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9/30/93 <br />Responses to Comments <br />San Luis Reclamation Evaluation Report <br />Page 2 <br />from the underdrain) flows from the water pool will be reduced <br />and the primary contributions to the underdrain will some from <br />the valley sides where deposition is thinnest and may still, in <br />places, be in direct contact with the underdrain. The purpose <br />of the underdrain is to prevent a head from formincg on the <br />liner, i.e. the development of a standing water table. As a <br />result, the design of the San Luis facility prevents a~standing <br />water table and as the tailings dewater and consolidate, the <br />material forms an impermeable barrier to downwaird flow <br />movement. <br />The assumption in the approved permit that no head is present <br />has been supported by the recent geotechnical investigation <br />completed for construction of the first embankment railse. The <br />results of this investigation were included in the report <br />entitled Phase II, Raise 1 Design Report prepared by Steffen tom,;,, ~~°{ <br />Robertson and Kirsten U.S. Inc. These results indicate that Y.~ <br />there is no water in(the piezometers installed in the maindd~~,'il7"~ S <br />embankment to monitor phreatic surface. <br />Once deposited, the tailings material is not saturated in the <br />sense that it remains a slurry, but rather it can be slaturated <br />in a geotechnical sense. By this we mean that the optimum <br />moisture content has been exceeded. The level of sa uration <br />for the tailings is approximately 26-27$ moisture cont~ nt based <br />on the tailings deposition modeling presented Appendix E of the <br />approved permit. In general, logging of the tailings samples '~ <br />for the Reclamation Report was performed usi~pg this aS ' ;,1~" <br />geotechnical definition of saturation. The logs indicated .~ ~= h~ <br />' <br />levels below 20$ moisture content to be dry or moist. Levels 1" ~ "' "` <br />I'~: <br />between 20-26$ were termed saturated, and anything a#~ove 26$ `'`~,,,~' <br />was termed supersaturated. The optimal moisture con ent for n ~~ <br /> <br />the tailings as determined in the mailings deposition yodeling ~ <br />^~,~{ <br />completed during the project design is 20$. The average ~,~,~'`~!~~~ <br />moisture content as measured during the recent geotechnical •~.!°' i~: ' <br />test work was 21-22$. °' <br />+~.~ ~ <br />{/~ ~~ <br />The impermeability of the tailings material was also measured 5 <br />«f"~~;~'~ <br />during the geotechnical test work recently completed for the px~'~ <br />embankment raise. Measured permeability values range from 5 x <br />10~ cm/sec to 7 x 10~ cm/sec (see Phase II, Raise 1 Design <br />Report, Appendix A). This is a lower permeability than <br />predicted in the permit application and should contkinue to <br />decrease at depth as additional tailings are added and <br />compaction occurs. <br /> <br />