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Revised 11/85 <br />Subsidence Effect on Unstable Slopes. Several ancient and modern <br />landslides have been mapped above the permit area of the Bear Coal No. 3 <br />Mine. This landslide material has been mapped and included on the <br />Geology/Hydrology Map (Map 6). As with many landslides in steep <br />terrain, they have not been known to be active in the recent past. Lack <br />of recent activity in the landslide area does not preclude future <br />activity. <br />Reactivation of ancient landslides of this type usually occurs as a <br />result of supersaturation of the landslide material or a combination of <br />supersaturated material with dynamic repeated loading. Reactivation of <br />the ancient landslide by either of these two mechanisms could occur in <br />the future. <br />Saturation of the slope could occur as a result of excess precipitation <br />in a given year or in a series of years. Subsidence at the Bear Coal <br />Mine will have no effect on the saturation of the material, as <br />• saturation is a function of precipitation. <br />Dynamic, repeated loading can occur as a result of certain types of <br />earthquake activity. In the repeated loading process, any solid <br />particles found within a saturated zone tend to become suspended within <br />the waters of the zone as a result of repeated vibrations. If the <br />vibrations are large enough in magnitude and close enough together in <br />frequency, the solid material can go into suspension and in effect <br />become a slurry material. This slurry material then provides a low <br />strength failure surface for potential landslide activity. The <br />prerequisites for this condition, however, are large magnitude <br />vibrations, combined with long-term repeated vibration. Individual <br />vibrations do not generally result in slope failures, as the pore <br />pressures have a chance .to subside between peak vibrations. <br />Subsidence at the Bear Coal No. 3 Mine will be areal type subsidence. <br />. This type of subsidence generally manifests itself on the surface as <br />gentle subsidence troughs over a large area. Since the topography above <br />the Bear No. 3 Mine is very steep, it is unlikely that troughs will <br />-26- <br />