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• 7.0 Discussion of Stability for Long-Term Conditions <br />(Post-Reclamation)-Revised September 21, 1998 <br />7.1 Introduction <br />The long-term stability of the West Elk Mine site is based upon the observational approach which <br />currently utilizes the analyses and measurements of movement for evaluating landslide <br />performance. The following discusses the validity of the analyses for long-term conditions, a <br />comparison of movements prior to correction and after correction, and a method to use for <br />determining the long-term stability with Bayes Theorem. <br />Validity of Geometry in Model <br />The currently planned post mining or reclamation topography of the site is shown on Map 58, <br />entitled "Post Mining Topography;' in Mountain Coal Company's (MCC's) existing permit. Long- <br />term reclamation will fill the ponds with soil and appropriate drainage will be constructed to not <br />add infiltration to the landslide. Figure 13 is a cross section of the currently planned post mining <br />• conditions. This figure indicates minor changes in the geometry so that the existing geometry is a <br />reasonable representation of the long-term topography. Since the existing geometry is <br />approximately the same as the final geometry after reclamation, the existing stability analysis is <br />suitable for estimating the long-term stability after reclamation. <br />Validity of FLAC Model <br />The FLAC analysis, included in Appendix D, shows that the toe of the landslide will remain stable <br />for the life of the pins. Based upon NBS Monograph 127-U.S. Department of Commerce/Nationa] <br />Bureau of Standards-NBS papers on "Underground Corrosion of Steel Piling;' 1962-1971, the <br />pins should not deteriorate for greater than 40 years. Even after some deterioration, the pins will <br />continue to be effective so that the pins may be considered along-term corrective measure. <br />Validity of Limit-Equilibrium Analysis <br />The results of the limit-equilibrium analysis, as shown on Figure 6, show that the landslide will <br />have a relative factor of safety of above 1.2 with the pins and greater than that with dewatering. <br />Long-term dewatering by the pond lining, stone columns (and horizontal wells), dewatering <br />trenches and reclamation activities that do not concentrate water flow will further increase the <br />• factor of safety to above 1.2. The likely increase in hydrostatic heads during wet conditions are <br />0626024\58991-1/YMH 15 Revised September 21, 1998 <br />