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iii iiiiiiiiiiiuiii <br />STATE OF COLORADO <br />MINED LAND RECLAMATION DIVISION <br /> <br />Department of Natural Resources of co <br />~~. ~\~Oy~ <br />1313 Sherman Sl.. Room 215 1`~S <br />Denver, CO 60203 + %. ~^ `j <br />303 666-3567 •~~•- %• <br />Fax: 303 632-8106 'ra'r6' <br />Roy Romer, <br />Governor <br />Fretl R Bania. <br />Drvrson D•retior <br />DATE: March 2, 1990 <br />T0: Berhan Keffelew <br />FROM: Jim Pendleton ~'`"`°' <br />RE: Rimrock rip Mine lication - Supplemental Adequacy Review <br />c nical Topics (Permit No: C-89-074) <br />I have completed a review of the geotechnical aspects of the supplemental <br />submittal information you forwarded to me. I return the following <br />observations for your consideration in preparing your findings. <br />Subsidence (Rules 2.05.6(6) and 4.20) <br />The applicant has modified the original submittal to include photography to <br />document the existing surface disturbance due to subsidence at and near the <br />proposed permit area. The application adequately addresses the regulatory <br />requirements with one omission. As I mentioned in my earlier adequacy <br />memorandum to Jim Stevens, dated April 18, 1989, I am concerned regarding the <br />possible stimulation of subsidence adjacent to the proposed strip pit. The <br />applicant proposes to excavate areas containing existing underground workings <br />to recover remaining coal resources. It is possible that in doing so, <br />subsidence may be stimulated in adjacent existing underground workings. <br />I recommend that we stipulate that the operator will be responsible for repair <br />of any subsidence which might occur in existing underground workings within <br />sane set arbitrary distance of their strip pit boundaries, such as 250 feet. <br />It would be necessary to complete an aerial fly over prior to their <br />commencement to document the pre-disturbance conditions at and near the site. <br />Use of Explosives (Rules 2.05.3(6)(a) and 4.08) <br />The supplemental submittal resolves one of the major concerns expressed in my <br />memorandum to Jim Stevens of April 18, 1989. The applicant requests <br />permission to blast within 50 feet of a buried 8 inch diameter natural gas <br />pipeline which crosses the property. Rule 4.08.4(7) requires that the <br />Division specifically approve a request to blast closer than 500 feet from <br />such a facility. The amended submittal contains supportive information <br />prepared by Greg Lewicki & Associates, Geosonics, Inc. and Vibra-Tech <br />Engineers, Inc. These consultants to the applicant demonstrate that this gas <br />pipeline should be capable of withstanding a peak particle velocity of 6.5 <br />inches per second with a factor of safety of 2. Further, they demonstrate <br />that if the explosive detonation is limited to less than 45.6 pounds per <br />