Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 7—Potential Failure Modes Associated with ConduitsFigure 103.—Failure Mode No. 4.—Internal erosion of the earthfill aboveand on either side of a conduit caused by concentrated flow in a hydraulicfracture or other preferential flow path in the compacted earthfill.Hydraulic fracture cracks in the embankment dam may result fromdifferential settlement caused by the presence of a conduit within theearthfill.5. This type of failure occurs most frequently on first filling of the reservoir sothat intergranular seepage rarely has had time to develop. One of therequirements for backward erosion piping to be defined in the context of thisdocument is that it results from intergranular seepage, which justifies thisconclusion.6. Another reason that backward erosion piping is seldom the cause for failures inthe earthfill above a conduit is that most embankment dams are notconstructed of soils susceptible to backward erosion piping without properdesign features to prevent the backward erosion piping.As previously discussed, Failure Mode No. 4 almost always involves the mechanismof internal erosion, and very rarely can backward erosion piping be correctlyattributed as the cause of such failure. This type of failure occurs most frequently onfirst filling of the reservoir, so that intergranular seepage rarely has had time todevelop. One of the requirements for backward erosion piping to be defined in thecontext of this document is that it results from intergranular seepage, which justifiesthis conclusion. Another reason that backward erosion piping is seldom the cause173