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Bruce Park Landslide Project No. 25-196 <br />Bowie Mine #2 <br />0 Observations and our subsurface investigation indicated the failure propagated upslope to a <br />point where the very hard, competent bedrock was near the ground surface. We also analyzed <br />the stability of the existing topography using the assumed softened strengths. Figures 7 and 8 <br />illustrate the existing landslide factors of safety ranging from 1.05 to 1.07 which indicates a <br />marginally stable slope. Our inclinometer measurements also indicated that slope movement <br />did occur over a several week period. <br />Seismic information from a previous longwall collapse event provided by Mr. Peter <br />Swanson (Appendix D), indicated the peak measured horizontal ground acceleration was <br />0.034g and the peak vertical ground acceleration was 0.0398 (g = acceleration due to gravity) <br />with magnitudes of 2.8 to 3.1 (ML). The data was collected at the West Elk Mine approximately <br />10 miles from the subject site. The values were measured at distances of about 2000 to 2800 <br />feet from the source. The data indicated that the peak ground accelerations were short duration <br />events in the tenths of a second range. Therefore, we estimated average horizontal ground <br />accelerations of 0.015g and average vertical ground accelerations of 0.017g. These longer <br />. duration magnitudes were used to analyze the influence of local seismic events on the landslide <br />stability. Typically, the peak ground accelerations are short duration (tenths of seconds) and do <br />not typically induce ground instability. Typically, the longer duration accelerations induce the <br />larger ground motions that influence landslide stability. The previous GEI Consultants report <br />(June 2002) estimated ground accelerations of about 0.30g for a site in Utah. Based on <br />information from the United States Geological Survey, the peak horizontal ground acceleration <br />in Utah for natural earthquakes is 0.20g with 10% probability of exceedance in 50 years and <br />0.03g to 0.07g for natural events in Colorado. Typically, a mine collapse would not create a <br />seismic event larger than a natural earthquake. Therefore, a horizontal ground acceleration of <br />0.015g was used for modeling the seismic effect on the landslide. Figures 9 and 10 illustrate <br />the effects of a seismic event and resulting factors of safety. The seismic factors of safety <br />ranged from 1.00 to 1.03 which indicated about a 3 to 5 percent reduction from the existing <br />stability. <br />We also reviewed work by Keefer (1984) which indicated that a minimum earthquake <br />magnitude of 4.0 (ML) was required to cause a landslide. Magnitudes less than 4.0 (ML), based <br />on the results of the paper, were not known to have initiated any landslide failures. The results <br />provided from Mr. Swanson indicated a maximum magnitude of 3.1 (ML) is anticipated. Based <br />on our analysis and the work by Keefer (1984), we believe the anticipated mining roof collapse <br />6