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12 The NRCS provides minimum sizes for embankment filters; however, the following recommendations should also be considered: • Inclined filter and drain zones which will be constructed at the same time as adjacent upstream and downstream zones should be designed with a minimum horizontal dimension of 5 feet. • Vertical filter and drain zones which will be constructed at the same time as adjacent upstream and downstream zones should be designed with a minimum horizontal dimension of 3 feet. • Inclined filter and drain zones which will be constructed against an excavated face should be designed with a minimum horizontal dimension of 3 feet. • Horizontal filter and drain zones should be designed with a minimum thickness of 1 foot. Filter and drain materials are not particularly amenable to conventional earthwork compaction density control. Typical filter sand materials do not exhibit the “standard” compaction curve shape, with a clear maximum dry density and optimum moisture content. Rather, these materials exhibit their maximum dry densities when either completely dry or nearly saturated. Drain materials are typically uniform gravels, which are not suitable for conventional compaction testing or conventional field density testing. Conventional end product compaction specifications (e.g. percent compaction specifications) have sometimes been used for filter and drain materials, however, they are difficult to apply in the field, for the reasons given above. End product compaction specifications based on relative density requirements have also sometimes been used. However, the relative density test is notoriously difficult to apply in the field. For most applications, it is desired that the filter and drain materials be compacted sufficiently to provide sufficient strength and to limit settlement. In locations subject to significant seismic loading, it is also necessary that the filter material be sufficiently dense to resist liquefaction if it is saturated. All of these requirements can be met by achieving densities that are greater than 70 percent relative density, which is not particularly difficult to accomplish with these clean materials. Further, it is desirable not to overcompact the filter material, because this can lead to excessive particle breakage and increased fines content, which is not desirable. In general, it is easier to use a method specification for filter and drain materials, in which minimum compaction equipment and minimum compaction effort (e.g. number of coverages with the equipment) are specified. In addition to the compaction equipment and effort, it is also recommended that the placement specification for the filter include thoroughly wetting the material (to near saturation) as it is being compacted. There are a number of practical ways to accomplish this, including 1) covering the material with a water truck immediately ahead of the compactor, 2) applying water to the material with a hose immediately ahead of the compactor, and 3) mounting a water spreader bar on the compactor ahead of the compaction drum. Vibratory compaction equipment is the most appropriate equipment for compacting filter and drain materials. A method specification requires close QC inspection during the work to assure that the method is being followed, but it is generally the easiest approach to use for these materials. If desired, the method specification can be combined with verification of the method by density testing in the initial production placements or in a test section. It is important to prevent contamination of the filter and drain materials during construction. To perform their functions as intended, the filter and drain materials must contain very limited amounts of fine materials. Contamination can occur if runoff carries fine-grained material into the filter and drain materials. To prevent contamination, it is recommended that filter and drain materials be maintained at least one lift higher than the adjacent materials that contain fine-grained soils, and the adjacent materials should be sloped slightly to drain away from the filter and drain materials. Should the filter or drain materials become contaminated despite efforts to prevent contamination, the contaminated materials should be removed and replaced. Conclusion Several guidance documents are available to assist the designer in developing a well-designed filter for an embankment dam. The designer must carefully