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' Sanborn Creek Subsidence Page 99 September 2, 1997 <br />room-and-pillar mining. The natural vegetation has apparently <br />• thrived in the subsidence areas in the past and should do the same <br />in the future. <br />PREDICTED LANDSLIDE SUBSIDENCE EFFECTS <br />_ Plate 4a and 9b (Panels #5 - #7) contain the outline of a <br />potential landslide, the dashed line. The westernmost part of the <br />potential landslide overlies the extreme northern 700-ft of Panel <br />#B. Surface inspection of part of the potential landslide did not <br />detect either a headwall crack or the hummocky surface normally <br />associated with active, or recently active, landslides. The <br />central portion of the potential landslide, which overlies Panels <br />#5 - #7, will be subjected to vertical subsidence and horizontal <br />strain. <br />The vertical subsidence accompanying longwall mining of third <br />panel group under the center of the potential landslide should <br />stabilize the potential landslide. Predicted worst-case lowering <br />of the center of a landslide should result in a minor decrease in <br />the slope of the sliding surface in the toe area by approximately <br />0. 6° (8-ft vertical over 780-ft horizontal), as shown by the <br />subsidence contours on Plate 4a. Predicted worst-case subsidence <br />of the center of the potential landslide should effectively <br />increase resistance to sliding at the lower southern downslope toe, <br />• i.e. buttressing and loading the toe of the slope. The slope at <br />the northern, higher, end of the potential landslide will, however, <br />be steepened by longwall mining of the second panel group by <br />approximately 0. 3° (6-ft vertical over 1075-ft horizontal). The <br />- effect of steepening a real landslide is to decrease its stability, <br />i.e. increasing the downslope thrust component. <br />~• The overall effect should be an increase in overall stability <br />of the potential landslide. If the potential landslide is simply <br />the vegetated slope it appears to be, subsidence should make the <br />slope more, not less, stable. This is true for all south facing <br />and sloping hillsides that will be undermined by the planned <br />!~ longwall B Seam mining. <br /> <br />I <br />I~l• <br />~- 44 <br />