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• <br />Ms. Alana Scott <br />July 28, 1993 <br />Page 4 <br />S~N ROBERTSON AND KIRS7EN <br />2. The displacement analyses presented assume liquefaction of the potentially saturated <br />tailings at elev 8552 and 8555 at the start of the earthquake ground motion and an <br />appropriately low shear strength for this material has been incorporated into the analyses. <br />While liquefaction substantially reduces the strength of this material, It still poses some <br />resistance to shearing and as demonstrated by the stability analysis is capable of <br />supporting the dam raise. Although not encountered in all of the CPT probes, this layer <br />has been assumed to exist beneath the entire raise. No other widesptlead saturated (or <br />nearly so) liquefiable tailings have been identified in the vicinity of the raise. <br />The deformation analyses account for cyclic loading pore pressure increases by assuming <br />that liquefaction has occurred and reduced the available resisting strength for this material . <br />to its post-liquefaction residual or steady-state strength (the lowest strength possible). <br />This is a common, widely accepted approach to dynamic deformation analytical <br />procedures (Ambraseys and Menu, 1988; Mazcuson, Hynes and Franklin, 1992). The <br />deformation analyses resulted in displacements which aze insufficient tb result in a loss <br />of freeboard which could result in escape of tailings. <br />3. The Mexico City earthquake cited has been extensively studied (Abbiss, 1989; Stone and <br />Yokel, 1987; Yang, Qi, Pavlin and Durelli, 1989). The correlation between that <br />earthquake and its effects and the San Luis Project's tailings impoungment, however, <br />cannot be made. The 1985 Mexico City earthquake involved approz)mately 35 to 50 <br />meters of very soft, saturated volcanic clay and a great earthquake (8+) at a considerable <br />distance from the site. As reported in the technical literature on the subject, the clay has <br />moisture contents which have been found to be higher than its liquid limit and in extreme <br />cases as high as 500 percent. The soft clay overlies additional sequences of denser <br />sediments. Amplification recorded at some Mexico City sites involved low strain low <br />frequency ground motions <br />In contrast, the San Luis tailings in the vicinity of the dam raise are predominately non- <br />saturated cohesionless materials with a maximum thickness of approxithately 25 ft and <br />the site seismicity consists of lesser magnitude events at close proximity to the site. Peak <br />ground accelerations close to an earthquake are typically associated wiflt higher strain, <br />high frequency ground motions. Thick sequences of soft cohesionless soils actually <br />dampen these ground motions rather quickly and the peak acceleradon at the surface of <br />the tailings could actually be lower than the peak bedrock acceleration (Seed, Ugas and <br />Lysmer, 1976; Justo, Jatamillo, and Garcia, 1989). <br />Furthermore, following liquefaction of any layer in a soil column, strong aground motions <br />are unable to propagate above this layer. The liquefied layer is similar ~in that effect to <br />base isolation systems used to protect structures in seismically active regions. Thus <br />following liquefaction of any tailings beneath the dam raise, the ground motions <br />propagating through the tailings, would be significantly reduced. Recent analysis of a <br />