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RATTLE MCUt1TAiN RESOURCES, Sriir L'uiS rnuJe~l CMLRD ADEQUACY LETTER RESPONSES Page 9 of 28 15. In order for the Division to make a more adequate determination of the long-term stability of the tailings disposal facility, a liquefaction potential investigation should be conducted. The stability of Phase 2 raises 1 and 2 in relation to the tails undergoing acceleration sufficient to produce this event should be specifically evaluated. Liquifaction Potential Liquifaction potential has been evaluated for the tailings facility in terms of the foundation, embankment, and tailings themselves. The facility is designed to retain the tailings solids and allow drainage of solution from the facility throughout operations. Liquefaction potential of a specific site is primarily governed by the moisture content, grain size, and density of the materials and the intensity and duration of the seismic loading. No liquefaction will occur if the material is not saturated or is at a sufficient density to inhibit collapse during seismic loading. At the San Luis tailings disposal area, the embankment will be constructed of permeable gravel materials with a drainage blanket on the upstream face and perforated pipes in a gravel blanket drain, throughout the impoundment and beneath the embankment itself. No phreatic surface will exist in the critical area of the embankment constructed for Phase 2 of the facility. The tailings depositional sequence will result in the deposition of relatively free draining sand adjacent to the embankment and will force the free water pool well away from the embankment. The deposition sequence and drainage system will result in a very low phreatic surface at the upstream toe of the embankment. The Phase 2 raises will be founded upon unsaturated tailings sand and the embankment itself, therefore, the design has assumed liquefaction in this area cannot occur. ,.~..__ .__..._ .n.,.,.. orde~to:moei~toc';the~acYual: field:performance.°o£'thrdrai~n[sysfea~ 1'nst'aTi`ed~fa. measure ~t~hreatfr3surface.during'~actua9;:alpeaai~i`d91 ofi>theAfaci,lfi:~. These piezometers will be installed at the base of the embankment, the upstream face of the embankment and within the tailings "beach" sands. Prior to construction of the Phase 2 raises, the location of the phreatic surface and density of tailing material will be established and its affect on stability evaluated to verify that liquifaction in this area will not occur.