Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 6Filter ZonesZones of designed filter material have become the accepted method of preventingfailures caused by uncontrolled flow of water through the embankment materials andfoundation soils surrounding a conduit through an embankment dam. This chapterdiscusses the theory behind the concept for using filter zones to prevent erosion ofearthen embankments near conduits caused by the uncontrolled flow of waterthrough soils surrounding conduits that penetrate the embankment.The type and configuration of the filter zone depend on site conditions and soilsused in the embankment dam. Three basic designations for filter zones associatedwith conduits are discussed: filter diaphragms, filter collars, and chimney filters.Examples of typical designs used by the major design agencies are included.6.1 Theory of filter seal developmentThe concept behind the function of a filter in sealing a concentrated flow wasdeveloped largely from laboratory experiments under the guidance of Sherard asreported in several references (Sherard, Dunnigan, and Talbot, 1984). Figure 86(top) is reproduced from this reference, and it illustrates filter cake development inthe laboratory experiments. Figure 86 (bottom) shows the action of the filter insealing a concentrated leak in an embankment dam.6.2 Federal agency policy on filters for conduitsThe following policy concerning filter zones has been summarized from three of themajor federal agencies that have been traditionally involved with embankment damconstruction:• Bureau of Reclamation.—From Reclamation’s, Embankment Dams—EmbankmentDesign (1992, p. 21):Structures through embankments should be avoided unless economics or sitegeology dictates their use. If they are used, the primary means of controlling131