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REPORT NO. MW06-065 SUMMARY As requested by Coal Mine Safety and Health (CMSH) District 9, an investigation of geological and geotechnical conditions was conducted at Trapper Mining, Inc.'s Trapper Mine on October 17, 2006, in response to a large mass wasting (ground failure) event that occurred on October S, 2006. The event encompassed approximately 300 acres of virgin area adjacent to the active mine pits. Overall, the event can be characterized as a massive rock slide because monitoring data suggest that failure occurred along one or more bedding horizons within or near the top of several coal seams. The toe of the slide exhibits characteristics of an earthflow, while the head of the slide exhibits chazacteristics of a giant slump. It is interpreted that when the G-5 strike pit was mined in 2004, support at the toe of the slope was undercut, and that initial downslope movement may have initiated at that time. The instrument data of the 2004 slide show that ground movement occurred far beyond the expected influence azea of the G-5 strike pit. When the recent G-i dip pit was excavated, additional confining stress was removed. It is likely that following initial, nearly imperceptible, downslope creep, tension fractures developed near the head of the eventual failure and provided conduits for water infiltration to lubricate or hydrostatically uplift potential slip surfaces along bedding horizons. Precipitation was much higher than normal prior to the ground failure. In order to reduce the likelihood of experiencing future large ground failures of this type and to mitigate the effects of the current event, it is suggested that the following actions be incorporated into the mining plan: 1. Up-dip strike pits should not be mined, especially where structure contours define a dip of greater than 10°; 2. Water infiltration should be controlled at high topographic elevations by the use of geo-liners or diversion ditches. Particulaz care should be taken to ensure that water is not running into open tension fractures that have already formed; 3. Dewatering axound the slide should be continued, recognizing that because the permeability of coal measure rocks is low, effective dewatering of the overburden requires a much longer time and larger area; 4. Overburden may have to be stripped from the topographic highs of the mass wasting area to reduce the weight of material contributing to the driving forces for movement; and 5. Slope stability monitoring, including the use of survey points, georeferenced airphotos, inclinometer measurements, and piezometers, should be continued.