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<br />Simple stand-pipe piezometers will be installed to verify <br />that the phreatic level is no higher than 10 ft above the <br />liner. The five existing vibrating wire piezometer will be <br />replaced by five stand-pipes spaced at approximately 200 ft <br />intervals across the 8590' crest of the existing main <br />embankment. <br />Tailings Consolidation <br />As outlined in your August 23, 1993 approval letter for <br />placement of the first 10 ft of fill material, settlement <br />markers are in place and are being surveyed daily. The <br />earthwork contractor's activities are more difficult to <br />accomplish with the settlement markers in the work area. <br />The primary concern with the settlement markers is the <br />possibility of markers being damaged due to equipment <br />hitting the stations and survey results being interpreted as <br />settlement. <br />Liquefaction and Stability Analysis <br />The following response to the Division's concerns regarding <br />the potential for deformation of the embankment caused by <br />earthquake shaking was prepared by Dave Hallman of Steffen <br />Robertson and Kirsten: <br />Stability analyses for the raised structure have <br />incorporated the residual or steady-state shear <br />strength for the potentially liquefiable tailings <br />identified in the CPT probes. As the shear strength <br />for a liquefied material cannot drop below this value, <br />this is a conservative 'first step' means for assessing <br />the stability of a tailings dam and the need for more <br />refined analyses. If the structure possesses adequate <br />stability utilizing this type of analysis (i.e. factor <br />of safety greater than unity) which represents the <br />worst possible condition that could develop for a dam <br />during an earthquake, more sophisticated analyses are <br />not required (Finn, 1982; Lo, et.al, 1982; Lo and <br />Klohn, 1990). Typically, the inertia forces caused by <br />the earthquake are not included in this type of steady- <br />state strength analysis; however, the more conservative <br />approach taken for the San Luis Project has been to <br />include the effect of the inertia forces in the steady- <br />state analyses. <br />Additional stability analyses have been performed for <br />the upstream slope of the dam raise to evaluate tlhe <br />factor of safety for failures which do not extends the <br />full height of the slope. These analyses incorpoirated <br />the same material properties and embankment geometry as <br />the stability analyses presented in the design report <br />and indicate that the factor of safety is higher for <br />potential failure surfaces which do not extend over the <br />full height of the slope. The attached Figures 1 and 2 <br />summarize the results of these additional stability <br />