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1 <br /> • • <br /> We have also investigated the risks of potential instability of the embankment <br /> from seismic problems. Information from the Nati onal Earthqu ake Center <br />' indicates that the latest earthquake in the Four Corners Area was c~picentered <br /> at North Latitude 37° and West Longitude 107°, on January 22, 1966. This quake <br /> <br /> had an intensity of 7.0 o n the Mercalli scale and was felt in the Ourango- <br />' Silverton Area. Modern practice in tailings disposal has shown that sand <br />tailings placed at over 60% relative density are not severely subject to sand <br />' liquifaction upon vibration. The data gathered during the field ami 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 density <br />' against a potential liquifaction problem. However, "circular" failures might <br />possibly result because of vibration. It must be noted that the earthquake <br />potential of the site is considered to be low and circular failures from these <br />' conditions are not likely. However, the possibility of vibration should be <br />incorporated into design for long term use of Pond 3. <br />' Based on the above analysis, it is our opinion that increasing the height <br />of the pond more than an additional 6 feet would reduce the factor oi' safety <br />' beyond acceptable limits. <br />If it is desired to continue use of the pond additional stability <br />should be added to the existing embankment. This can be accomplished by one of <br />' the three following methods. <br />1. The existing embankment can be stabilized by placing sand tailings at <br />the front of the earthen berm. Placement of sand tailings without compaction <br />' should be approximately 2.5 to 1, (i.e, horizontal to vertical). This will <br />increase the safety factor for use of the pond to the additional 6 foot crest <br />height above 1.5. However, this is not considered a "long term" solution for <br />increasing the total height above 6 feet as the safety factor would de~:rease <br />' significantly with increasing height. <br />_7_ <br />