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<br />' San Luis Mine Phase n, Raise 2 Design Report <br />' Although initial liquefaction triggering is predicted by the method of Seed and Harder, this method <br />does not distinguish true liquefaction from cyclic mobility behavior (McRoberts and Sladen, 1992). <br />Castro (1975) presents a methodology for determining whether or not a material is dilative or <br />' contractive from consolidated-undrained (CU) static triaxial test results and hence, if the material <br />is susceptible to liquefaction or cyclic mobility. From the results of the CU triatcial tests conducted <br />' on samples of the tailing sand and slimes from the San Luis facility presented in Section 2.5, the <br />tailings exhibit dilative behavior and should exhibit cyclic mobility rather than true liquefaction <br />i behavior. This is consistent with observations by Troncoso (1995) that tailings are frequently <br />' slightly dilative due to the angularity of the particles. Troncoso further states that the undrained <br />shear strength for such tailings is about equal to the drained strength. <br />' Nevertheless, for conservatism in the raise design it will be assumed that all saturated portions of <br />the tailings will exhibit true liquefaction behavior under the design earthquake loading. <br />' Accordingly, those portions of the tailings will be assigned a drastically reduced shear strength for <br />incorporation into the dynamic stability analyses as described in the following section. <br />4.2 Material Properties <br />In order to assess the stability of the proposed embankment raise conservative material properties <br />have been adopted in all cases to ensure that the evaluation of the stability of the structure is <br />conservative. <br />' I Consistent with the Amendment, the embankment fill has been conservatively modeled as a <br />' cohesionless fill with a friction angle of 35 degrees. Direct shear testing of two samples of the <br />Raise 1 fill material resulted in friction angles of 37.6 and 35.5 degrees with 364 and 618 psf <br />cohesion, respectively. If the apparent cohesion is neglected, i.e. a c=0 analysis, the two tests <br />result in friction angles of 40.8 and 39.7 degrees. In addition, comparison with typical published <br />ranges in friction angle for gravelly sand (Winterkorn and Fang, 1975; U.S. Navy, 1982) indicates <br />' that the use of 35 degrees with no cohesion is a conservative assumption. <br />For potential failure surfaces of the downstream slope which may involve sliding along the VLDPE <br />' liner, the liner has been modeled as a 0.2 fr thick layer with a friction angle of 18 degrees and no <br />cohesion. Review of technical literature on the subject and the results of direct shear testing of the <br />' soil to VLDPE liner interface for other projects indicates that a friction angle of 18 degrees is <br />conservative for VLDPE liner. A friction angle greater than 25 degrees is typical for the interface <br />between this type of liner and soil materials (ICriofske, et. al., 1990). Nevetheless, for lack of <br />' specific test data, a friction angle of 18 degrees was utilized to maintain consistency with previous <br />design analyses. <br />1 <br />~ Y 4-Z ro~ect o. <br /> <br />