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-, J <br />Ms. Alana Scott <br />July 28, 1993 <br />Page 2 <br /> <br />Liquefaction and Stability Analyses <br />$~N ROBERTSON AND KIRSTEN <br />The cone penetrometer (CPT) soundings were located within the central portion of the <br />raise foundation as this area has the highest potential for containing saturated tailings and <br />tailings slime due to the deposition methods practiced at the mine. Other azeas closer <br />to the periphery of the existing impoundment contain a greater percentage of tailings sand <br />and overall denser better-drained tailings as determined through the su~cial density test <br />work and hand auger borings discussed in Section 2.1 of the report. <br />I The numerical factor of safety (FOS) computed through any analytical procedure is <br />directly related to the conservatism which is employed in selecting inputl parameters. For <br />the same stability problem, incorporating non-conservative parameters will result in a <br />higher factor of safety than conservative parameters but the actual stability will be <br />identical. <br />As discussed below, the acceptance of a FOS of 1.1 is mitigated by several compounding <br />conservative assumptions. A FOS of 1.1 in this case conforms to generally accepted <br />values. As discussed in the Permit Amendment adequacy response No, 25, the stability <br />analysis approach for the San Luis Project has been to adopt nservative input <br />parameters and the lower calculated factors of safety which result. conservatism has <br />been imparted in the stability analyses for the dam raise through the f¢llowing: <br />• The embankment fill has been modeled with a friction angle of 35 degrees and <br />no cohesion. For the "dirty" sand and gravel used for construction of the main <br />embankment and proposed for construction of the raise, the friction angle is <br />probably several degrees higher and the material should exhibitl cohesion on the <br />order of several hundred pounds per square foot (psf). <br />• The inhomogeneous and anisotropic nature of the tailings has been considered in <br />the analyses by modeling all tailings as the weaker tailings slimes. A sample of <br />the slimes was tested for undrained shear strength in a triaxial shear test. As <br />discussed on page 3 of the report, the test results indicate a friction angle of 31 <br />degrees and 358 psf cohesion at 5 percent strain and a friction angle of 32 <br />degrees with 486 psf cohesion at 10 percent strain. The tailings were modeled <br />with a friction angle of 30 degrees and cohesion of 350 psf. <br />• The embankment fill to VLDPE liner interface friction angle was modeled as 20 <br />degrees. When tested in direct shear, sand and gravel typically exhibit a friction <br />angle of 25 degrees or higher relative to sliding along VLDPE liners. <br />• The tailings from elev 8552 to 8555 were indicated by the CPT work to be <br />saturated or neazly so. For all dynamic loading, this layer has been assumed to <br />