Laserfiche WebLink
<br />' San Luis Mine Phase H. Raise 2 Design Report <br />' The complexly layered and inhomogeneous nature of the tailings has been considered in the stability <br />analyses by modeling all tailings as the weaker tailings slimes. The unsaturated non-liquefiable <br />tailings have been modeled with conservative shear strength parameters consistent with the triaxial <br />' shear test data for drained (effective stress) shear strength. Accordingly, these tailings have been <br />modeled with a friction angle of 30 degrees and cohesion of 300 psf. For static and pseudostatic <br />' stability analyses the saturated tailings have been assigned a conservative shear strength with a <br />friction angle of 16 degrees and cohesion of 300 psf consistent with the undrained (total stress) shear <br />strength for the tailings slimes. <br />' Comparison with the laboratory derived shear strength parameters for the slimes presented in Section <br />' 2.5 indicates that these values are reasonably conservative. Lacking actual tailings materials and <br />therefore specific test data, design analyses presented in the Amendment conservatively utilized a <br />friction angle of 28 degrees and no cohesion. <br />' As presented in Section 4.1, the saturated tailings have been assumed to liquefy under the design <br />' earthquake loading. Due to the small ratio of cyclic shear strength to cyclic shear stresses developed <br />under the MCE earthquake loading, it is probable that initial liquefaction in the tailings would be <br />triggered prior to the conclusion of earthquake shaking for this design event. Accordingly, the <br />dynamic deformation analyses presented in Section 4.6.2 of the report conservatively assume that <br />liquefaction has been triggered in the saturated tailings at the start of the earthquake rather than at <br />some point part way through the earthquake shaking. <br />'. <br />Although the tailings are indicated to be dilatent and exhibit cyclic mobility behavior, a conservative <br />post-liquefaction residual or steady-state strength for this layer for use in the stability analyses was <br />selected on the basis of procedures recommended by Seed and Harder (1990) for deposits which <br />' have truly liquefied. <br />Following these procedures, the average SPT N-value normalized to 1 tsf overburden pressure and <br />' 60 percent of theoretical hammer energy, (N,)~, is determined for the material in question. This <br />(N,)~ value is then corrected for fines content to determine the average equivalent clean sand (N,)~ <br />' value, (N,)~s, which is used to estimate the post-liquefaction residual strength from empirical data. <br />The CPT data indicates average (N,)~ values of 9.9, 7.3,and 6.3 for the tailings sand, mixed tailings <br />and slimes, respectively. Laboratory testing indicates that these materials contain roughly 30 to 50, <br />' S0 to 70, and greater than 70 percent fines, respectively, and respective fines corrections of 3, 4, <br />and 5 have been employed. These fines corrections result in an average (N,)~~ value of 12.4. <br />Utilizing the average recommendations for residual strength presented by Seed and Harder for <br />liquefied deposits, the resulting residual strength is approximately 475 psf. <br />1 <br />u y 4-3 rotect o. <br />