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Memo to Erica Crosby 2 March 23, 2005 <br />Modified Lunton Pit Mining Setbacks Permit No. M-2004-078 <br />highway. A discussion of the selection of a sufficient safety factor is applicable to these slope stability <br />analyses. These analyses are highly conservative in that they were designed to simulate the worst-case <br />scenario relative to stability. In particular, the shear strength input for the weathered claystone assumes that <br />this stratum has degraded to its residual strength. It is unlikely that this would occur in the field, and even if <br />it did happen, the degradation of the stratum would only occur for a few feet inward from the toe of the <br />slope to a point that will still be buried under a significant thickness of the sand and gravel deposit. The <br />pressure on the weathered claystone created by the weight of overlying sand and gravel would prevent <br />complete degradation to residual strength. <br />In order to analyze the effect of two geotechnical principles discussed above, DMG created analyses nos. 4 <br />and 5. The principles to be analyzed are the fact that lower safety factors result for deep-seated failure <br />surfaces under shallower pit slopes, and the fact that weathered claystone that remains under pressure from <br />overlying sand and gravel will not degrade to residual strength. In reality, weathered claystone subject to <br />pressure from the overlying material will maintain some component of cohesive strength. Analysis no. 4 <br />shows that if the pit wall were to be sloped from the toe of slope 100-foot setback location to the property <br />line, an unacceptable safety factor of 0.90 results. For purpose of this analysis, the location of the property <br />line is assumed to be 20 feet from the edge of the highway, and the resultant safety factor is for a failure <br />surface with a scarp at the edge of the highway. For analysis no. 5, DMG inputs assume residual strength <br />weathered claystone to a point 15 feet in under the sand and gravel body. From this point and for the <br />weathered claystone stratum further in under the sand and gravel a cohesion factor of 400 pounds per squaze <br />foot was applied. This is a highly conservative analysis relative to the protection of the highway, and it <br />yields an acceptable safety factor of 1.02. <br />Based on the Applicant's and DMG analyses the setback to Highway 85 proposed in the Mazch 22, 2005 <br />Civil Resources, LLC submittal is adequate. DMG did not prepare independent analyses of the other <br />setback proposals contained in the March 22, 2005 submittal, but has reviewed the Applicant's analyses and <br />has determined that these other proposed setbacks aze sufficient. All that remains for satisfaction of Rule <br />6.5 and the Geotechnical Stability Exhibit to the application is for the Applicant to provide revised maps as <br />committed to in the March 22, 2005 submittal. Once DMG has reviewed and accepted the revised maps, <br />the Geotechnical Stability Exhibit will be adequate, and need not deter the DMG from a recommendation to <br />approve the application. <br />attachment(s) <br />ca Carl Mount, DMG (via email and w/o attachments) <br />c:\Documen[s and Settings~acsVvty Documents\lupron pit stability final.doc <br />