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10 effectively lowering embankment phreatic levels and protecting the critical zones of the embankment. The size or thickness of the filter zone is important to ensure it meets necessary capacity requirements and also provides ample thickness to assure continuity during placement and to prevent contamination during construction. Let’s focus first on the filter gradation design. Detailed guidance documents for gradation design for soil filters are readily available from three federal agencies: the U.S. Department of Agriculture, Natural Resources Conservation Service (NRCS) [NRCS (1994)]; the U.S. Department of the Interior, Bureau of Reclamation (Reclamation) [Reclamation (2007)]; and the U.S. Army Corps of Engineers (USACE) [USACE (2004b)]. This article does not include a repetition of the detailed guidance included in the three documents referenced above, all of which are readily available. Rather, this article presents a general discussion of the NRCS method, highlighting some of the important practical aspects of the guidance. The NRCS method for filter gradation design is summarized in 11 steps. The 11 eleven steps are not reiterated in this article; however a brief discussion presenting the goals of the various steps is provided below. Steps 1 through 5 of the procedure establish the criteria that must be met to provide a filter that will prevent movement of soil particles from the base soil (the soil being protected) into the filter – the filter function. Mathematical regrading of the base soil is performed in these steps and is a critical part of the filter design process. Step 6 establishes criteria to assure that the filter is significantly higher in permeability (hydraulic conductivity) than the base soil – the drainage function. Steps 7 and 8 are intended to prevent the filter from being gap graded. A gap graded filter is a soil composed of particles of two different gradation ranges, e,g, gravel and fine sand, with very little if any of the intermediate grain sizes, e.g. coarse and medium sand. Gap graded soils can be internally unstable; that is the coarse fraction does not serve as a filter to the fine fraction, and the fine fraction can be eroded out through the coarse fraction. Steps 9 through 10 are intended to produce a filter gradation that will limit the likelihood of particle size segregation during placement of the filter. Segregation of the filter into coarser and finer zones can result in coarse zones which do not provide the required filter function. If a particular design does not require that the filter meet permeability requirements, the permeability criterion, Step 5, can be relaxed, as long as the filter criterion, the gap graded criteria, and the segregation criteria are met. An example of where this might apply would be a filter for a core, with a very permeable, filter-compatible shell downstream of the filter. In this case, the downstream shell would serve the drainage (permeability) function, lowering the phreatic surface immediately downstream of the filter. If it is typically desired that the filter has high permeability (hydraulic conductivity),it is recommended that the filters have less than 3% nonplastic fines (material finer than the No. 200 sieve size), in place, before compaction, and at most 5% nonplastic fines, in place, after compaction. Permeability of the filter decreases dramatically as the fines content increases above these levels. It is very rare to find a case where natural materials can satisfactorily serve as filters, without significant processing. Natural materials are typically not suitable as filters for the following reasons: • The required gradations requirements for filters are relatively narrow, and the variation in gradations in natural deposits is typically too great to be confident that all of the material obtained from a natural source will be within the specified narrow limits. • It is generally desirable for filters to have very low fines contents, less than 3 to 5 percent, as discussed above. It is very unusual to find natural deposits that reliably have such low fines contents. • Natural deposits often have enough coarse particles that they do not meet the filter requirements to prevent segregation during placement. It is not necessary that the exact gradation limits resulting from the filter calculations be used in the project specifications. Rather, the calculated gradations can be used to select and specify readily-