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2 <br />In addition to the authors, the following persons were present during the field <br />investigation and related discussions: <br />Raymond DuBois, President/General Manager, Trapper Mine <br />Kevin Brown, Senior Mine Engineer, Trapper Mine <br />Karl Koehler, Safety Director, Trapper Mine <br />Sam Anderson, Miners' Representative, Trapper Mine <br />Billy Owens, Supervisory Roof Control Specialist, CMSH District 9 <br />Kathleen Kelleher, Mining Engineer, CMSH District 9 <br />BACKGROUND <br />As requested by CMSH District 9, an investigation of geological and geotechnical <br />conditions was conducted at Trapper Mining, Inc.'s Trapper Mine on October 17, 2006. <br />The Trapper Mine extracts coal from the Cretaceous-aged H, I, J, L, M, and Q seams by <br />the strip mining method, utilizing several draglines. The request was prompted by the <br />occurrence of a lazge ground failure event on October 8, 2006, that encompassed an azea <br />of approximately 300 acres adjacent to the active mining pits. <br />The Trapper Mine utilizes draglines to extract coal from several seams by sequentially <br />excavating a series of rectangular pits, which aze subsequently filled as mining <br />progresses across the reserve area. Broadly, two kinds of pits are developed based on <br />their orientation with respect to the moderately dipping strata. A "strike pit" represents <br />a pit that is mined approximately parallel to the strike of the coal seam, whereas a "dip <br />pit" is a pit that is mined pazallel to the dip direction of the coal seam. By definition, a <br />"strike pit" is approximately horizontal and follows the contour of the coal seam, <br />whereas a "dip pit" is inclined along the surface of the dipping coal seam. <br />The mine experienced a similaz issue in 2004, when highwal] degradation occurred in a <br />strike pit that was mined in the vicinity of the current failure. That investigation was <br />documented in a Technical Support memorandum to the District dated May 2, 2005. <br />The mine maintains a slope stability monitoring system and records a variety of <br />information including any movement of survey pins, crack displacements, piezometric <br />levels, dewatering volumes, and the inclination of underlying strata at which planaz <br />shear is occurring. The use of these instruments allowed the operator to withdraw <br />equipment and personnel from the G-5 pit prior to the October 8, 2006, failure. <br />OBSERVATIONS <br />Observations began at the northeast corner of the G-1 dip pit where the toe of the slide <br />is exposed, and proceeded up-dip to the reaz-bounding scarps of the slump azea of the <br />