Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 1.—Nonpressurized outlet works.Figure 2.—Pressurized outlet works.This discontinuity can cause settlement to be different adjacent to the conduit than itis in the rest of the embankment dam. Earthfill may also be compacted differentlyaround a conduit than for the rest of the embankment dam. These factors can causecracking of the earthfill and lead to other consequences. Failures of embankmentdams caused by the uncontrolled flow of water through the dam or foundation are acommon problem. A conduit can develop defects from deterioration, cracking fromfoundation compressibility, or joint separation due to poor design and construction.Water leaking from defects in conduits can contribute to seepage pressuresexceeding those that occur solely from flow through soils in the embankment damfrom the reservoir. When preferential flow paths develop in the earthfill throughwhich water can flow and erode the fill, severe problems or breaching type failuresoften result. The reasons that conduits contribute to these failures are discussedmore extensively in several sections of this document.Historically, a single term, “piping,” has been commonly used in literature todescribe all erosional processes involved in embankment dam failures. The reasonfor this is that frequently after a failure, a tunnel-shaped feature resembling a pipe isobserved. See figure 5 for an example. In this document, two terms will be used todescribe failures of embankment dams associated with uncontrolled flow of water,rather than using a single generic term. The two terms that will be used in thisdocument are:• Backward erosion piping and• Internal erosion2