Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 5.—Embankment dam failure caused by internal erosion of earthfillnear the conduit. Flow was not directly along the contact betweenearthfill and conduit, but in the earthfill away from conduit. Hydraulicfracture in highly dispersive clay embankment soils caused the failure. Theembankment design included antiseep collars, but not a filter diaphragm.These two terms are more descriptive of the distinctly different mechanisms bywhich water can damage embankment dams. In this document, the term “backwarderosion piping” will be reserved to describe conditions where water flows notthrough preferential flow paths, such as cracks in the soil, but through the pores of asoil. The flow causing the mechanism of failure termed “backward erosion piping”is solely that from intergranular flow causing excessive seepage forces at an exit face.These seepage forces cause a boil condition or particle detachment at an exit face, ifit is not protected by a properly designed filter. The term “backward erosion piping”is used in an attempt to define this precise condition of failure. The term “internalerosion,” discussed in the following paragraph, describes the more common way thatwater can damage embankment dams, as it flows through cracks, discontinuities atthe interfaces between conduits and earthen embankments or their foundations, orother preferential flow paths. Seepage flow for internal erosion is typicallyconcentrated.The term “internal erosion” will be used in this document to describe all conditionsother than “backward erosion piping” by which water flowing through embankmentdams or foundations erodes the soils and causes a failure. Internal erosion occurswhere water flows through a discontinuity in the embankment dam and/orfoundation, and erodes the sides of the crack to enlarge it and cause a failure. Theterm “internal erosion” is used in lieu of a number of terms that have historicallybeen used to describe variations of this generic process including scour, concentratedleak piping, and others. This term will also be used for another type of conditioncalled suffosion. Suffosion is the type of erosion where the matrix of the soil mass is4