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SHANNON 6WILSON. INC. <br />deposits in the valley to thin and grade into thinner colluvial deposits on the slopes above the <br />flatter valley floor. The estimated width of the deeper soil deposits in the valley floor ranges <br />from about 150 to 200 feet. <br />The bedrock consisted of very low to moderate strength interbedded sandstone and claystone. <br />Our experience at the mine indicates it is not uncommon for a relatively thin carbonaceous <br />mudstone (CMS) or carbonaceous shale (CS) layer to be present below coal seams. These layers <br />represent the weaker rock types with respect to shear strength that are commonly found in this <br />bedrock formation at the Colowyo mine. The CMS or CS layers are typically 1 to 2 feet thick <br />but can be thinner or thicker. <br />Groundwater was measured at depths of approximately 16 to 22 feet below the existing ground <br />surface during drilling in borings C- 101,103 and 104. Boring C -102, located further from the <br />creek, was dry. It should be noted that groundwater conditions during drilling are not a static <br />measurement, but rather reflect short-term conditions. Groundwater fluctuations may occur <br />depending on the time of the year and local conditions particular to the site. <br />We reviewed a seep and spring survey by Greystone Environmental Consultants, Inc. (2003). <br />This report indicates that three seeps /springs are located within the proposed footprint of the <br />spoil fill. The locations of the mapped seeps and springs are shown in Figure 4. <br />6.0 STABILITY ANALYSIS MODEL AND PARAMETERS <br />The following sections present our method of stability analysis including: <br />• The selection of two basic models to analyze (one with clay at the base and one <br />with CMS /CS at the base) and the geometry of the layers, <br />• Selection of shear strength and density values for the model layers, <br />• Groundwater conditions to be considered, <br />• Selection of pseudo- static acceleration parameters, <br />• Selection of variable combinations of these factors to include in a parametric <br />analysis, and <br />• Evaluation of the results of the stability analysis and development of <br />recommendations. <br />23- 1- 01101- 400- R1.docx/wp /Ikn <br />0 <br />23 -1- 01101 -400 <br />Exh. 23 -1 -13 Revision Date: 08/05/09 <br />Revision No.: PR -03 <br />