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Evaluation of Bear No. 3 Mine Landslide <br />landslide, however, coal in this area was mined more than ten years ago and surface strains <br />would have occurred prior to the recent instability. <br />Slopes in materials which have sheaz strengths only ntazginally higher than the slope angle <br />experience "creep." Creep occurs because the material in the slope must strain significantly to <br />mobilize the peak strength necessary to support its own weight. As the strain progresses, some <br />weak materials such as those found on this hillside, experience a decrease in shear strength from <br />the peak to the residual value. No change in moisture content is necessary for this stress-strain <br />reversal to occur. However, typically, the movement results in tension cracks which open in the <br />slide mass and allow the entry of surface water. Based on the available data, it is possible that <br />the recent landslide occurred in a neazly dry over-steepened slope. Available laboratory shear <br />strength data and limit equilibrium calculations on nearby slopes with similar angles indicates <br />they aze at or below a factor of safety of 1 without hydrostatic forces resulting from groundwater <br />flow. <br />The mechanism ofren responsible for initiation of landslide movement is water. Water has a <br />tendency to both increase the forces which drive a landslide and by way of saturation decrease <br />the soil strengths which resist the downslope movement. There appear to be three possible <br />sources of groundwater at this site: 1) surface precipitation which has infiltrated through the slide <br />mass to the colluvial bedrock contact, 2) flow within the B- or C-Seam workings behind the <br />landslide, and 3) groundwater flow independent of the mining operations. <br />Based on the available data, there is no evidence to support the last two sources of groundwater. <br />All observers of the landslide have reported no seeps, springs, or wet azeas on the slide mass. In <br />addition, with the exception of the lower over-steepened slopes which continue to ravel, the <br />major landslide movement took place in a relatively short period of time and has since stabilized. <br />If the slide was being driven by a significant hydrostatic pressure, we would anticipate that <br />movement would continue and/or a surface reflection of the seeping groundwater could be <br />observed. <br />831-032.411 Wright Water Engineers, Inc. Page 13 <br />