Technical Manual: Conduits through Embankment Dams (FEMA 484)

Glossary• Backward erosion piping is associated with intergranular seepage throughsaturated soil zones, not along concentrated flow paths (such as cracks).• Backward erosion piping begins at a seepage discharge face where soil particlescan escape because of the lack of a filter or an improperly designed filter at theexit face. As particles are removed, erosion progresses backward toward thesource of seepage.• The material being piped must be able to support a “pipe” or “roof,” or mustbe adjacent to a feature such as an overlying clay layer or concrete structure thatwould provide a roof.• For backward erosion piping to progress to the point where a failure occurs,soils susceptible to backward erosion piping must occur along the entire flowpath.• Backward erosion piping requires a hydraulic gradient high enough to initiateparticle movement in soils that are susceptible to this phenomenon. Piping canbegin with relatively low gradients for horizontal flow. For flow exiting adeposit vertically, if gradients are very high, the soil may be loosened, creating acondition sometimes termed heave.• The term blowout is used to describe backward erosion piping that resultswhen a sand horizon is overlain by a clay horizon with a defect in it, and anexcessive hydraulic gradient causes backward erosion piping through that defectin the blanket. Defects in the blanket may consist of crayfish holes, fence postholes, animal burrows, and drying cracks. The transported sand forms a conicaldeposit on top of the surface clay horizon that itself is resistant to backwarderosion piping.In this document, the term “backward erosion piping” is used to describe the condition where pipingoccurs as defined above. The term “internal erosion” is used to describe all other erosional processeswhere water moves internally through the soil zones of embankment dams and foundations.Bedding: Concrete used to provide transverse and lateral support under precastconcrete conduits. Bedding generally comes up to about 25 percent of the conduitheight.Bedrock (FEMA, 2004): Any sedimentary, igneous, or metamorphic materialrepresented as a unit in geology; being a sound and solid mass, layer, or ledge ofmineral matter; and with shear wave threshold velocities greater than 2,500 ft/s.Bentonite: A type of clay derived from weathered volcanic ash that expands whenwet; commonly used as well drilling mud and annular seal.391