2012-09-06_REVISION - M2008070 (25)

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Design and Operations Plan October 24, 2011 Western Gravel E & P Waste Disposal Facility Page 28 of 51 3.4.9 Final Cell Cover Placement 3.4.9.1 Final Cover System When a substantial portion of the cell has been filled to final grade the final soil cover system will be established. The final cover grade 6 (Horizontal) to 1 (Vertical) has been designed so that erosion of final cover is minimized, make sure that underlying waste materials remain stable, and prevent precipitation migration through the waste that could result in leach ate generation. The finish contours for Cell A, which will be typical for all of the cells are provided in Figure 7 . The vertical profile /cross Section for Cell A in a west to east direction is provided in Figure 8. Slope stability analyses completed on the final cell configuration provided above indicates that the finished slope will have a minimum factor of safety against failure of 4.3, which exceeds the recommended minimum factor of safety of 1.5. Slope stability calculations for the finished soil cover are provided in Appendix B. A waiver from CDPHE solid waste regulations Section 3.5.3(A)(1) regarding fmal cover permeability not exceeding that of cell liner is requested in accordance with Section 1.5 of the Regulations. Waiver request information is provided in Appendix A. The final cover system will be constructed with four components having a minimum aggregate thickness of 4 % feet. From bottom to top, final soil cover will consist of a minimum 6 -inch foundation layer; a geosynthetic clay lin er (GCL) membrane consisting of Bentomat CL or equivalent product (barrier layer); a biotic barrier consisting of a layer of 16 -ounce geotextile underlying 18- inches of cobbles and boulders or broken concrete fragments from 4 to 18 - inches in diameter and a layer of 6 -ounce geotextile placed over the top of the rock surface; a minimum two foot thick layer of unsaturated cover soils; and a minimum 6 -inch thick vegetative layer. Final cover system details are discussed below and shown on Figure 8. • The soils used in final cover will be obtained from future cell excavations and/or Phase 2 mining area. The soil balance calculations discussed above and estimated fmal soil cover volumes are provided in Appendix B. • The foundation layer and frost barrier layer will be constructed of on -site or imported soils free of organics, frost, or other deleterious materials. It has been assumed that these materials will consist of the sandy clay and/or clayey sand overburden materials located above site sands and gravel s. The foundation layer may be placed in one lift and will be track compacted using a dozer to obtain a suitable and stable base, prior to placement of the GCL barrier layer. • The barrier layer will consist of a GCL consisting of Bentomat CL or equivalen t product and prevent precipitation migration into the waste. The GCL will be installed directly over the foundation layer and extend a minimum of 2 feet beyond the anchor trenches. The GCL will be seamed by overlapping adjacent panels a minimum of 9 -inches. Seams sealing will be enhanced using a continuous bead of bentonite between panels. The GCL will be installed as described in the CQA/QCP and as shown on Figure 8 details. • A biotic barrier will be installed to prevent damage to the barrier layer from root penetration and burrowing animals. The barrier will also serve as a drainage layer above the barrier layer (GCL). The biotic barrier will be a minimum of 18 - inches thick and consist of 4 to 18 -inch diameter cobbles, boulders, or broken concrete fragments. A layer of 16 -ounce geotextile will be placed over the GCL and under the cobbles and boulders to protect the GCL from damage during placement of the cobbles and boulders. In addition, a layer of 6 -ounce geotextile will be placed over the gravels and cobbles to prevent soil infiltration and provide additional protection from root penetration. NWCC, Inc.