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q• <br />•1 y <br />filtering efficiencies for sandy sediments, there is considerable variation <br />l amonc both woven and non -woven fabrics when filtering the finer silt and clay <br />_^ particles. <br />1'. <br />Design Criteria. <br />1. No formal design is required. <br />2. Filter barriers shall have an expected usable life of 3 months. They <br />are applicable in ditch lines, around drop inlets, and at temporary <br />locations where continuous construction changes the earth contour and <br />runoff characteristics and where low or moderate flows (not exceeding I <br />cfs) are expected. <br />3. Silt fences, because they have a much lower permeability than burlap <br />filter barriers, have their applicability limited to situations in which <br />only sheet or overland flows are expected. They normally cannot filter <br />the volumes of water generated by channel flows, and many of the fabrics <br />do not have sufficient structural strength to support the weight of <br />water ponded behind the fence line. Their expected usable life is 6 <br />months. <br />kConstruction Specifications <br />2. Burlap shall be 10 ounces per square yard fabric. <br />?. hosts for Silt fences shall be either 4 -inch diameter wood or 1.33 <br />pounds per linear foot steel with a minimum length of 5 feet. Steel <br />posts shall have projections for fastening wire to them. <br />6- 30 <br />Materials <br />-. <br />1. synthetic filter fabric <br />shall be a pervious sheet of propylene, nylon, <br />polyester or ethylene yarn and shall be certified by the manufacturer or <br />supplier as conforming <br />to the following requirements: <br />PHYSICAL PROPERTY <br />TEST REQUIREMENTS <br />i ter►ng td ency <br />VTM-51 minimum <br />Tensile Strength at <br />VTM-52 Extra Strength - <br />20% (max.) Elongation* <br />50 lbs./lin. in. (minimum) <br />Standard Strength - <br />30 lbs./lin. in. (minimum) <br />Flaw Rate <br />VTM-51 0.3 gal./sq- ft./min. (minimum) <br />*Requirements reduced <br />by 50 percent after 6 months of installation. <br />Synthetic filter fabric shall <br />contain ultraviolet ray inhibitors and <br />stabilizers to provide a minimum <br />of 6 months of expected usable construction <br />life at a temperature range <br />of 0° F to 120° F. <br />2. Burlap shall be 10 ounces per square yard fabric. <br />?. hosts for Silt fences shall be either 4 -inch diameter wood or 1.33 <br />pounds per linear foot steel with a minimum length of 5 feet. Steel <br />posts shall have projections for fastening wire to them. <br />6- 30 <br />