2010-11-12_REVISION - M2009076 - Laserfiche WebLink

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-- , f V Coarse Fractio Before Test - Saturated Wet Density 113.9 lb/ft3 (1.5 ton/yd3) Moisture Content 25.3% Percent Saturation Level 81.7% Dry Density 90.9 lb/ft3 (1.2 ton/yd3) n Consolidation After Test - Under-saturated 118.5 lb/ft3 (1.6 ton/yd3) 25.5% under-saturated 94.5 lb/ft3 (1.3 ton/yd 3) Slimes Fraction Consolidation Before Test - Saturated After Test - Under-saturated Wet Density 110.7 lb/ft3 (1.5 ton/yd3) 121.4 lb/ft (1.6 ton/yd3) Moisture Content 43.3% 30.4% Percent Saturation Level 100.5% under-saturated Dry Density 77.2 lb/ft3 (1.0 ton/yd) 93.1 lb/ft (1.3 ton/yd) Important Impoundment Stability Measure - The consolidation data above provide for a standard of comparison for evaluating consolidation during operations, utilizing a core sampling procedure that has already been described earlier in Exhibit C. Core samples shall be determined under-saturated when testing reports Moisture Content data less than the Before Test - Saturated conditions in the above table. This condition demonstrates the impoundment is consolidating and that saturated, phreatic zones are being mitigated through proper dewatering and drying. Additionally, we have elected to incorporate standpipe piezometers into the design as a simple and effective means of measuring the saturated, phreatic surface level. Standing water will be measured in these standpipes if saturated zones exist and an embankment profile can be mapped. See revised Exhibit C for the standpipe piezometer testing method. Relating to this information, the DRMS asked a question on November 8`s: "Where did you source the Unit Weight Soil Data in the Synteen stability analysis?" Soil density is based largely on three variables: the mineral content, water content, and the effect of applied energy. A single, definitive soil density value is not a firm number and must be considered in a particular environment. To provide for a conservative design, Synteen utilized the TRI/Environmental data reported in the Proctor Density curves. The Proctor Curves report a maximum dry density (7d,y max) having an optimum moisture content (%MC). Maximum wet density, a more pertinent value, is calculated: ywet max ° (ydry max) (%MC), for coarse fraction ywat max = (104.3 pef) (1.114) = 116.0 pcf, max. wet density. for slimes fraction ywet max = (109.1 pcf) (1.161) = 126.5 pcf, max. wet density. This approach uses the worst case, heaviest wet densities to provide for the most conservative design. DRMS Statement in Review Item #2: "Analysis of impoundment with unsaturated slimes yields a safetyfactor of 1.33 under static conditions and 1.03 under pseudostatic conditions. The pseudostatic coefficient input was set to the peak ground acceleration for an earthquake with two percent in fifty years return period at Hukill Gulch, as obtained from earthquake. usgs.gov/hazards. These safety factors are unacceptably low, and complete dewatering of tailing is unlikely. " Venture Resources Response to Review Item #2: Venture Resources and Synteen Technical Fabrics have considered the above requirement and have used a value of 0.12 for the peak ground acceleration of an earthquake with 2% in a 50 year return period in the seismic stability analyses. Please seethe USGS Peak Ground Acceleration Map, 2% in 50 year return period data attached as anew appendix to the revised Exhibit C. Refer to the new design table of factors of safety contained in Item #I above. Our professional knowledge and experience (Venture Resources' staff has two Certified Professional Engineers and a Certified Professional Geologist) tells us that the tailings will dewater, dry and consolidate and that the impoundment stability only increases over the long term. Offhand, unsubstantiated comments such as "... complete dewatering of Page 3 of 5