1990-01-09_REVISION - M1988112 (6)

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?i~ 01 05 l8:[@ E 377 985 591? SFe DEU~!ER ~ 0: • BATTLE MOUNTAIN RESOURCES, SAN LUIS PROJECT CMIRD ADEQUACY LETTER RESPONSES Page 9 of 28 l5. 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 Phese 1 raises 1 and 1 in re)ation to the tails undergoing acceleration sufficient to produce this event should be specifirally evaluated. 6~~? f~.t is D~enlLta] 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 y~ver'ned 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, tf~roughout 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 a~.sumed liquefaction in this area cannot occur. in order to monitor the actual field performance of the drain system and stability of the embankment, pneumatic piezomet~rs will be Installed to measure the phreatic surface during actu,rl operation of the facility. These piezometers will be installed at tite base of the embankment, the upstream face of the embankment ami 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 to this area will not occur.