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~~ <br />_~ <br />3 <br />I_ <br /> <br /> <br />Woodward-Clyde <br />Consultants <br />3.6 KEY ASSUMPIYONS MADE FOR SLOPE STABILITY ANALYSES <br />Various assumptions have been made in performing stability analyses for the axisting <br />height Dam No. 1 and for the raised emban]]mzent. The key assumptions that )have been <br />made include: internal geometry of tailing materials, in particulaz the width of tailing <br />sand; similarity of tailing materials that wmprise the tailing sands and tailing slimes; <br />phreatic surface and pore water pressure conditions; .overall slope of the raised <br />embanlonent; and that effective stress analyses aze appropriate for analysis of Dam <br />No. 1. Each of these assumptions should be confirmed and evaluated as Darn No. 1 is <br />raised. The classification and distribution of tailing materials should be invet~tigated as <br />the embanlonent is raised. We suggest that beach slope monitoring and profile: sampling <br />be performed frequently enough to monitor changes in conditions that may result from <br />changes in operations. This should also be done in conjunction with future piezometer <br />installations. These investigations should include sampling of tailing materials from <br />piezometer drill holes and along profile lines, and laboratory testing of samlrles. <br />The current practice of regulaz monitoring and review of the phreatic surface location <br />and pore water pressure rnnditions as indicated by piezometer water levels should <br />con:roue. Additional piezometers should be installed as the embankment is raised. We <br />recommend that the development of the dam monitoring system be continued in a <br />manner similaz to how it has thus faz. Additional piezometers should bentstalled at <br />about 25-foot vertical spacings along the existing study sections as the dam. is raised. <br />Additional piezometers may be appropriate to monitor specific azeas of the dam. At the <br />time of the next piezometer installation and tailing sampling work, we recommend that <br />tailing samples be tested to establish undrained strength properties, and that the stability <br />of the embanl~ent be evaluated using undrained strength chazacterization for the tailing <br />materials. Recent work in the geotechnical literature suggests that the stability of tailing <br />dams should be evaluated using tmdrained strengths. For Dam No. 1 we believe that <br />the stability using undrained strengths should not vary greatly from that depicted in this <br />study where drained strengths were used. <br />nssva» mm-ryerr 3-7 <br />