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Mine Waste and Geotechnical Engineering Division memorandum, which indicated <br />that ground movement occurred far beyond the expected influence zone of the recently <br />mined strike pit. The presence of water is interpreted to have been a necessary <br />contributing factor because the maximum 11.7-degree dip of the strata is slightly less <br />than the 15-degree internal angle of friction reported for intact shale. Kinematic <br />analysis with a stereonet suggests that sliding failure should not have occurred; based <br />on these values, unless some other effect reduced the internal angle of friction. <br />Similarly, a simple limiting equilibrium calculation suggests a very high safety factor <br />for the ratio of resisting forces to driving forces. The safety factor can be reduced when <br />a term of buoyancy or artesian pressure is included as the uplifting force of water. In <br />addition, these equations do not account for the effects of time and may also be <br />unsuitable for the very large volume of material involved in this slide. It is suggested <br />that imperceptible creep and water infiltration may have been occurring for nearly two <br />years, since the G-5 strike pit was mined in 2004. It is unknown when the first tension <br />cracks developed in the slump area a# the head of the rockslide, but they would have <br />acted as conduits for accelerated water infiltration, reducing cohesion above and below <br />aquicludes by lubrication and by buildup of hydrostatic pressure that may have <br />provided an uplift force. Subsequently, when the G-1 dip pit was mined, the potential <br />rockslide area lost confining stress on a second down-dip face. <br />Prior to the ground failure, the mine local had precipitation on October 2, 3, 5, 6, and 7, <br />totaling 1.04 inches. The precipitation data has been recorded at a measuring station <br />neaz Craig at latitude 40°30' N and longitude 107°32'W. In addition, the mine Locale <br />experienced higher-than-normal rainfall during the month of September 2006, the <br />month preceding the slide. The precipitation data shows that the region had a rainfall <br />of 1.36 inches on September 15 and .80 inches on September 16. The local weather <br />station recorded rainfalls on September 7, 8,10,14, 20, 22, and 22, for a total September <br />rainfall of 3.07 inches. This amount is more than twice that of September 2005 (1.33 <br />inches). Mine personnel further indicated that the September 15-16 rainfall represented <br />a hundred-year-storm event. The excessive moisture due to the rainfall could be a <br />contributing factor to the slide. <br />The local wet condition of ground is illustrated by the stream flow record of the USGS <br />stream gauging station located near Craig at latitude 40°28'51"Nand longitude <br />107°36'49" W. The stream flow of the Yampa River prior to the slide was more than <br />twice that of the 2005 measurement (figure 10). The plotted data is the daily mean <br />value and shows that the river flow of September 2006 remained higher than that of <br />September 2005 after the precipitation of September 15 and 16 2006. <br />RECOMMENDATIONS <br />The massive October 8, 2006, ground failure event at the Trapper Mine is interpreted to <br />have occurred due to a combination of undercutting the slope toe, water infiltration <br />