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<br />through surface fractures at the head of the failure (the area had precipitation much <br />higher than normal), and local changes in the strike and dip of the coal seams and coal <br />measure rocks. In order to reduce the likelihood of experiencing future lazge, ground <br />failure events and to mitigate the effects of the current event, it is suggested that the <br />following steps be taken: <br />1. Up-dip strike pits should not be mined, especially where structure contours define <br />a dip of greater than 10 degrees. By definition, a strike pit follows the contour of <br />the coal seam, and undercuts the toe of a broad portion of a slope directly below <br />the maximum dip angle of the slope. A change in mining method or mining <br />sequence is only way to avoid this kind of ground failure. <br />2. Water infiltration should be controlled at high topographic elevations by the use of <br />geo-liners or diversion ditches. Particular care should be taken to ensure that <br />water is not running into open tension fractures that have already formed. The . <br />area had precipitation unusually higher than normal prior to the ground failure. <br />3. Dewatering around the slide area should be continued in order to remove water <br />that could reduce the internal angle of friction and increase hydrostatic pressure <br />along critical potential failure horizons. Because some coal measure rocks are <br />chazacterized by low permeability, a dewatering program may take much longer to <br />be effective. Judging from the slide area, the dewatering program should be <br />expanded far beyond the boundary of an active pit. <br />4. Overburden may have to be stripped from the topographic highs of the current <br />failure area, to reduce the weight of material contributing to the driving forces for <br />movement. This could be an opportunity to contour a slope that diverts water <br />from the slump area of the slide. <br />5. Slope displacement monitoring, including the use of survey points, georeferenced <br />airphotos, inclinometer measurements, and piezometers, should be continued. The <br />monitoring program has already proven its effectiveness by alerting mine <br />personnel to movement, thus allowing equipment and miners to be withdrawn <br />from the affected area prior to failure. <br />Report Prepazed by: <br />f~1~ ~ . <br />Dai S. Choi <br />Mining Engineer <br />'~Ii~rn~~l_,~ c <br />Sandin E. Phillipson <br />Geologist <br />