2018-10-12_REVISION - C1982056 (2)

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maximum recommended for access and maintenance, satisfies all slope stability design criteria. Factors of safety (FOS) for the various stability analysis conditions are provided in Table 6. Table 3-2: Computed Factors of Safety of 3H:1V Upstream and Downstream Stopes for Considered Analysis Conditions Analysis Condition Slope Factor of Safety Design Criteria (Minimum Fos) End -of -construction Interior 1.89 1.3 End -of -construction Exterior 157 1.3 Steady-state seepage Exterior 2.23 1.5 Pseudostatic seismic stability Exterior 1.91 1.0 3.3 Geotechnical Recommendations 3.3.1 Borrow Suitability Based on the observed field conditions during the test pitting program, the soil graded gradually from residual clay soils appropriate for embankment fill material to the shale bedrock below. In all but one test pit, the transition from clay to shale was encountered at a depth of 4 to 8 feet. Within the transition zone, an intermediate weathered weak shale was encountered that crumbled easily to a clay rich soft gravel type material that does not compact to a water tight material in the same way as the clay soil above. The material used as borrow for the embankment should be restricted to the overlying clay soil, and not extend to include the intermediate material that crumbles into a clay rich material. 3.3.2 Embankment Foundation Preparation The impoundment site is currently covered with grass. In addition, the uppermost 6 to 12 inches of the soil profile contains soil with organic matter that is unsuitable for embankment construction. Embankment foundation preparation will require stripping of the surface vegetation (roughly 6 inches) for disposal. The removed topsoil can be stockpiled for use as topsoil upon completion of the embankment. 3.3.3 Embankment Lining Prior to the geotechnical investigation, the conceptual study proposed that the constructed wetlands would be lined with high density polyethylene (HDPE) geomembrane. However, results from the investigation have identified that a clay liner will be suitable for this application to prevent leakage of water through the berms and bottom and to maintain separation of residuals within the passive treatment systems. For clay lined impoundments, it was determined that the maximum recommended permeability of materials used as fill is 1 x 10' cm/s. Based on the results of testing conducted in 2007 and the index properties of the materials tested within the project footprint in 2016, properly compacted borrow material is expected to meet permeability requirements. In addition, the native clay and shale underlying the proposed impoundments both have low permeability. Practical methods for ensuring adequate permeability performance include the following design considerations: • Construct the homogeneous embankment from clay compacted to 95% of standard proctor maximum density at a moisture content ranging from 1 point below to 3 points above optimum moisture content. PEABODY-FINAL_DESIGN_TM-20181010 !-i COPYRIGHT 2012 BY CH2h6 HILL INC.. COMPANY CONFIDENTIAL