Laserfiche WebLink
1 ~ <br />1 ~ ROCKY MOUNTAIN GEOTECHNICAL, INC. <br />Achieving Harmony with Earth <br />,January 10, 1984 <br />1 <br />Mr. Larry Reschke <br />' Grand Mesa Coal Company <br />P.0. Box 226 <br />Delta, Colorado 81416 <br />Refuse Disposal Area <br />Re Si Canvon Mine <br />' Dear Larry: - -~ - <br />It is our understanding that you require some additional <br />' information pertaining to stability of the above referenced <br />project. Our original information pertaining to the stability of <br />the refuse disposal area is contained in a report dated August <br />' 25, 1981 and in a letter to Ute Engineering and Surveying <br />Company, dated August 31, 1983. The factors of safety contained <br />in the original report are based on shear strength values for a <br />' sample of refuse compacted to an initial dry density of 78.7 pcf. <br />It is our understanding that some materials have been placed in <br />the refuse disposal area at densities well under 78.7 pcf and <br />that there is some concern with respect to the impact of these <br />' lower density soils on stability. <br />To date, we have not performed any actual shear strength tests on <br />' loose samples of the refuse material. The tri-axial tests <br />performed earlier is representative of a compacted material of 95 <br />percent standard proctor density-plus. Past experience with <br />' materials from other mine sites in Colorado would indicate that <br />residual strength parameters (those associated with low density <br />of loose soils conditions) are significantly lower than the peak <br />values typically obtained from compacted specimens. Our <br />' experience indicates that on the average friction angles are <br />reduced to approximately 75 percent of the peak value and <br />cohesion to approximately half of the peak value. Assuming this <br />' relationship would prevail fot this material, the friction angle <br />would fall from 28.5 degrees to 21 degrees and the cohesion would <br />drop from 200 psf to 100 psf. Using these values and stability <br />' charts to assess the potential impact on stability of the refuse <br />pile, indications are that the factor of safety for the proposed <br />slope would drop from 2.4 to 1.7 and, that for a 3:1 slope <br />condition the factor of safety would drop from 2.1 to 1.6. <br />' Therefore, based on this past experience, it is unlikely that the <br />lower density materials would cause factors of safety to fall <br />below 1.5, however, at this time these results are not based on <br />' either specific shear strength testing or rigorous computer <br />analysis. <br />' The Railroad Station • 555 E. Pikes Peak Avenue, Suite 107 • Colorado Springs, Colorado 80903 • (3037 634-1999 <br />