Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesFigure B-6.—The downstream section of the CMP spillway remainedpartially intact, but deep troughs were visible directly above each side ofthe conduit.location of items of interest (elevation of top of conduit, conduit invert, locations ofjoints and antiseep collars, etc.) as directed by the engineers.The conduit included three joints. When the hugger band at the downstream jointwas removed, one of the o-ring gaskets was observed to have been displaced into theconduit, and there was debris from the pool under the band. This indicates thatwater from the pool may have flowed along the outside of the conduit, despite theantiseep collars, and into the joint. Figure B-7 shows the spillway conduit andportions of the soils that were carefully removed and documented during a forensicinvestigation about 2 weeks after the failure. Figure B-8 shows the two largeexcavators that removed the majority of the embankment dam. Figure B-9 showsthe o-ring gasket that was found to have been displaced.The forensic investigation confirmed that the CMP was installed in a trench withnear vertical sides. In addition, it appeared that the trench may have beenoverexcavated and backfilled with poorly compacted fill material, which was quicklyeroded away by flow along the outside of the conduit and/or into open joints. Lossof soil support would have caused additional conduit deformation, further openingthe joints, resulting in an ever-increasing cycle of leakage and loss of earthfill byinternal erosion. Figure B-10 shows the presence of undisturbed foundation soilsB-7