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999 <br /> The application of the filter criteria rules is indicated in Figure 8. <br /> The computations for applicaticns of filter criteria are shown in Figure 8a <br /> and the grain size distribution curves are shown in Figure 8b. The shaded <br /> zone on the right in Figure 8b shows the grain size distribution of typical <br /> mill tailings. The shaded zone on the left is the allowable range of grain <br /> size distribution for filter material to be placed adjacent to the tail- <br /> ings. The zone for the filter materials was established by application of <br /> the rules as indicated. <br /> The drain material adjacent to a tailings zone must fall within the <br /> shaded zone. If the sand and gravel to be utilized in the drain is too <br /> coarse and does not fall within that zcne, an intermediate zone may be <br /> required. In that case, filter criteria must be met between the sand and <br /> the intermediate zone and the tailings. Filter criteria must also be Bret <br /> between the intermediate zone and the sand and gravel in the drain. In dam <br /> construction, it is not uncommon for as many as three or four zones to be <br /> used in order to ensure filter criteria between all zones in the dam. <br /> Rule 6 is intended to prevent drain material or tailings from entering <br /> the holes or slots in the perforated pipe in the drain. Other criteria <br /> have sometimes been specified (Cedergren, 1977) to reduce the value of the <br /> ratio to 1.2 for slotted pipe and 1.0 for pipes having circular holes. <br /> In some cases, filter cloth or filter fabrics have been used to <br /> prevent migration of fine material into drains. Scme uncertainty exists <br /> regarding the potential clogging of filter fabric due to migration of <br /> slimes through the tailings and accumulation on the filter fabric. At the <br /> present time, filter criteria for filter fabrics have not been established. <br /> -16- <br />