Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsincluded gravelly clay gully wash deposited in the form of an alluvial fan, as well ascolluvium. Fine grained material was intended to be reserved for the designated corezone. The embankment dam design did not include provisions for either filtering ordrainage of the core.Tests showed that the earthfill material was dispersive. During construction, gypsumwas added to portions of the core as a treatment for dispersivity. The new outletconduit was laid roughly along the original ground surface, under the highest sectionof the new dam. A pipe was placed in the bottom of a wide slot cut through the oldembankment dam. The pipe was laid in a trench dug into the lowest layers ofcompacted fill that had already been placed, and then encased in reinforced concrete.Across the new core’s foundation cutoff, plus along one other section, the outlettrench was deepened to weathered bedrock prior to filling with concrete, to improvebearing upon soft material present under those sections. Concrete antiseep collarswere found to have been cast over only the top and upper sides of the outletencasement. The collars did not extend below the pipe encasement.Breaching took place soon after filling began and before the reservoir wascompletely filled. This suggests that one or more concentrated leaks must haveexisted, to enable flow to reach the downstream toe so soon, long before thesaturation front could have advanced very far into the earthfill. Failure started at thedownstream toe in the vicinity of the outlet conduit, and the erosion tunnelterminated immediately adjacent to the upstream end of the outlet encasement.The initial concentrated leaks alongside the outlet conduit are postulated to haveoccurred due to hydraulic fracture of the earthfill by the rapid rise of the reservoir.The conduit appears to have allowed for a low stress zone to occur in the earthfillnext to the wall of the encasement. In essence, the wall “shielded” the adjacent fillfrom the full weight of the overlying embankment. The stresses in this area werelikely lower than the reservoir head. A somewhat compressible foundation materialbeneath the conduit could have assisted in the formation of the crack completelyalong the embankment dam’s cross section. Once concentrated flow started, thedispersive nature of the embankment fill would have allowed for rapid erosion fromthe downstream exit of the crack, progressing upstream. Lack of any defensivedesigns for embankment cracking, such as a filter and drain, contributed to thefailure.Also contributing to the failure was the poor compaction of the earthfill materialadjacent to the encasement wall that was found during the forensic investigations.Also thought to contribute to the failure were the potential for differential settlementof the new and old earthfill.Figure B-64 shows a cross section through the conduit area after failure.B-82