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0 <br />6.1 Model Development and Geometry <br />The proposed downstream slope face, 875 foot high, was considered the critical slope for <br />stability analysis. Below the toe of this slope, the X -seam mining will remove soil deposits and <br />bedrock from the underlying valley side slopes. However, the X -seam floor elevation is slightly <br />above the elevation of the valley floor in the center of the drainage. Therefore, material within <br />the lower, flatter portions of the valley floor will not be removed; this zone contains the deeper <br />clay soil deposits found in our borings. We therefore evaluated two general models - one based <br />on conditions near the valley centerline as found in our borings and the other based on the <br />conditions created to each side from mining of the X -seam. <br />The two analysis sections were developed based on Sections A -A' and B -B' at the locations <br />shown on Figure 2. The A -A' analysis section includes deeper clays below the spoil fill (model <br />designated as - Clay Layer at Base of Spoil). The B -B' section includes a weaker CMS /CS layer <br />remaining at the base of spoil after mining of the X -seam (model designated as — CMS /CS at <br />Base of Spoil). Following are details regarding each model. <br />6.1.1 Clay Layer at Base of Spoil Model <br />We generalized the soil conditions found in our borings to represent conditions along the <br />topography of the valley centerline. The generalized conditions included a 35 -foot thick clay <br />layer underlain by 5 feet of clayey gravel that is then underlain by bedrock. Water levels were <br />generalized to be located at a depth of 15 ft below ground. <br />As previously discussed, the width of the alluvial clay deposits in the valley is relatively <br />small, on the order of 100 to 150 feet. Within this zone, the thickness and depth of the clay is <br />deepest near the centerline of the valley and rapidly decreases from the creek towards the side <br />slopes. Therefore this model is appropriate when considering the stability of a small zone of <br />spoil about 150 feet wide located near the center of the valley. <br />Preliminary model calibration indicated two general failure modes were critical <br />depending on various combinations of model parameters (primarily groundwater). One <br />mechanism involved failure paths near the toe of the slope; this was designated as Smaller -Scale <br />Toe Failure. This is considered the most appropriate failure mode for this model. <br />The other failure mode we evaluated involved failure paths that entered further up- slope, <br />generally near the crest. This was designated as Larger -Scale Crest Failure. In our opinion, the <br />Larger -Scale Crest Failure mode in our two- dimensional model will significantly underestimate <br />23- 1- 01101- 400- R1.docx/wp /Ikn 23 -1- 01101 -400 <br />7 <br />Exh. 23 -1 -14 Revision Date: 08/05/09 <br />Revision No.: PR -03 <br />