Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesFigure B-3.—Initial construction of embankmentdam and outlet conduit (upstream is to left).Note presence of antiseep collars surroundingconduit.Figure B-4.—Initial construction. Note handtampers being used to compact earthfill adjacentto outlet conduit.The cause of the failure was likely the combination of the dispersive clayembankment material, hydraulic fracture, antiseep collars (figure B-3) around theconduit that required the use of hand tampers (figure B-4), “flowable” backfill forconduit support in lieu of a continuous concrete conduit support, and lack of a filterand drain around the outlet conduit in the downstream portion of the embankmentdam. Cold air flowed through the conduit during the winter preceding the failure.This caused lenses of frozen material in the conduit’s backfill. These lenses mayhave provided a path for concentrated leakage when they were thawed by the initialflow of water in the conduit during the runoff.Lessons learnedEven though antiseep collars were utilized, a major leak occurred along the conduit,causing rapid erosion of the dispersive clays used to construct the embankment dam.B-3