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2.7 - Design of Filter B1BDket A filter b1aDket is usually needed beneath the riprap cover to prevent the water from removing bBDk material through voids in the rip- rap (~, p. 713). Removal ot bank material leaves cavities behind the riprap cover llDd failure of the cover might result, particularly if the riprap cover is rigid and cannot slump to continue contact with the sup- porting soi1. Whether a filter blanket is needed will depend upon the gradation of the bank material and the openings or voids in the riprap cover. For dUlDped riprap, a filter ratio of 5 or less between layers will usually result in a stable condition (12). The 1'11 ter ratio (~), is defined as the ratio of the 15 percentp&rtic1e size (D:i.5) at the coarser layer to the 85 percent particle size (>>as) of the finer layer. An addi- tional requirement for stability is that the ratio of the 15 percent particle size of the coarser material to the 15 percent particle size at the finer material should exceed 5 but be less than 40. This requirement can be stated thus: D:i.5 (ot riprap) ~5 (ot riprap) <40 Da5 (01' bank) < 5 < Da5 (01' bank) If a single layer ot tilter material will not satisfy the filter requirements, one or more additional layers of f11 tel' material must be used. The filter requirement applies between the bank material and the tilter blanket, between successive layers of filter blanket material if more than one layer is used, and between the f11 ter blanket and the stone cover. In addition to the filter requirements, the grain size curves tor the various layers should be approximately parallel to minimize the infil- tration 01' the fine material into the coarser material. lot more than 5 percent of the filter material should pass the No. 200 sieve. The thickness of the filter blanket ranges from 6 inches to 15 inches for a single layer or from 4 inches to 8 inches tor individual layers of a multiple layer b1BDket. Where the gradation curves 01' adjacent layers are approximately parallel, thickness of the b1BDket layers should approach the minimum. Thickness of individual layers should be increased above the minimum proportionately as the gradation curve of the material comprising the layer departs frOll1 a parallel pattern. Requirements for filters have been investigated by Bertru (14), the Waterways Experiment Station (~, 1.2, and 16), and the Bureau of Rec1lU118tion (17). "Design of Small Dams (1, p. 175) contains a typical filter design. An example of a filter design for dumped riprap tallows. It is assumed that riprap is to be used to protect a streambank with the grada- tion in the filter shown in figure 7. The gradation curves of the r1prap and of the sand and gravel available tor use are also shown on figure 7. 11-16 . ! : . # ~ .