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13 1. In-place material gradation, including material quality and durability specifications; 2. Moisture (wetting requirements) – generally requiring the addition of water to the material during handling and prior to compaction; 3. Compaction effort (number of passes with specified equipment) for a method specification or % compaction/relative density for an end-result specification (see discussion later in this article concerning method and end-result specifications); and 4. Geometry (alignment, width, and vertical continuity). Filters used to protect the core are generally specified to be constructed of sand-sized materials. Gradations should be designed in accordance with current dam practice to provide for both filtration of the base soil and for drainage of collected water. To accomplish this, more uniformly graded sand is preferable to broadly graded materials. In practice, commercially available concrete sand produced in accordance with ASTM C33 is applicable in most cases for protection of a fine-grained base soil. However, this information should always be verified by analysis. Aggregate quality and durability are other requirements which should be specified for granular filter materials, and those in ASTM C33 are applicable for filters, as well. Specifications for filter sands typically require that filter aggregates shall be “sound, strong, durable, clean, and minimally affected by chemical alteration and physical breakdown, meeting durability requirements for concrete sand.” These requirements can be verified by use of the various testing methods for friability, clay lumps, soundness, and impact resistance listed in ASTM C33. To ensure permeability and the self-healing nature of filters, the presence of fines (-#200 sieve size materials) in filter sand should always be limited to no more than about 3-4% at the source, and the presence of plastic fines should be prohibited altogether. Typical practice requires that particle breakdown during handling and compaction should result in no more than about 5% fines in place. This limit on break down is generally achievable with the typical durability and compaction requirements discussed herein. 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 at specific optimum moisture content. Rather, these materials exhibit their maximum dry densities when compacted either completely dry or nearly saturated. Drain materials, usually uniform gravels, are not influenced in their compactability by the presence of water, and are not suitable for conventional compaction testing or conventional field density testing. Conventional end-result compaction specifications (e.g., percent compaction specifications such as ASTM D698) are sometimes used for filter and drain materials, but they can be difficult to apply in the field. End-result compaction specifications based on relative density requirements (e.g., ASTM D4254) are also sometimes used, but the relative density test is notoriously difficult to apply in the field. Consequently, method specifications are often used for filter and drain materials. The difference between method and end-results compaction specifications are discussed in a subsequent section of this article. Photo 1. Placement of a 3-stage filter showing use of hand-held plywood shield to limit cross contamination. For most applications, the desired degree of compaction of filter and drain materials is such that sufficient strength is attained and settlement is limited. In locations subject to seismic loading, it is also necessary that filter materials be sufficiently dense to