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7.0 Discussion of Stability for Long-Term Conditions <br />(Post-Reclamation) <br />The post mining or reclamation topography of the site is shown on Map 58, entitled `Post Mining <br />Topography," in Mountain Coal Company's (MCC's) existing permit. Long-term reclamation will <br />fill the ponds with soil and with appropriate drainage so they will not add infiltration to the <br />landslide. Figure 13 is a cross section of poet mining conditions. This figure indicates minor <br />changes in the geometry so that the existing geometry is a reasonable representation of the long- <br />term topography. As such, the long-term stability is based on the analysis of the existing <br />landslide. <br />The FLAC analysis shows that the toe of the landslide will remain stable for the life of the pins. <br />Based upon NBS Monograph 127-U.S. Department of Commerce/National Bureau of <br />Standards-NBS papers on "Underground Corrosion of Steel Piling," 1962-1971, the pins should <br />not deteriorate for greater than 40 years. Even after some deterioration, the pins will continue to <br />be effective so that the pins may be considered along-term corrective measure. <br />The results of the limit-equilibrium analysis, as shown on Figure 6, show that the landslide will <br />have a factor of safety of above 1.16 with the pins and greater than that with dewatering. Long- <br />term dewatering by the pond lining, stone columns (and horizontal wells), and reclamation <br />activities that do not concentrate water flow will further increase the factor of safety to above 1.2. <br />The likely increase in hydrostatic heads during wet conditions are shown on Table 2 and, based <br />upon the results of the stability analyses (as shown on Figure 6), the landslide will have a long- <br />term factor of safety greater than 1.1 and is stable. Therefore, the analyses indicate that the <br />overall landslide is stable in the long term with the existing corrective measures. <br />A comparison of the results of the surface movement monitoring information from April 1997 with <br />April 1998, as shown on Figures 14 and 15 from HZA, indicate that the toe of the landslide has <br />been stabilized where corrective measures have been implemented and that the landslide <br />movement in this region will remain constant. The piezometer responses in the stone columns and <br />the dewatering wells are inconclusive at this time to evaluate long-term conditions but will <br />continue to be monitored and evaluated. If short-term corrective measures are implemented (i.e., <br />buttress or portal retaining structure) they will tend to further stabilize portions of the landslide. <br />• <br />0626024\58991-LABM 14 <br />