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:; , ~ ~ III IIIIIIIIIIIIIIII STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Urpdnnu•m of N~lural Resources I l l 4 Sperman 51., Room 215 Denver, Qrluraclo 80207 D I V I S I O N O F Fhnne: 13us1 nr,a-7567 MINERAL $ rAx: an71 as?-nlob & G E O L O G Y March 3, 2000 R E C L A M A T I O N ' MINING•SAFETY 1vtr. Mike Refer CAMAS CO. Inc. sill ovens Governor 3605 S. Teller St. ere,; E. wakner Lakewood, CO 80235 Eveculive Director Mghael B Long RE: Third Technical Adequacy Review of Slope Stability Analysis for New 112 Reclamation D.~sion Direcor Permit Application, Permit No. #M-99-098~Riverview Resources Dear Mr. Refer, This letter is being written to address technical issues still outstanding in the Slope Stability Analysis submitted January 12, 2000, for the new 112 application submitted by your company, Permit No. #M-99- 098, Riverview Resources Pit. These issues must be resolved before the Division of Minerals and Geology (DMG) can approve this application. Please address the following concerns prior to the due date of March 3, 2000. I) It is understood that the software available for the analysis of slope stability conditions is limited in its ability to represent the true conditions at the site. However, DMG requests that no altering of the model mine face height or the represented phreatic surface be done in order to compensate for this software shortcoming. In our review of [he next data analysis, we will simply allow for the lack of free water surface in the de-watering trench (and the lack of de-watering trench) at the toe of the slope, and use our software program to find any differences in the safety factor caused by this omission. The phreatic surface should still intersect the tce of the mined face, as it would under real- life mining conditions. Please re-submit the data analyses wi[h the true heights and phreatic surface. 2) DMG is constrained from analyzing the slope stability of the proposed mining faces and suggesting setbacks based on such an analysis. We are required by policy and the regulations to review proposed setbacks and mining slopes submitted to us by the Applicant, and offer our technical opinion on the adequacy of the analyses. At this time, the analyses as submitted have omitted the weathered claystone soil layer which resides above the claystone bedrock. This layer is clearly shown in the Terracon soil borings as varying in depth from 0.5" to 2" across the site. Any slope stability analysis done for this site must include this soil layer. Tn the absence of site-specific soil testing data, the values for weathered claystone's cohesion, strength, and internal angle of friction as explicitly stated in our last technical adequacy review letter of March 1, 2000 must be used. Please re-submit the slope stability analysis and include this information. Target safety factors would be 1.15 for analyses considering residual strength, 1.25 for analyses considering fully softened strength, and I.5 for peak strength analyses. The Applicant may submit a permit revision to the Division for lesser setbacks once the pit is opened up and the weathered bedrock is exposed for observation and testing. The other alternative for decreasing the amount of required setback would be to obtain written waivers from the owners of structures in accordance with 34-32.5-115(4)(e), C.R.S. 3) It is understood that the fences do not add an ex[ta load to the soil stability analysis., However, they still qualify as permanen[ structures that must be included in geotechnical stability analyses if the