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s <br /> STATE OF COLORADO <br /> MINED LAND RECLAMATION DIVISION oF.0 <br /> Department of Natural Resources <br /> 1313 Sherman St., Room 215 <br /> Denver,CO80203 * a� <br /> 303 866-3567 ** * <br /> FAX: 303 832-8106 <br /> Roy Romer, <br /> Governor <br /> Fred R Banta, <br /> DATE: January 22, 1991 Division Director <br /> TO: Harry Ranney <br /> FROM: Jim Pendleton <br /> RE: National g Co Mine - Refuse Pile Stability Analysis, PR-02 <br /> (Permit No: C-81-035) <br /> I have reviewed the stability analysis which you forwarded to my attention. <br /> This analysis, prepared by Fred Johnson, consultant geologist to NKC, was <br /> submitted in response to our earlier aaequacy comments. <br /> The methodology applied by Mr. Johnson consists of a manual evaluation of the <br /> original Terzaghi and Rankine pile stability equations. This technique is <br /> valid, but has several serious constraints which might render it inconclusive <br /> in this situation. The analytical technique graphically divides the body of <br /> the pile into two zones, that portion applying the load acting to displace the <br /> pile, and that zone supplying the resistance to displacement. In its simplest <br /> form, as applied by Mr. Johnson, the technique assumes a level and planar <br /> foundation beneath the pile. In addition, the evaluation of the equations <br /> requires the determination of values for the refuse density, the coefficient <br /> of friction of the waste material , and the angle of repose of the refuse. Mr. <br /> Johnson has uniformly applied a refuse material density of 86 <br /> pounds/cubic-foot, a coefficient of friction of 0.93, and an angle of repose <br /> of 42 degrees. No explanation is provided to document the derivation of these <br /> important mechanical properties for the waste. The applicant should provide a <br /> thorough discussion of the derivation of each of these mechanical properties , <br /> including a discussion of the expected uniformity of these properties <br /> throughout the refuse. <br /> Nay recollection of the Nat King Coal mine's surface facility area suggests <br /> that the foundation of the proposed refuse pile is neither level nor planar. <br /> In fact I believe the area dips away from the high wall of the face up. If my <br /> recollection is correct, this configuration is not conducive to stable <br /> placement of refuse. Further, the simplistic analysis performed by Mr. <br /> Johnson would not be adequate to determine the projected mass stability of the <br /> proposed refuse pile. Since Terzaghi and Rankine developed the calculation <br /> techniques in the early 1900's, considerably more comprehensive techniques <br /> have been developed. Slope stability analytical techniques, such as the <br /> Bishop Method and the Simpson Method, have been developed which can evaluate <br /> the stability of more complex pile configurations. <br />