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11 available, commercially-produced aggregates. Use of readily available materials can significantly reduce project costs. It is very unusual when readily-available commercial materials cannot be found to meet filter requirements. Typical readily-available commercial materials include ASTM, AASHTO, and state transportation department standard gradations. After the required filter gradations are calculated, gradations of readily-available materials should be reviewed for compliance. The availability of local suppliers producing the desired gradations should be verified before the gradations are specified. For most mixtures of sands, silts, and clays found in dams and foundations, ASTM C33 fine aggregate will meet filter requirements. Although, ASTM C33 fine aggregate is a suitable filter for a wide range of soils, the filter calculations should always be completed for the particular base soils being protected, to verify the suitability of the specified filter. If ASTM C-33 fine aggregate is suitable as a filter, then ASTM coarse aggregate gradation No. 8, AASHTO coarse aggregate gradation No. 8, or a similar transportation department specification is a suitable, filter- compatible drain material. If a drain pipe is included in the filter and drain system, the slots or perforations in the pipe must be sized to be filter-compatible with the soil material that surrounds the pipe. The guidelines published by the three federal agencies referenced above provide criteria for appropriately sizing pipe slots or perforations, although there are some variations among the three documents in this regard. Currently, the guidelines and policies of the principal federal agencies involved in dam design, construction, and operation indicate that geotextiles are not to be used for critical filter functions in dams and at locations that could not be relatively easily accessed for replacement. This includes geocomposite drains in lieu of sand filters. This is due to the potential for geotextiles to clog over time, be damaged during installation, or deteriorate over time. Clogging can lead to increased pore pressures within the dam, which may be unacceptable. Damage or deterioration could compromise filter function. For zoned embankments the chimney filter should be located immediately downstream of the core. In recent years the application of risk analysis to dam seepage issues has led many practitioners to design chimney filters that extend up to an elevation equal to the normal pool. Based on the potential for cracking and dispersion, filters may be extended to the flood pool level or even the dam crest. Now let’s look at some items to consider during construction of the embankment filter. Constructing Embankment Filters In design of a chimney filter drain, analyses are normally completed to determine the thickness of the filter and drain zones required to convey the estimated seepage flow rates. Normally these calculations result in relatively thin filter and drain zones and layers. In reality, the design thicknesses of the filter and drain layers are normally controlled by consideration of constructability, not seepage flow capacity requirements. In considering constructability, the designer must address the question of how thick must each zone be to ensure that the zone is continuous, with no interruptions. In typical filter and drain construction, the filter and drain materials are delivered to the dam in dump trucks and moved into the final location by loaders, dozers, or graders, after which they are compacted. Placement of chimney drains using this methodology is subject to what has been called the “Christmas tree effect.” This effect can result in portions of the filter not meeting the minimum specified thicknesses. As discussed earlier in this article, filter and drain materials are most commonly commercially-produced, processed materials, and, therefore, are expensive. As a result, there are always pressures to reduce the thicknesses of these materials and reduce cost. It is essential to resist any pressures to reduce filter and drain zone thicknesses to dimensions less than those that will reasonably assure satisfactory construction. Photo 1: “Christmas Tree” Effect