2000-07-31_GENERAL DOCUMENTS - M1978305

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DATE: July 31, 2000 TO: Bob Oswald FROM: Allen Sorenson RE: Highwall Stability, Gunnison Gravel,Dickerson Pit, Permit o. M-78-305 The proposed mine plan for the Dickerson Pit would leave a 200 foot high quarry face with an overall slope of 1:1 by creating five 40 foot '/2:1 highwalls and 20 foot wide catch benches. Geologic information provided are statements from Celia Greenman of the Colorado Geological Survey that the unweathered granite and West Elk Breccia exposed in the pit are "very resistant, hard rock." Ms. Greenman did not evaluate the weathered and decomposed granite or the quartzite that are also present in the quarry. The Rules of the Mined Land Reclamation Board require that"If not eliminated, all highwalls shall be stabilized," (Rule 3.1.5(3)). The information provided by Gunnison Gravel to this point is insufficient to make a determination that the proposed final highwall configuration will be stable. This memorandum outlines the type of information that will be required. The Division of Minerals and Geology must make a finding that the geologic conditions in the weathered and unweathered granite and the West Elk Breccia of the Dickerson Pit are favorable for steep pit slopes in order to approve the final configuration proposed by the Operator. Favorable geologic conditions relate not only to the nature of the intact rock, but also to the aspect of the final highwall,the orientation and condition of the geologic structure, and to the excavation practices used to create the final highwall. To facilitate a finding by the Division that conditions are favorable and to approve the highwall proposal, certain geologic investigations are required. 1. Fracture mapping is required to evaluate the influence of natural fracture weakness in relation to the intact rock. Natural fractures,joints, faults, and foliation normally control slope failure in rock slopes. Adverse orientation of these structures occurs when down dipping fractures or fracture wedges daylight in a highwall. This condition is illustrated in the attached figure 1. The Operator must measure the strike and dip of fractures and foliation exposed in the current pit to determine if any adverse orientations exist. A person experienced in measuring fracture orientations must be employed to conduct the mapping. Typically the results of the measurements are depicted on a Schmidt equal-area net to evaluate the dominant fracture orientations. 2. The strength of the rock exposed in the pit wall must be evaluated. The attached table 1 provides a useful rock classification system. Any areas of rock that fall below the R2 strength classification are not suitable for reclamation as a benched or terraced highwall, and should be reclaimed like an