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0 <br />B-4. For the north wall the minimum factor of safety was 1.4 at a slope angle of 65 <br />degrees. <br />The sidewalls of the box cut (east and west) were modelled as 350 foot high <br />slopes to include excavation from the ground surface to the F-Seam. At a slope <br />angle of 65 degrees the factor of safety approaches 1.2, the minimum considered <br />acceptable. A 100 foot high intermediate bench slope was also analyzed. Factors <br />of safety calculated ranges from 2.3 to 1.8 for slopes ranging from 60 to 80 degrees. <br />At steeper slope angles the effects of toppling and ravelling will control. The effects <br />of the spoil pile were then evaluated for the north wall configuration wfth a slope <br />angle of 60 degrees,l /2 water pseudostatic conditions. The model indicated minimal <br />effects on the factor of safety at distances greater than 150 feet. <br />Rockfall/Bench Width <br />As we understand, the minimum bench width used in the East pit is 25 feet. <br />this width combined with bench heights on the order of 80 to 100 feet has performed <br />satisfactorily. We simulated a 100 foot high bench and 25 foot width using the <br />Colorado Rockfall Simulation Program14. The rockfall is expected to be very angular <br />and slab or cubical in shape. The benches are typically placed at the base of a coal <br />seam which results in loosened, fractured surface on the bench which will tend to <br />absorb energy. Based upon estimated properties, the results of the rockfall <br />simulation indicate 85 to 95 percent of 1 foot diameter rockfall will be caught by the <br />first 25 foot bench and up to 99 percent by the next, lower 25 foot wide bench. In <br />our opinion the 25 foot bench width is reasonable, provided a low risk of rockfall <br />passing the bench is acceptable. <br />Summary of Stability Analysis <br />The results of both the stepped wedge model and the circular model were <br />considered to develop the slope recommendations. The models were intended to <br />GOLOWYO COAL COMPANY, L P. <br />crLrt za,ses 15 <br />