Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams7.4 Failure Mode No. 4: Internal erosion of hydraulic fracture cracks in theearthfill above, below, or adjacent to the conduitConduits are one of the primary causes of differential settlement of an embankmentdam that can result in hydraulic fracture of the embankment in the vicinity of theconduit. When an earthfill experiences hydraulic fracture, a pathway is created alongwhich water from the reservoir can flow easily and erode the soil in contact with thecrack.Failure Mode No. 4 is one where hydraulic fracture of the embankment dam in thevicinity of a conduit is attributable to the differential settlement caused by theconduit, and flow through the crack erodes the embankment to the point where abreaching type failure occurs. Hydraulic fracture of earthfill is discussed extensivelyin section 5.2. This failure mode differs from Failure Mode No. 3, since the seepagepath forms at a location away from the soil-conduit interface.These kinds of failures are most common when a reservoir fills suddenly shortly aftercompletion of the embankment dam, and the earthfill is highly erodible. The sidesof cracks may erode very quickly when water from the reservoir flows through thecrack. The eroded failure path can enlarge to a size that can empty a reservoirrapidly.If a crack is not intercepted with a filter zone, an embankment dam failure can resultwhen the crack enlarges from erosion. Even high plasticity clays that are notdispersive can erode over time. The sequence of failure for Failure Mode No. 4 isdescribed as follows and is illustrated in figure 103.1721. After a crack forms in the soils surrounding the conduit, if the embankmentsoils are highly erodible, the crack rapidly enlarges from erosion of the sidewallsof the crack. The water discharging at the downstream face of theembankment dam is muddy, and a vortex may form at the entry point on theupstream slope.2. The erosion tunnel enlarges to the point that the reservoir is emptied and thebreaching process is completed.3. A tunnel-shaped hole will exist after the failure, if the eroded tunnel is narrowenough to support the roof of the tunnel. If the tunnel collapses from erosionand widening caused by a lack of support for the roof, the failure will have theappearance of an open breach in the embankment dam.4. As previously discussed, Failure Mode No. 4 almost always involves themechanism of internal erosion, and very rarely can backward erosion piping becorrectly attributed as the cause of such failure.