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• Ms. Alana Scott July 28, 1993 Page 3 S~EN ROBERi50N AND KiRS7EN liquefy and to have a reduced shear strength at the start of the earthquake which is equivalent to its post-liquefaction residual or steady-state strength. Since the strength of a soil cannot fall below this value, this represents the worst condition possible during undrained loading (Finn, 1982) such as would arise during an earthquake. • Dynamic deformation calculations utilized a M=7.5 earthquake rather than the M=6.8-7.0 maximum credible earthquake event determined for the project site. Using the greater magnitude earthquake results in higher deformations. • Pseudostatic analyses assume that the earthquake ground motions act perpendicular to the longitudinal axis of the dam and out of the dam face. Actual earthquake ground motions are random and in all directions. Reducing the conservatism of the input parameters results in increasing the factor of safety. Utilizing a friction angle of 38 degrees and cohesion of 200 psf for the embankment fill and increasing the friction angle for non-saturated tailings to 31 degrees while maintaining all other parameters as per the previous analyses, the pseudostatic (a=O.15g) factor of safety for the upstream face rotational and wedge failure surfaces depicted on Figure 4.1 in the report increase to 1.39 and 1.21, respectively. These factors of safety correspond to the stability of the structure immediately following construction of the raise and prior to deposition of any tailings upstream of the raise. As subsequent tailings deposition raises the tailings surface elevation, stability of the upstream face will increase due to the buttressing effect of the tailings. With 5 ft of tailings deposition after construction of the raise, the pseudostatic factor of safety for the wedge failure surface depicted on Figure 4.1 in the report increases to 1.35. The probability that a large earthquake will occur at the site prior to tlhe accumulation of sufficient tailings to buttress the raise and increase the factor of saftity above 1.3 is extremely low. Assuming 1 year following construction of the raise is required to increase the tailings level by 5 ft, there is only a .00006 to .001 probability of experiencing O.bg at the project site during this period as per the original seismological site assessment. Prior to deposition of 20 ft of tailings upstream of the dam raise, the tailings surface will be below the elevation of the crest of the main embankment (elev 8590). Even in the event of a total failure of the raise prior to this point all tailings would Vbe contained by the main embankment. As the tailings level rises above elev 8590 the minimum pseudostatic factor of safety for an upstream failure using a seismic coefficient of O.15g will be well above 1.3. The minimum factors of safety for potential downstream failures will approach the values indicated in the repoR as the tailings level continues to rise.