Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsdesirable self-healing characteristics than broadly graded, coarse filter materials.However, segregation of broadly graded filters can be a serious problem.The no erosion filter test described by Sherard and Dunnigan (1989, pp. 927-930) isuseful in evaluating whether a specific filter gradation is filter compatible with aspecific base embankment soil. While existing filter criteria generally ensure filtercompatibility, site-specific testing using the no erosion filter test is advisable forsome situations. For high hazard embankment dams, where designers wantadditional documentation on the filter compatibility between a specific filter sourceand the embankment soils, or where designers wish to explore whether relaxing strictfilter criteria would be safe, conducting laboratory filter tests with actual sitematerials is advisable.Other considerations for the filter and drainage zone materials include:148• Granular materials should be hard and durable, so they will not break downduring transportation, placement, and compaction. Overcompaction canreduce permeability and increase fines.• The specified gradation should filter the embankment dam’s core material andbe permeable enough to avoid excess water pressure buildup.• The filter gradation should be designed to avoid segregation during placement.• A two-layer filter may be required for zoned embankment dams where the coreand shell zones are very different.• Designs for drainage zones favor permeability considerations over filteringcriteria. Ordinarily, filters that are separate from the other embankment damdrainage zones are not expected to convey much flow, because their purpose isto intercept and prevent flow through cracks.• The most likely damage that can occur to sand and gravel filter/drainage zonesor material during construction is contamination or segregation.• Segregation will occur when the filter or drain material is dumped from an enddumptruck or other hauling unit, where the material falls more than about2 feet. Close inspection is always necessary to make sure segregation is notoccurring. The use of narrowly graded materials helps to prevent this problem.• Wide gradations (gradations that include a wide range of particle sizes) can beinternally unstable. This is a problem when the finer portion of the filter canpass through the coarse portion with water flow; washing the finer portion outand leaving a very coarse soil matrix that will not serve as a filter for the base