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shall have the appropriate engineering properties, so as to provide a <br />relatively unyielding surface upon which to place and compact the low - <br />hydraulic -conductivity layer. <br />• Low -Hydraulic -Conductivity Laver: A minimum one -foot thick layer of <br />clean low -hydraulic -conductivity soil containing no waste or leachate <br />placed over the foundation layer. The low -hydraulic -conductivity (or low <br />through -flow rate) soils shall be placed on top of the foundation layer and <br />compacted to attain a hydraulic conductivity, which is the lesser of either; <br />-6 <br />X 10 cm/sec. <br />The hydraulic conductivity of any bottom liner system or underlying <br />natural geologic materials. <br />Erosion Resistant Layer: A minimum one -foot thick layer of soil <br />containing no waste or leachate placed on top of all portions of the low - <br />hydraulic conductivity layer. Vegetation root depths must not exceed the <br />topsoil layer thickness. Vegetation is to be replanted, as needed, to <br />provide effective erosion resistance. <br />The final cover should be designed to allow for minimal maintenance. The <br />final grading design for areas flatter than 5:1 (horizontal:vertical) shall have a <br />gradient of at least three percent, to prevent ponding and accommodate <br />settlement. <br />Federal Final Cover Prescriptive Design Standard <br />The minimum final cover standards for the Forward Landfill, as outlined in the <br />closure criteria of 40 CFR, Subpart F, Section 258.60, include: <br />• A cover with a permeability less than or equal to the hydraulic -conductivity <br />of any bottom liner system or natural sub -soils present, or a permeability <br />no greater than 1 x 10-5 cm/sec, whichever is less. The barrier layer shall <br />consist of a minimum 18 inches of earthen material. <br />• A cover which minimizes erosion of the final cover by the use of an <br />erosion resistant layer that contains a minimum six inches of earthen <br />material and is capable of sustaining native plant growth. <br />3.2.1.2 FINAL COVER DESIGN <br />Introduction <br />Several factors were taken into consideration in determining the final cover <br />Forward Landfill PFCPCMP 3-2 <br />Z:\PROJECTS\Allied Waste\Forward\Reference Files Prior JTDWune 05 Stage IA Partial FCPCMP\FL-sec3.doc: 6/16/2008 <br />