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~ III IIIIIIIIIIIII III ~ STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Deparhnenl of Nawral Resources 1 31 3 Sherman SL, Room 215 Den.-er, Colorado 80203 Phone: 13031 a66-3567 FAx:13o31a3z-slob DATE: March 28, 2001 DIVISION OF MINERALS GEOLOGY RECLAMATION MINING•SA FETY TO: Tom Schreiner Bill Owens Governor FROM: Allen Sorenson Greg E. Walther EvecNwe Dnecmr RE: Protection of Nearby Structures Aggregate Industries West Central Region Inc. Michael R. Long ~ + > >Divisinn Direclar Fredstrom Resources Pit, File No. M-2001-016 I have reviewed Exhibit S to the above referenced permit application, including the slope stability analysis prepared to demonstrate that nearby swctures will not be damaged by mining. The Application states in Exhibit S that agreements will be forged with ditch owners to allow mining in close proximity or directly through existing irrigation laterals and ditches. As such, the permit should stipulate that no mining will occur within 200 feet of ditches and laterals until the agreements have been provided to the Division of Minerals and Geology (DMG) and accepted as part of the reclamation permit. As for the structures other than irrigation ditches, the critical area for analysis of slope stability is at the northwest comer of the proposed gravel mining operation. In the northwest comer, the excavation w-ill be at its deepest, 23 feet according to the drilling logs, and mining will occur at the closest point (50 ft.) to any of the stmctures In the mine vicinity. The analysis provided in [he application models [he shale bedrock underlying the gravel layer using relatively high shear strength. Thls analysis yields a safety factor of 2.1 for the northwest a~mer of the operation Typically, the shale bedrock in the area of the proposed operation will be weathered at the graveVshale contact, and the weathering will extend to a depth of several feet. The residual shear strength of the weathered shale may be much lower than the shear strength input to the Applicant's analysis. The DMG has completed an independent analysis of the critical cross section for stability at the Fredstrom Pit. DMG's analysis differs from the Applicant's in that a three foot thick layer of residual strength weathered shale is modeled at the graveVshale contact. The shear strength parameters input for the weathered shale layer are zero cohesion and an internal friction angle of 14 degrees. Also, the DMG's model assumes a near vertical highwall ac opposed to the Applicant's modeled O.SH:I V slope. The results of the DMG analysis are attached to this memo. A safety factor of 1.34 is output. Although this safety factor is much less than the 2.1 estimated by the Applicant. it is still sufficient to assure the protection of stmctures. Particularly since the 1.34 safety factor results from analysis of the worst reasonable case scenario for both shear strength and highwall angle. Based on the DMG's analysis, the mining limits depicted on the Exhibit C, mining plan map, may be approved. However, since the mining limits do not coincide with the proposed permit boundary, it is important to emphasize that the mining limits depicted on the ntap are not considered by DMG [o be generalized or flexible. Rather, these mining limits and the setbacks from structures stated in the application will be enforceable components of the reclamation permit. As such, the mining limits should be marked in [he field prior [o commencement of excavation. attachment(s) cc: Carl Mount. w/attachments c wmdows~persnnalVredsuom.doc