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Evaluation of Bear No. 3 Mine Landslide <br />Trigger Mechanisms <br />This section evaluates the conditions which may have changed in the vicinity of the slide and <br />created either an increase in the driving forces and/or a decrease in the resisting forces which <br />control the static stability of the landslide mass. The potential mechanisms which will be <br />discussed are: <br />• Change in slope geometry, <br />• Subsidence, <br />• Over-steepened dry slopes, and <br />• Water. <br />Changes in the geometry of the slope can result in instability. This can occur if the toe of a slope <br />is excavated removing resisting force and/or if the head of the slide is loaded with additional <br />forces caused by new geometry or structural loads. A June 1990 high water photo by CDMG <br />shows bank erosion cutting the toe of the westernmost raveling slope. Field evidence and logic <br />support the thesis that 1997 high flows of the North Fork would have eroded the base of the <br />westernmost two raveling slopes. Some minor reconfiguration of the coal pile on the east end of <br />the recent landslide was done by Beaz Coal Company during mine closure. However, this <br />regrading does not appear to be significant in quantity when compared to the landslide mass. <br />More importantly the observed failure surface is well above the elevation a[ which [lte regrading <br />occurred. Little evidence exists which indicates a change in slope geometry triggered the recent <br />landslide movements. <br />Strains caused by subsidence as a result of coal mining can induce slope instability. This <br />mechanism was not evaluated as part of this investigation because monument survey data <br />necessary to conduct the evaluation was not collected in the slide area. However, mining <br />induced subsidence does not appear to be a trigger mechanism for this landslide. By evaluating <br />cross section A-A' on Figure 8 it appears sufficient B-Seam coal is present to support the slope in <br />the area of the current landslide. The remnant C-Seam coal is limited in extent under the <br />831-032.411 Wright Water Engineers, Inc. Page 12 <br />