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<br /> <br /> <br />Memo to Larry Oehler 3 July 21, 1993 <br />design to ensure tailing containment if deformation of the <br />embankment were to occur. <br /> 3. The operator should address the possibilit of amplified <br /> ground motions occurring within the tailing. 6ata from the <br /> 1985 Michoacan Earthquake in Mexico shows that certain <br /> combinations of soil column height and soft soil consistency <br /> can greatly amplify the base rock motions in the overburden. <br /> The San Luis Project tailing impoundment will contain zones <br />that are similar in consistency and height of overburden that <br /> greatly (up to 4 times) amplified bedrock motions in Mexico <br />City during the Michoacan Earthquake. If this effect was to <br /> occur at the San Luis Project, the seismic coefficient used in <br /> the stability analysis, which is based on an appropriate <br /> design earthquake, would not accurately model the ground <br /> motions within the tailing and would result in an inflated <br /> <br />// <br />~, ~,: ~~ ~ "''~' t <br />~`"' ~f i,~ <br />~~~~~^' ~ factor of safety. The simplest way to deal with the potential <br />for amplified ground motions within the tails, a though not a <br />rigorous solution, would be for the operator to ubmit design <br />modifications that would increase the dynamic fac, or of safety <br />for the embanlanent, as detailed in item 1 above. <br />SUMMARY; . <br />The open"~~'tbr has provided results of field investigations and <br />stabilitx.analyses adequate to demonstrate that the initial dam <br />raise will not ''compromise embankment .stability under static <br />loading, however, the information. supplied is '~~nsufficient <br />assurance of embankment stability under seismic loading. The <br />,~ operator must provide design changes that will '.satisfy the <br />criterion iletaYie3 above. ~~ " <br />The piezometers within the embatil®ent are not functioning properly <br />to provide reliable data. The piezometers are an integral <br />L component of the monitoring program to assure proper function of <br />the embanl~ent and must be replaced with reliable instruments. <br />cc: James Stevens, DMG <br />