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.~ <br /> STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Drpanmenl of ~Nawral Rexnrces <br />111 1 Shaman SL. Room ! 15 <br />D~•m cr. ~p10lal10 00!11.5 <br />D I v 15 1 0 N O F <br />Pln,nr~ i;uJ. Sbn~}567 MINERAL S <br />cn; ljtnl++r_-slur, & <br /> GEOLOGY <br />IvlaCCh 1, 2000 RECLAMATION <br /> MIN IN G•$AFETT <br />Mr <br />Mike Refer <br />. o,,,,~„,^, <br />CAMAS CO. [nc. c°~°'^^' <br />3605 S. Teller St. c"" F "'''`he' <br /> F.r~„t~~e D~~e~t~r <br />Lakewood <br />CO 80235 <br />, ,,,~h,el a Long <br /> Division D6reaor <br />RE: Second Technical Adequacy Review of Slope Stability Analysis for N ew 112 Reclamation <br />Permit Application, Permit No. #M-99-098, Riverview Resources <br />Dear Mr. Refer, <br />This letter is being written address technical issues still outstanding in the Slope Stability <br />Analysis submitted ]anuazy 12, 2000, for the new ] 12 application submitted by your company, <br />Permit No. #M-99-098, Riverview Resources Pit. These issues must be resolved before the <br />Division of Minerals and Geology (DMG) can approve this application. Please address the <br />following concerns prior to the due date of March 3, 2000. <br />1) The applicant states that overestimating the height of the stability model will make up for not <br />including the presence of the de-watering trench at the toe of the excavated slope, and that <br />altering the phreatic surface throughout the analysis will account for any effects to the water <br />surface. We disagree. The increase in height may distort the stability analysis through the <br />layers. Please analyze these slopes with an open water surface at the toe and no increase to <br />the height of the excavation. <br />2) Three potential strength conditions aze possible in claystone, referred to as peak, fully <br />softened, and residual strength. Under short-term conditions for unsheared claystone, peak <br />strength governs behavior. Removal of overlying gravel changes pore water pressure and the <br />overall stress regime (stress relief). This leads to an increase in pore water pressure, <br />following an initial drop, and a corresponding drop in cohesion. Over time, the cohesion <br />drops to zero without a significant change in the friction angle leading to the fully softened <br />condition. If a sheared plane or zone is present within a claystone, as a result of faulting, <br />slippage between beds due to folding, or stress relief, the cohesion along the shear zone drops <br />to zero and the angle of internal friction is reduced due to alignment of clay minerals parallel <br />to the shear plane or zone. <br />