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_17_ <br />• the sedimentary rocks (overburden) a significant portion of the <br />available shear strength is represented by the cohesive nature of <br />the materials. The total shear strength available at any point <br />within the mass is represented by the equation ~=N tan p + C. <br />Determination of single design values for the angle of internal <br />friction and cohesion for the materials occurring within the pit slope <br />is not possible. A range of values for both p and C will occur de- <br />pending on the depth under consideration and the nature of the <br />materials. The ultimate length of the pit will be several thousand <br />feet long. The strength will vary along the length of the pit as well <br />as depth. The preliminary strength determinations are based on in- <br />formation from approximately 10 borings with a very limited amount of <br />laboratory testing. Them is enough variation from point to point <br />within the slope that an exact value of the shear strength for each <br />• layer is not warranted. In our evaluation of the available shear <br />strength, we considered the data available from laboratory shear tests <br />and triaxial shear tests made on5amples obtained in the drilling pro- <br />gram as well as back calculation of the existing slopes. <br />Analysis of Existing Slopes <br />The previous operator of this mine had made excavations up to 200 <br />feet in depth along the coal seams through material similar to those <br />which will 6e encountered in expansion of the pit. The previous ex- <br />cavations were conducted with angle on the west wall equal to the angle <br />of dip along the bedding. The east wall had been excavated an average <br />• <br />