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<br />Memo to Larry Oehler <br /> <br />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 possibility of amplified <br />ground motions occurring within the tailing. Dada 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 gverburden. <br />The San Luis Project tailing impoundment will contain zones <br />that are similar in consistency and height of overlDurden that <br />greatly (up to 4 times) amplified bedrock motions in Mexico <br />City during the Michoacan Earthquake. If this affect 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 />factor of safety. The simplest way to deal with the potential <br />for amplified ground motions within the tails, although not a <br />rigorous solution, would be for the operator to submit design <br />modifications that would increase the dynamic factor of safety <br />for the embankment, as detailed in item 1 above. <br />SUMMARY <br />The operator has provided results of field investigations and <br />stability analyses adequate to demonstrate that the initial dam <br />raise will not compromise embankment stability under static <br />loading, however, the information supplied is insufficient <br />assurance of embankment stability under seismic loading. The <br />operator must provide design changes that will satisfy the <br />criterion detailed above. <br />The piezometers within the embankment are not functioning properly <br />to provide reliable data. The piezometers are an integral <br />component of the monitoring program to assure proper function of <br />the embankment and must be replaced with reliable instrL~ments. <br />cc: James Stevens, DMG <br />