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\ • <br />• <br />Memo to Larry Oehler 4 August 6, 1993 <br />amplified ground motions at Mexico City can have a correlation at <br />Battle Mountain's San Luis facility. The arguments presented by <br />SRK for tailing in the vicinity of the dam are valid only in the <br />immediate vicinity of the dam. Approximately 600 feet upstream <br />from the dam face, where decant water is ponded, there As a mass of <br />tailing that is analogous to the soil columns that exhibited <br />amplified ground motions at Mexico City. These tails aYe more than <br />75 feet thick, saturated, and consist of materials similar to those <br />SRK has reported to have cohesion between 358 anii 486 psf. <br />Although the dam raise is not founded on this type of tailing, <br />amplified ground motions within these tails would be dilsruptive to <br />all tailing within the impoundment. Also, while tlhe Division <br />concurs that an earthquake on the nearby Sangre de C$~isto fault <br />complex would not produce low frequency, low strain ground motions <br />at the tailing. impoundment, there are numerous potentially active <br />seismic sources within 200 miles of the site, with MC~'s greater <br />than magnitude 7, that could produce such motions (Rirkham and <br />Rogers, 1981). <br />Regardless of the fore-going discussion, the Division Will accept <br />the design of the facility as submitted without further hnalysis of <br />the potential for amplified ground motions based on the following <br />points: <br />1. The tails in the immediate vicinity of the upstream raise <br />should not be subject to amplified ground motion. <br />2. A factor of safety of 1.3 will be achieved tlhrough the <br />buttressing affect of spigotted tails. <br />SUMMARY AND CONCLUSIONS <br />~ The operator should be required to define the location of the <br />phreatic surface within the embankment before proceeding with <br />construction. ~Phe operator should monitor and report on~the degree <br />of consolidation that occurs when tailings are loaded, at-d complete <br />remedial actions as necessary. s'The operator should address the <br />potential for lowering of the dam crest elevation due tJo settling <br />of the upstream portion of the raise during strong shaking.~The <br />Division should further require that construction of thle upstream <br />raise be supervised and certified by a Registered Prpfessional <br />Engineer, and that daily observation sheets be loggled by the <br />engineer. <br />cc: James Pendleton, DMG <br />James Stevens, DMG <br />attachments <br />