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<br /> <br />1 <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br />San Luis Mine Phase U, Raise 2 Design Report <br />4.0 STABILTTY ANALYSIS <br />4.1 Liquefaction Triggering <br />If a saturated granular material is subjected to cyclic loading, such as that produced by an <br />earthquake, it tends to densify. If drainage is unable to occur, the tendency to decrease in volume <br />results in an increase in pore water pressure. If sufficient load cycles are applied the pore water <br />pressure can build to the point that it is equal to the confining pressure. Thus the effective stress <br />becomes zero and the material loses virtually all of its strength . This phenomenon is frequently <br />referred to as liquefaction but actually includes two distinct types of behavior and is more correctly <br />referred to as initial liquefaction triggering. <br />As the pore pressures build up and the effective stress reduces the material will deform if subjected <br />to shear stresses. If the material is loose, shearing and deformation results in a tendency for <br />volume reduction or contractive behavior. This tendency causes the pore pressures to remain equal <br />to the confuting pressure and the material can undergo large deformations with very little resistance <br />and shear strains as high as 20 percent. This can occur even after the cyclic load applications have <br />ceased. This type of behavior, where the material can flow and deform with very little sheaz <br />resistance, is true liquefaction. <br />On the other hand, if the material is dense, shearing and deformation results in a tendency for <br />' ' volume increase or dilative behavior. This tendency causes a decrease in pore pressure and a <br />substantial increase in shear strength. Additional load cycles can cause the pose pressure to again <br />' increase to the confining pressure but further deformation causes a drop in the pore pressure. Thus <br />the pore pressure cycles up and down during the period of cyclic load application with momentary <br />' periods of zero effective stress and limited deformation and shear strains. This type of behavior has <br />been referred to as "cyclic mobility" by Castro (1975) or "peak pore pressure ratio of 100 percent <br />with limited strain potential" by Seed (1979) and does not entail a loss in sheaf strength. <br />' The liquefaction triggering evaluation method presented by Seed and Harder (1990) has been used <br />' to evaluate the liquefaction potential of the tailings which will underlie the embankment raise. <br />This method gives results that aze bounded by the liquefaction evaluation methods prescribed by the <br />Japanese code for tailings dam design (after Ishihara, et. al., 1981) and the Japanese code for bridge <br />' design (after Ishihara, 1993). Following the Seed and Harder procedure, liquefaction triggering in <br />significant portions of the saturated tailings is anticipated during a magnitude 7.0 earthquake <br />producing peak ground accelerations at the tailings dam site of approximately 20 percent of gravity <br />(0.2g). Liquefaction is anticipated to be triggered in virtually all of the saturated tailings during the <br />MCE design event, i.e. a magnitude 7.0 earthquake resulting in a peak ground acceleration (PGA) <br />of 0.6g. The liquefaction triggering calculations are presented in Appendix E. <br />n y q_~ rotect o. <br />