Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams10, 15, 20, and 25 feet from the riser. The conduit joints were to consist of 13-inchwide “hugger bands” with o-ring gaskets installed in “re-rolled” corrugations at theends of each conduit section.A review of a videotape of the site during construction (which was made for trainingpurposes, not for documentation of construction, and only incidentally containedfootage of the dam construction) indicated that a substantial portion of the damembankment had been placed prior to delivery and installation of the spillway CMP.The spillway conduit was then apparently installed into a narrow trench with verticalsides, excavated through the partially completed embankment dam and into thefoundation soils. The design engineer was not required to be onsite duringconstruction, and construction inspection was at the discretion of the contractor.Site personnel noted that just before failure, the pool level was at the upper weirelevation, and a vortex (whirlpool) was observed in the pond adjacent to thespillway. Failure occurred at about midday on March 4, 1993. After the failure,about 26 feet of the upstream section of the CMP was observed to have completelycollapsed. The bottom of the collapsed portion of the conduit exhibited an inverted“V” shape. A large amount of upstream portion of the embankment had washedout through the downstream portion of the CMP, which remained partially intact.Deep, vertical troughs were visible on the downstream slope directly above the sidesof the CMP. The sediment level in the channel below the dam obscured the bottomhalf of the CMP. Figure B-6 shows the downstream section of the CMP spillwayremained partially intact, but deep troughs were visible directly above each side ofthe conduit.An unauthorized grating with small openings (i.e., chain link fence), which had beenbolted to the downstream end of the conduit by the owner to prevent vandalism,was observed to be nearly plugged over its entire area with debris, indicating that theCMP probably was full of water at the time of the failure. The grating was detachedfrom the end of the CMP during the failure, and an o-ring joint gasket was observedentwined in the grating and debris. Grass growing on the dam embankment at thedownstream toe near the spillway outlet was bent downstream, indicating that waterhad flowed along the outside of the conduit during the failure.About 2 weeks after the failure, a team of geotechnical engineers, state and localofficials, surveyors, lawyers, and the local pipe manufacturer observed excavation ofthe failed spillway conduit in order to determine the cause of failure and who wasresponsible. Engineers from at least three companies were present, representing theembankment dam owner, the contractor, and the original designer.Two large excavators carefully removed soil from above the conduit. The sides ofthe excavation were sloped as required for stability. A surveyor documented theB-6