Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 7Potential Failure Modes Associated withConduitsWater flowing through conduits can escape through defects in the walls or betweenseparated joints of the conduit. Soils can also be carried into a conduit through thesedefects. If a conduit is flowing under pressure and water is forced out of defectswithin the conduit, this can lead to a very serious problem that must be addressed byemergency action, since catastrophic embankment dam failure could result. If aconduit is not flowing under pressure, defects within the conduit may allow soilssurrounding it to be carried into the conduit by seepage and hydraulic fracture.Water escaping through defects from within nonpressurized conduits will probablyhave a lower velocity and lower pressure and should be less damaging to thesurrounding soils, than if the conduit were pressurized. This may allow for remedialmeasures to be undertaken in less of an emergency mode. Generally, defects inconduits are much more serious for conduits designed for pressure flow than fornonpressurized flow.Attempting to place a filter on the outside of a conduit at a defect is likely to beineffective, particularly for a pressurized conduit. The quantity of flow from thedefect in a pressurized conduit will likely exceed the capacity of a filter designed toprotect adjacent soils. In a nonpressurized conduit, the filter designed for a givensize defect may be inadequate when the defect increases in size. Replacing orrenovating conduits with defects are the only reliable long term solution topreventing damage to surrounding soils. See chapters 12 and 13 for guidance onreplacement and renovation of conduits.Water flowing through soils surrounding a conduit may also cause failure of theembankment dam. A conduit within an embankment dam is a discontinuity thatmay create stresses in surrounding soils that are conducive to hydraulic fracture. Aconduit may impede uniform compaction of soils in its vicinity. The various waysembankment dams may fail (where conduits are the sole or primary contribution tothe failure) are discussed in this chapter. Many other types of failure modes forembankment dams exist that are not associated with conduits and are outside thescope of this document.157