conduitscontributed to differential settlement, which led to hydraulic fracturing (Sherard,1972; Sherard, Decker, and Ryker, 1972a). Another example of this type of failure isthe Anita Dam case history in appendix B. Investigators attributed one possiblecause for the formation of a flow path for water to be freezing and thawing of soilsadjacent to the conduit. Hydraulic fracture could also have contributed to thefailure.7.4.1 Design measures to prevent failureSeveral design measures are available in preventing this type of failure mode fromdeveloping.The first design measure involves reducing the potential for cracking and internalerosion of the fill. The mechanism responsible for this type of failure mode is174