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Geotechnical Evaluation of Mine-induced Earthquakes on Bruce Park Dam <br />Bowie Resources, Limited <br />June 2002 <br /> <br />4.2.2 Material Properties <br />Except for the failure zone, the materials at the landslide are similar to the materials at the <br />main dam. Therefore, the shear strengths of glacial till and bedrock at the landslide were <br />considered to be the same as those under the main dam. Based on an assumption that <br />considerable deformation has occurred in the failure zone we considered the failure zone <br />material to be at residual strength. We estimated the residual strength, which is expressed as <br />a drained residual friction angle 0,' using published data relating residual friction angle of <br />cohesive soils to liquid limit, clay fraction, and effective normal stress (Stark and Eid, 1994). <br />We also back-calculated the mobilized shear strength of the failure zone material based on <br />the following: <br />• The average effective mobilized cohesion of the failure zone is zero. <br />• Movement of the landslide was triggered by an increase in pore pressure caused by <br />precipitation and seepage through the slope, which resulted in a phreatic surface that <br />coincides with the ground surface. <br />• Adjusting the ?, of the failure zone until the computed factor of safety for a failure <br />surface along the top of bedrock was equal to 1.0. <br />• <br />The material properties used in the back-calculation analysis are summarized in Table 4.1. <br />TABLE 4.1 <br />MATERIAL PROPERTIES USED IN BACK-CALCULATION OF BEDROCK <br />SHEAR STRENGTH <br /> Strength Parameters <br /> Used in Back- <br /> Moist Saturated Calculation Analysis <br /> Unit Unit Cohesion Friction <br />Material Weight Weight C9 Angle ?' <br /> (Pcfl (Pcfl s de rees <br />Glacial Till 130 132 390 26 <br />Failure Zone 126 128 0 22* <br />Bedrock 133 139 2000 30 <br />Note: <br />* See Section 4.2.3. <br /> <br />4.2.3 Stability Analysis Results <br />The effective mobilized friction angle (residual) in the failure zone was back-calculated to be <br />22 degrees. The computer output of the back-calculated stability analysis is included in <br />02-05-22 Geotechnical Evaluation of Nlinnlnduced Eanhquakn <br />GEI Consultants, Inc. <br />13