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-16- <br />• 3. It is also necessary to establish a range of expected <br />shear strengths far the materials occurring within the <br />slope. <br />4. The geometry of the existing and final slope and any <br />surcharge loads must be defined. <br />Our field investigation was designed to develop a general pic- <br />ture of the orientation of bedding and the sequence of formations <br />occurring within the pit area. The thickness of overburden soils which <br />occur over bedrock and zones which either are very moist or carrying <br />free water significantly affect stability. On the basis of the condi- <br />tions encountered in the drill holes, evaluation of aerial photography <br />and observations of conditions exposed within the existing pit, we <br />developed a general subsurface profile for purposes of analysis. The <br />general profile used in the stability analysis is presented on Figs. <br />• 4 and 5. <br />After typical soils and bedrock had been identified, we made lab- <br />oratory tests and estimates of the strength characteristics of the <br />typical materials. For purposes or our analysis, we have used the <br />Mohr-Coulomb strength criteria. The soil or sedimentary rock is assumed <br />to possess a shear strength which can be evaluated in terms of an angle <br />of internal friction and cohesion. The angle of internal friction is <br />normally defined as 0• The frictional portion of shear strength avail- <br />able at any point is a function of the tangent of the angle of internal . <br />friction and the normal stress existing within the ground at the point <br />under consideration. Because of the clayey nature of the soils and <br />• <br />