Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 7—Potential Failure Modes Associated with ConduitsThere are four main potential failure modes involving conduits through embankmentdams. These failure modes are discussed in the following sections.7.1 Failure Mode No. 1: Backward erosion piping or internal erosion of soilsinto a nonpressurized conduitFor this failure mode, the conduit is surrounded over at least part of its length by soilwith a low resistance to backward erosion piping. If the conduit develops a defectfrom deterioration, or a joint in the conduit becomes open from movement andseepage is occurring through the surrounding embankment, seepage forces may carrysoil particles into the conduit. For this failure mode, the conduit is presumed tohave an interior pressure lower than the seepage pressures in the surrounding soil.Figures 96, 97, and 98 show conduits with defects where water is entering theconduit. In figures 96 and 97, the defects are separated joints in a conduit. Infigure 98, the defect is a poorly constructed joint in a CMP.If the soil surrounding the conduit defect is resistant to backward erosion piping andthe defect in the conduit is small, the time for serious erosion of surrounding soils todevelop could be lengthy. Inspections of the conduit should disclose the presence ofdefects and allow for timely repair before serious problems develop. However, if thereservoir head is high and the defect in the conduit is large enough, the potential foreither backward erosion piping or internal erosion is significant. Backward erosionFigure 96.—Leaking joints in a 60-inch diameter RCP spillway.Several large voids were also observed in the adjacentearthfill on the upstream slope.159