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unyielding surface upon which to place and compact the low-hydraulic- <br /> conductivity layer. <br /> • Low-Hydraulic-Conductivity Layer: A minimum one-foot thick layer of clean <br /> low-hydraulic-conductivity soil containing no waste or leachate placed over <br /> the foundation layer. The low-hydraulic-conductivity (or low through-flow <br /> rate) soils shall be placed on top of the foundation layer and compacted to <br /> attain a hydraulic conductivity, which is the lesser of either; <br /> - 1 x 106 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 containing <br /> no waste or leachate placed on top of all portions of the low-hydraulic <br /> conductivity layer. Vegetation root depths must not exceed the topsoil layer <br /> thickness. Vegetation is to be replanted, as needed, to provide effective <br /> erosion resistance. <br /> The final cover should be designed to allow for minimal maintenance. The final <br /> grading design for areas flatter than 5:1 (horizontal:vertical) shall have a gradient <br /> of at least three percent, to prevent ponding and accommodate 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 of <br /> any bottom liner system or natural sub-soils present, or a permeability no <br /> greater than 1 x 105 cm/sec, whichever is less. The barrier layer shall consist <br /> of a minimum 18 inches of earthen material. <br /> • A cover which minimizes erosion of the final cover by the use of an erosion <br /> resistant layer that contains a minimum six inches of earthen material and is <br /> 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 /> design for the Forward Landfill, including the geometry of the existing landfill, <br /> Forward Landfill PFCPCMP 3-2 <br /> ):\Allied\Forward\2005.0051 Partial Closure\Partial Closure Plan\FL-sec3.doc:6/15/2005 <br />