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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 /> - 1 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:\PROJECTSWIIied Waste\Forward\Reference Files Prior JTD\June 05 Stage IA Partial FCPCMP\FL-sec3.doc:6/16/2008 <br />