Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsX AxisFigure B-41.—Idealized cross section of Hernandez Dam.trench condition under the upstream shell and the first half of the downstream shell;it was constructed in the positive projecting condition in the sections under the coreand second half of the downstream shell. The impervious core backfill in the keytrench was brought up to ½ foot above grade; the earthfill was then excavated downto grade prior to the installation of the outlet conduit.In February 1997, San Benito County Water District (District) staff attempted tooperate the valves to reduce the water release and found that the right valve wasinoperable in the open position. Later inspection revealed that the flexible ¾-inchhydraulic hose connector to the hydraulic cylinder was severed by the operation ofthe left butterfly valve. The reservoir emptied through the outlet conduit becausethe open gate was the only control for the conduit. Once the reservoir was emptied,staff from the California State Division of Safety of Dams, the District, and aconsultant hired by the District inspected the valves and the outlet conduit.Observations of water flowing through the conduit, showed it to have a sag in itsprofile. This was confirmed in a subsequent survey of the outlet conduit.The outlet conduit was found to have settled more at locations over the cutofftrench than it had at locations upstream and downstream. The sharp differential inthe thickness of compressible materials at that point is depicted in figure B-41. Thedifference in settlement was attributed to the condition where the center portion ofthe outlet conduit was underlain by up to 24 feet of compacted fill in the cutofftrench while the adjacent sections under the embankment shells had rock or gravelfoundations that were much less compressible. In addition to the problems with thesag in the outlet conduit, some welded joints had cracked open and others were invarious stages of corrosion. The cracked joints had corroded through the rupturedB-46