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• -16- <br />which will provide entry protection. Considering that the formations above <br />the massive gray sandstone have weathered and slaked to relatively flat <br />angles, we believe that a bench should be provided at the interface between <br />the brown and gray sandstones. The Colorado Department of Highways has <br />found that catch benches tend to reduce long -term maintenance costs. We <br />recommend that the bench be at least 15 feet in width to accommodate construc- <br />tion equipment. We believe that entry liners should extend about 10 feet past <br />the footwall into the open pit to provide protection from rockfall in the <br />formations below the upper catch bench. Excavation of the entry wail by <br />pre - splitting rather than mass blasting can result in a cleaner face and <br />further reduce risks associated with rockfall. <br />At the time of our field investigation, most of the soil above the bed- <br />rock had been removed in the vicinity of the highwalll. It is expected that <br />at least some soil will occur above bedrock if the highwall is moved northwest. <br />In our opinion, stability of the soil cut must be given consideration. Based <br />upon previous work done within the mine, we believe that soil slopes will <br />be stable at ratios no steeper than 2:1 (horizontal to vertical) to heights <br />of about 15 feet. Flatter slopes may be desirable to reduce erosion and <br />maintenance. <br />Conclusions and Recommendations <br />Based upon our analysis, we recommend the following criteria for <br />construction of the highwall. Figure 7 graphically presents our recom- <br />mended configuration. <br />