2011-04-18_REVISION - M1981185 (20)

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Response to CN-01 Preliminary Adequacy Issues for a 112(d) Application 6.5 Geotechnical Stability • Bench stability including rock storage areas (May Day No. 1, May Day No. 2, Lamb, and the augmentation pond rock storage adjacent to the Idaho historic mill building. • Augmentation pond foundation and • The La Plata River bridge May Day No. 1 Portal and Mill entry collapse and head scarp The unconsolidated material (landslide material) located behind and above the existing steel sets at May Day No. 1 portals failed in the first quarter of 2010. Approximately 2000 cubic yards of unconsolidated material will be removed using an excavator. The material will be temporarily stored on the May Day bench while steel sets are extended into bedrock immediately below the contact of the unconsolidated material and the bedrock. The proposed construction workplan is summarized in attachment A. May Day No. 1 steel set stability analysis The May Day portal steel set geotechnical stability analysis is presented in attachment B. The soil bearing capacity is approximately 4500 pounds per square foot (psf) whereas rock has a bearing capacity of approximately 16, 0000 psf. Based on these criteria, the steel sets can be buried under a six foot soil cover. New Access Road-road stability including cut slope The new access road is designed by CLC Associates, Greenwood Village, Colorado 80111 under the direction of Mr. Daniel Madruga P.E. LEED AP. In addition, a geotechnical reconnaissance report as outlined in a May 29, 2010 memo (Attachment D) are adopted by reference and the road construction work plan as presented in section (Mine Plan Road Exhibit D). Bench Stability Bench stability including rock storage areas (May Day No. 1, May Day No. 2, Lamb, and the Idaho augmentation pond rock storage area Instability has not been noted since the Idaho, Lamb and May Day mine benches and rock storage areas were constructed. Future bench and storage areas will be limited to existing configuration where the outslopes will be limited to 1.5 (horizontal) to 1 (vertical) slope configuration. Drainage control design will be incorporated into all constructed structures and appropriate BMPs will be adopted to mitigate offsite erosion and sedimentation and to prevent the constructed structures from saturation. All future benches and rock storage areas will be located where ponding or material saturation is not possible. Augmentation pond stability When field access is granted, and as necessary, a geotechnical study of the augmentation construction activities will be conducted. Pond and rock storage foundation geotechnical and hydrogeologic characteristics will be determined prior to finalizing augmentation pond May Day Idaho Mine Complex 112(d) Permit Application Revised: April 14, 2011 2