Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 6—Filter ZonesFigure 89.—Typical design used for embankment dams with distinctlydifferent materials in the core zone and exterior shells of the dam.• Zones of widely varying grain size are not used adjacent to one another in thecross section. Examples include core zones of fine grained soil with exteriorzones of coarser gravel or rockfill.• Abrupt changes in the cross section transverse to the centerline are not present.Examples are bedrock ledges, closure sections, and excavated trenches.• Good oversight of construction is provided, including quality assurance andcontrol that are consistent and effective.• Special care is taken in following recommendations for compaction of fillsurrounding the conduit, as discussed in section 5.3.2.If a chimney drain is not included in the design of a low hazard embankment dam,the design measures for preventing internal erosion or backward erosion pipingalong the conduit may consist of three basic choices:1. A filter diaphragm is a valuable defensive design measure even for low hazardclassification embankment dams with favorable site conditions. The filterdiaphragm will intercept flow along discontinuities along the pipe or through137