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beneath the slide areas. The report concludes that the principal <br />• mechanism involved in reactivation of the slides is as follows, <br />"It is this progressive differential movement in the surface <br />colluvial on a steep slope which triggers the reactivation of <br />relatively unstable soil. The process of subsidence weakens the <br />support from the toe areas of the old landslide deposits and <br />allows the toe itself to drop and fall away from the rest of the <br />old landslide deposit. Most landslides are reactivated whenever <br />the support and the groundwater in the toe area is altered". It <br />should be noted that the landslides described in this study were <br />relatively small each having a width on the order of 200 feet and <br />a length of between 100 and 200 feet. Both were positioned <br />immediately above a creek at the toe of a steep slope. Although <br />10 separate old landslide areas (all larger than the two <br />• reactivated areas) existed above the longwall panels, only the <br />two small features were reactivated by mining. Aowever, the <br />report did conclude that the reactivation in the two areas was <br />clearly tied to subsidence. <br />8.3.2 Short-Term Effects <br />In our opinion subsidence on this site will have both short-term, <br />or temporary, effects and long-term, or permanent, effects. The <br />short-term effects are primarily associated with the development <br />of tensile ground strain at the surface. These zones of tensile <br />ground strain result in the development of tension cracks. <br />Considering the magnitude of subsidence movement versus the <br />• degree of topographic relief naturally present on-site, we would <br />anticipate no significant changes in surface drainage patterns <br />24 <br />