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• For SAIC Zones 5 and 6 (Zones 3 through 9 in this Studyj <br />• Hazards: <br />- zones are presently quite stable. <br />- there is some potential for large rock wedges. <br />- effect of subsidence will be to increase the continuity of joints and <br />create new joints. <br />- large numbers of blocks and wedges are likely to come down over a <br />period of years <br />- some blocks will reach the road <br />- a block (10 ft x 10 ft x 10 ft) almost reached the road in Zone 5. <br />- potential for large number of block and wedge failures <br />- impact not likely to be severe since most blocks will stop before <br />reaching road <br />- occasional rocks can be expected to reach road <br />Recommendations: None in text. <br />In May, 1994, an area of the Twentymile sandstone cliff failed over longwall panel 6. This <br />• area is Northeast of the limits of the previous study area and was not previously <br />evaluated due to the distance of the road from the cliff and the generally favorable <br />foreslope topography. The locations of the two major failure areas and boulders that <br />emanated from each area are shown on Drawing 1. <br />1.2 Approach <br />The approach was primarily defined by the requirements of the Colorado Rockfall <br />Simulation Program (CRSP), that was chosen to model boulder travel from the cliff <br />towards the road, and previous experience. Several assumptions were made to <br />constrain and simplify the study; assumptions included: <br />Although the previous analysis by SAIC indicated that no mining related <br />• increases in the likelihood of failure were present in SAIC zones 1 through 4 <br />3 <br />