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~ <br />,. ~:~ ~ ~ <br />~ ii <br />ii i ii <br />i iii iii i ~ <br /> <br />`I ~ STATE OF COLORADO <br /> <br />DIVISION OF MINERALS AND GEOLOGY <br />Depanmenl of Natural Resources <br />131 3 Sherman SL, Rwm 215 <br /> <br />Denver, Colorado 80203 DIVISION O F <br />Phnne: t3o3~ a66-3sg7 MINERAL S <br />FA.1: (3011 R32-8106 & <br /> GEOLOGY <br />DATE: April 23, 1999 NENINGMSAFEOTY <br /> <br />TO: Tom Schreiner Bdl Owuns <br /> Governor <br /> Greg E. Walther <br />FROM: Allen Sorenson Ete<olivC D,re<rnr <br /> Mii hael B. Long <br /> <br />RE: Mining Setbacks for the Protection of Structures, CAMAS Colorado, Inc., Drvi.ion Dne~lor <br />Cooley Reservoir and Fulton Wildlife Area, File No. M-99-034 <br />The above referenced permit application states the applicant's intention [o mine within 200 feet of <br />certain manmade structures. The applicant has provided slope stability analyses to demonstrate that <br />these structures will not be damaged by potential land sliding into the proposed pit. The purpose of this <br />memo is to describe the results of the Division's review of the stability analyses, and to provide a <br />tabulation of the type of additional information that will be required to approve mining within the 200 <br />toot setback established in the Construction Materials Act at 34-32.5-1 l5(4j(e), C.R.S. <br />1. Public Service Comaany Towers <br />The applicant proposes a 50-foot setback for the line of electrical transmission towers that bisect the <br />site. However, the applicant indicates that the minimum setback will be closer to 85 feet due to the <br />current configuration of the easement boundary and the location of the towers within the easement. The <br />statement that the actual minimum setback would be 85 feet appears to conflict with the discussion in <br />the stability analyses indicating that mining would occur within the easement. The applicant should <br />address this apparent conflict. <br />The enclosed stability analysis prepared by the Division, using the applicants proposed critical cross <br />section, soil strength values, and minimum safety factor of L5, demonstrates that a slope failure could <br />occur to within 12 feet of one of the towers. A second analysis was conducted on an approximation of <br />the slope configuration that would result from the initial failure in order to analyze the potential for a <br />progressive failure. The progressive failure analysis demonstrates that a failure surface with a safety <br />factor below the established minimum of 1.5 will develop subsequent to the initial failure and will <br />intersect [he transmission tower location. Based on these analyses, the Division cannot approve mining <br />to a 50-foot setback from the towers, <br />In order to establish an acceptable mining setback relative to the towers the applicant may either: <br />a) Commit to a 200-foot setback. <br />b) Conduct a geotechnical evaluation to deterniine the actual soil conditions present at the site and <br />input the values established through the evaluation into an updated stability analysis. If the <br />