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BATTLE MTN. GOLD SL I~19-672-3363 JUL ~93 1244 No .008 P.05 <br />STEEPEN ROBERTSON wND KiRSTEN <br />MS. Alana Scot[ <br />July 28, 1993 <br />Page 4 <br />The displacement analyses presented assume liquefaction of the potentially saturated <br />railings .at elev 8552 and 8555 at the scan of the earthquake ground motion and an <br />appropriately low sheaz 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 analysik is capable of <br />supporting the dam raise. Although not encounoered in all of the CPT probes, this layer <br />has been assumed co exist beneath the entire raise. No other widespread saturated (or <br />neazly 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 trsidual or steady-state strength (the lowest strength possible). <br />This is a common, widely accepted approach to dynamic deformation analytical <br />procedures (Ambtaseys and Menu, 1988; Marcuson, Hynes and Franklin, 1992). The <br />deformation analyses resulted in displacements which aze insufficient t0 result fn a loss <br />of freeboard which could result in escape of tailings.c~ N.,. ~~` f f~.f ff.~,~~-y ~,~., <br />a~ ~.-.x~..r ? <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 wrrelation between that <br />earthquake .and its effects and the San Luis Project's tailings impoundment, however, <br />cannot be made. The 1985 Mexico City earthquake involved approximately 35 to 50 <br />meters of very soft, saturated volcanic clay and a great earthquake (8+) m 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 S00 percent. The soft clay overlies additional sequences of denser <br />sedimenu. Amplification recorded at some Mexico City sites involved low strain low <br />frequency ground motions ~ <br />~f ~~• <br />In contrast, the San Luis tailings in the vicinity of the dam raise are predominately non- 5° +~~I,,,1~ <br />saturated cohesionless materials with a maximum thickness of approxitirately 25 ft and v <br />the site seismicity consists of lesser magnitude events at close proximity tlD the site. Peak <br />ground accelerations close to an eanhquake are typically associated wilt higher strain, <br />high frequency ground motions. Thick sequences of soft cohesionle$s soils actually <br />dampen these ground motions rather quickly and the peak acoeletation 3t the surface of <br />the tailings could actually be lower than the peak bedrock acceleration (Seed, Ugas and <br />Lysmer, 1976; Justo, Jaramillo, and Garcia, 1989). <br />Furthermore, following liquefaction of any layer in a soil column, strong ground 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. Recant analysis of a <br />