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Il v&4 e",J- D, U c7.14-s <br /> �f J <br /> 5.13 <br /> 5.2.7.2 Plastic Filter Cloths <br /> Plastic filter cloths are being used beneath riprap and other revetment <br /> materials, such as articulated concrete blocks, with considerable success. <br /> However, filter fabric is by no means a total substitute for granular filters. <br /> Filter fabric has the limitations of ( 1 ) filtering action is provided in only <br /> one direction, (2) only one Equivalent Opening Size (EOS) is maintained <br /> between the bed material and riprap, (3) fabric is less resistant to stone <br /> movement because of its relatively smooth surface, (4) additional of care must <br /> be exercised in placing riprap over plastic cloth filters to prevent damage, <br /> and (5) long term durability has not been proven. <br /> The Denver Urban Flood Control District Drainage Design Criteria Manual <br /> (in print) specifies that plastic filter cloths should not be used when slopes <br /> are greater than 2.5 to 1. This is due to the reduced resistance to movement <br /> afforded by the smooth fabric. A six- to nine-inch layer of granular material <br /> is also recommended to be placed over fabric cloths to prevent tearing during <br /> placement of the riprap. <br /> Care must be exercised in filter fabric installations where the seepage <br /> forces could be oriented parallel to the fabric. This could result in piping <br /> along the underside of the fabric and possible stability problems. Durability <br /> of filter cloths has not yet been established because they have been in use <br /> only since around 1967. However, inspections at various installations indi- <br /> cate little or no deterioration had occurred in the few (one to four) years <br /> that have elapsed since test installations. <br /> Applications for filter fabric should be evaluated in terms of the speci- <br /> fic advantages and/or disadvantages as compared to granular bedding. The eco- <br /> nomics associated with granular material availability, excavation, and <br /> placement of granular filters should be weighed against the economics of <br /> filter fabric. Obviously numerous site-specific factors determine the rela- <br /> tive merits of each method; however, for long term design on a surface mine <br /> granular filter layers are generally preferred. <br /> The following design criteria for plastic filter cloths were given by <br /> Normann ( 1975) in Federal Highway Administration Hydraulic Engineering <br /> Circular No. 15. <br /> For filter cloths adjacent to granular materials containing 50 percent or <br /> less by weight fines (minus No. 200 material) : <br />