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STATE OF RADO DIVISION OE MINERALS AND GEOLOGY Department of Natural Resources 1313 Sherman St., Room 215 Denver, Colorado 80203 Phone. 1303) 866-3567 FAX: (303) 832-8106 DATE: July 31, 2000 TO: Bob Oswald FROM: Allen Sorenson RE: Highwall Stability, Gunnison Gravel, Dickerson Pit, Permit No.1VI-78-305 Bill Owens Governor GregE Walther Executive Director Michael B. Long Division Director 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'/::1 highwalls and 20 foot wide catch benches. Geologic information provided aze statements from Celia Greenman of the Co]orado Geological Survey that the unweathered granite and West Elk Breccia exposed in the pit are "very resistant, hazd 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 Boazd require that "If not eliminated, all highwalls shall be stabilized," (Rule 3.1.5(3)). The information provided by Gunnison Gravel fo 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 geolo>ic 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. 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 aze depicted on a Schnudt equal-azea net to evaluate the dominant fracture orientations. DIVISION O MINERAL. GEOLOG' RECLn MATIOt MIN IN G•SAFET` The strength of the rock exposed in the pit wall must be evaluated. The attached table 1 provides a useful rock classification system. Any azeas of rock that fall below the R2 strength classification aze not suitable for reclamation as a benched or terraced highwall, and should be reclaimed like an