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~. Memo to Joe Dudash Sanborn Creek Mine TR-32, Geotechnical Adequacy page 2 In my opinion, Oxbow should be required to demonstrate that the proposed Elk Creek mine portal fill will be stable. An appropriate slope stability analysis should be performed and submitted to the Division for review and approval. In order to complete this analysis, Oxbow will first need to complete appropriate material strength laboratory analysis of the proposed fill material. These material values may already have been determined during previous design exercises within the adjoining surface facility areas. It will be important to determine saturated effective stress properties for the proposed fill materials. Further, the Williams Fork claystones commonly demonstrate expansive characteristics upon wetting. For this reason shrink/swell analysis should also be performed to determine the appropriate density for determination of sheaz strengths under the loading conditions projected to exist within the portal fill. Once appropriate material strength properties have been determined, the analyst will have to project an appropriate phreatic surface for completion of the stability analysis. The analyst should project a plausible phreatic surface. The chosen phreatic surface should be hydrogeologicallyjustified, as a portion oC the analysis. It would also be informative to perform sensitivity analyses using various elevated phreatic surfaces, in order to determine whether the configuration of the phreatic surface might be critical in this situation. L` the phreatic surface proves to be potentially critical to the stability of the fill, remedial design techniques could be implemented to prevent the development of potentially detrimental saturation of the fill. Soil Stockpile above Elk Creek Portal Bench Oxbow proposes to place a soil stockpile on the valley slope east of the northern end of the Elk Creek Portal Fill. Given the demonstrated tendency of the steep slope valley slopes adjoining Elk Creek to fail, I am concerned that the imposition of this load may result in failure of the foundational slope. For this reason, I recommend that Oxbow be required to complete a slope stability analysis to demonstrate the stability of the proposed soil stockpile. The necessary material strength properties may be obtained simultaneously with those required for the analysis required for the Elk Creek portal fill. Surface Water Diversion -Elk Creek Borrow Area Oxbow proposes to excavate an existing landslide mass immediately west and up slope from the Elk Creek portals to control encroachment of the landslide and to provide borrow for the construction of the portal fill. Oxbow believes that excavation of the failed mass will remove the hazard oY failure onto the facilities. In the short term I believe their conclusion may be correct. In the longer term, perhaps several decades, the excavated Williams Fork bedrock will deteriorate from exposure to the physical and chemical weathering of precipitation and freeze- thaw. Infiltration of precipitation and melt water is particularly detrimental to slope stability of