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Page 2 <br />Homestake Tailings Dam <br />Meeting notes - 6/12/80 <br />i <br />wet. Probably involves some sort of sprinkler system. <br />Geotechnical Investigation (Ralph Mock and Curt Seally of Chen & Associates) <br />1. Extensive boring program. <br />28 holes beneath dam embankment. <br />26 holes throughout reservoir area. <br />14 test pits in reservoir area, 23 holes in proposed embankment borrow <br />area, 8 holes in proposed liner borrow area, 12 holes outside the area <br />of impoundment. <br />2. Basic Geology. <br />A. Basement rock consists of Paleozoic carbonates and shales. <br />B. Tuffaceous siltstones, channel sands and some airfall ashes overlie paleozoic <br />section. <br />C. Andesitic filled channels eroded into tuffaceous material. Andesitic <br />materials consist of andesite flows and colluvial deposits eroded from <br />the flows. <br />D. Left and right abutements both underlaying by andesitic channel fills. <br />(appears to have been topographic inversion due to armoring of channel <br />fills). <br />E. Andesitic channel deposits as deep as 300 feet beneath the existing <br />surface on both abutements. <br />G. Very large Landslide feature veneering the entire face of the backside <br />of the right abatement ridge. Now being extensively drilled by Chen and <br />Associates in order to closely define its limits and mechanics. Preliminary <br />analysis suggests feature now stable, estimated factor of safety for static <br />case 1.7. Further irk being completed and investigation should be <br />completed within the month (landslide feature on valley wall of Indian <br />Creek drainage basin). Channel fills appear to consist of unified soil <br />classification (GC-stiff(?]). <br />H. In valley bottom of tailings dam, Tuffaceous materials well saturated, <br />weathered, relatively soft-will be excavated prior to construction. <br />J. Alluvium deposited in valley bottom, varies from 2 to 20 feet in thickness- <br />wi11 also be removed prior to excavation. Approximately 60,000 yards of <br />material will be excavated prior to the construction of the dam and the <br />liner. <br />K. Permeability within the (GC) soils ocassionally are very high. Channels <br />filled with alternating andesitic flows and mud flows. Maximum permeability <br />approximately 1,500 feet per year. <br />3. Seismic Stability Analysis <br />A, Short term intensity 6 to 7 modified Mercalli scale. <br />B, Long term anticipated intensity 8 on modified Mercalli scale. <br />C. Source of seismic activity anticipated to be the Sangre deCristo fault <br />zone. <br />D, Assuming potential max. of Richter scale 7.2 earthquake, with a recurrence <br />interval between 3 to 5,000 years. Fault zone now considered quiescent. <br />Most recent quake appears to have been approximate 2,000 years before present. <br />No immediate quake threat recognized, <br />