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We have also investigated the risks of potential instability of the embankment <br /> from seismic problems. Information from the National Earthquake Center indicates <br /> that the latest earthquake in the Four Corners Area was epicentered at North <br /> Latitude 370 and West Longitude 1070, on January 22, 1966. This quake had <br /> an intensity of 7.0 on the Mercalli scale and was felt in the Durango- <br /> Silverton Area. Modern practice in tailings disposal has shown that sand tailings <br /> placed at over 60% relative density are not severely subject to sand <br /> liquifaction upon vibration. The data gathered during the field and laboratory <br /> work indicates relative densities of the sand tailings of 67% to over 100% <br /> were encountered. It is our opinion that sand tailings have sufficient <br /> density against a potential liquifaction problem. However, not all earthquake <br /> failures are of the sand liquifaction type, and a "circular" failure could <br /> possibly result because of vibration. <br /> Based on the above analysis, it is our opinion that increasing the <br /> height of the pond more than an additional 6 feet would reduce the factor <br /> of safety beyond acceptable limits. <br /> E? <br /> If it is desired to continue use of the pond additional stability should <br /> be added to the existing embankment. This can be accomplished by one of the <br /> three following methods. <br /> 1 . The existing embankment material and/or sand tailings can be built <br /> pp at a flatter angle to provide additional stability. This angle should <br /> be approximately 2.5 to 1 , (i .e. horizontal to vertical ), and the sand <br /> tailings should be compacted as they are placed to at least 95% of Proctor <br /> Density ASTM D-1557. <br /> r^. <br /> 2. An additional berm of earth could be placed at the toe of the fill <br /> embankment to increase the stability. This addition should make the <br /> • -7- <br />