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Evaluation of Bear No. 3 Mine Landslide <br />' Geotechnical Analyses <br />This section compares the results of previous stability analyses conducted on a neazby hillside <br />with the field observations made at the subject landslide. The theoretical calculations confirm <br />' the field observations that the current landslide is a shallow translational failure with a failure <br />surface in the upper and middle portions, which is located near the contact of the colluvial <br />overburden and the underlying bedrock surface. <br />' The stability of the slopes in the vicinity of portals and access roads for the Bear No. 3 Mine site <br />just east of the subject landslide was the focus of an investigation conducted for Bear Coal <br />' Company by Rocky Mountain Geotechnical submitted on April 16, 1982. The results of this <br />report which included the collection of field data at five test hole locations and laboratory testing <br />to develop shear strengths are summarized in Figure 9, Sensitivity Analysis. <br />The subsurface conditions encountered generally consist of a varying thickness of colluvial soil <br />overlying interbedded sandstone, siltstone, and shale bedrock. The vertical thicknesses of <br />' colluvium utilized in the stability analysis typically was on the order of 30 Y'eet. However, <br />thicknesses of the colluvial layer varied, being thinner at higher elevations and thicker at the base <br />' of the slope. No free water was observed in any of the colluvium materials on site at the time of <br />drilling. <br />' Rocky Mountain Geotechnical analyzed the stability of five proposed topographic cross sections <br />(A through E) using peak and residual shear strengths in the colluvium and two different <br />groundwater conditions. The location of Sections A and D, which represent the range of slope <br />angles evaluated, are shown on Figure 8. The residual shear strength was a combination of a <br />' friction angle of 16° and a cohesion of 210 pounds per square foot. Peak shear strengths were <br />based on a friction angle of 23° and a cohesion of 360 pounds per square foot. Assumed <br />' groundwater conditions included no seepage and a 4 foot seepage zone at the bedrock/colluvial <br />interface. Bedrock was assumed to have a friction angle of 33° and a cohesion of 4000 pounds <br />per square foot. <br />831-032.411 Wright Water Engineers, Inc. Page 9 <br />