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STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Department or Natural Resources I } 13 Sherman St.. Room 215 Denier, Colorado 80.01 D l v l s l O N O r Phone: 13031 8b6-7567 M[ N E RA L 5 FAC: 1303) 83?-8106 & GEOLOGY RECLAMATION MINING•gAFE TY DATE: December 26 2000 gill Owens , Gavemor Greg E. Walther TO: Tom Schreiner E"`°"ve D""`« Michael B. Long D~•~swn O~.ecca FROM: Allen Sorenson RE: Stability Analysis, Hall-Irwin Corporation, North La Poudre Resource, File No. M-2000-144 I have reviewed the stability analysis provided with the above referenced application. The closest structure to the proposed near vertical pit walls is a Sinclair gas pipeline. The application specifies a mining setback of 50 feet from the pipeline. The application further states that the pit depth will generally not exceed 20 feet. Typically, any mining setback in an alluvial gravel pit that is equal to or greater than two-times the maximum pit depth may be considered a safe, conservative, and prudent protection measure. Also, the Applicant provided limit equilibrium stability analyses yielding a slope stability safety factor of 1.5 for the gas pipeline. Although I do not necessarily agree that the sheaz strength for the claystone bedrock input to the analyses is sufficiently conservative for a critical cross section of the proposed slope, the Applicant's demonstration is additional evidence that the proposed 50 foot setback for the pipeline is satisfactory. (It is the Division's experience that residual strength weathered shale or claystone at the toe of a pit wall may be the weak layer that causes slumping or slope failure; residual strength weathered shale may be modeled using c=0 and ~=14 degrees). In summary, the Division can approve the mining setbacks proposed in the application. The Applicant should commit to mazking the underground utilities and establishing the setback lines oa the ground prior to mining. Of greater concern than the setback distances, is the potential for the lateral berm that will support the Sinclair pipeline following mining to be washed out during a flood. The Applicant must provide a design for the Division's review and approval that will demonstrate and ensure equalization of hydrostatic pressure between the two mining pods during a flood. This design must be approved prior to consideration of the reclamation peanit application for approval. c:~windowstpersanal~noM la poudrc dot